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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.
**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:
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.
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.
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_):
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.
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](https://unix.stackexchange.com/questions/454708/how-do-you-add-cap-sys-admin-permissions-to-user-in-centos-7), [this ](http://manpages.ubuntu.com/manpages/bionic/man5/capability.conf.5.html)and [this ](https://stackoverflow.com/questions/1956732/is-it-possible-to-configure-linux-capabilities-per-user)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`.\
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).
The **capability-aware binaries won't use the new capabilities** given by the environment, however the **capability dumb binaries will us**e them as they won't reject them. This makes capability-dumb binaries vulnerable inside a special environment that grant capabilities to binaries.
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:
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.
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:
[**CAP\_SYS\_ADMIN**](https://man7.org/linux/man-pages/man7/capabilities.7.html) 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.
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
**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**](https://man7.org/linux/man-pages/man7/capabilities.7.html) 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](https://gist.github.com/thejh/8346f47e359adecd1d53) or the **following PoC**:
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`
3. Find a **program** to **inject** the **shellcode** into a process memory ([https://github.com/0x00pf/0x00sec\_code/blob/master/mem\_inject/infect.c](https://github.com/0x00pf/0x00sec\_code/blob/master/mem\_inject/infect.c))
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`)**.**
[**CAP\_SYS\_MODULE**](https://man7.org/linux/man-pages/man7/capabilities.7.html) 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.\
In the following example the binary **`kmod`** has this capability.
```bash
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.
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
**The code of this technique was copied from the laboratory of "Abusing SYS\_MODULE Capability" from** [**https://www.pentesteracademy.com/**](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](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**](https://man7.org/linux/man-pages/man7/capabilities.7.html) 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](https://medium.com/@fun\_cuddles/docker-breakout-exploit-analysis-a274fff0e6b3) exploit.\
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
You can learn how the following exploiting works in [https://medium.com/@fun\_cuddles/docker-breakout-exploit-analysis-a274fff0e6b3](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](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.**
```c
#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
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:
**The code of this technique was copied from the laboratory of "Abusing DAC\_READ\_SEARCH Capability" from** [**https://www.pentesteracademy.com/**](https://www.pentesteracademy.com)
**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**](payloads-to-execute.md#overwriting-a-file-to-escalate-privileges).
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
First of all read the previous section that [**abuses DAC\_READ\_SEARCH capability to read arbitrary files**](linux-capabilities.md#cap\_dac\_read\_search) of the host and **compile** the exploit.\
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**](https://www.pentesteracademy.com)
Lets suppose the **`python`** binary has this capability, you can **change** the **owner** of the **shadow** file, **change root password**, and escalate privileges:
There are a lot of files you can **overwrite to escalate privileges,** [**you can get ideas from here**](payloads-to-execute.md#overwriting-a-file-to-escalate-privileges).
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:
In this case the group shadow was impersonated so you can read the file `/etc/shadow`:
```bash
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](./#writable-docker-socket).
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**:
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**](https://man7.org/linux/man-pages/man7/capabilities.7.html) 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](http://lkml.iu.edu/hypermail/linux/kernel/9907.0/0132.html). As per the man page, this also allows the holder to descriptively `perform a range of device-specific operations on other devices`.
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.
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.
```bash
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
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)
[**CAP\_NET\_RAW**](https://man7.org/linux/man-pages/man7/capabilities.7.html) 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.
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/](https://attackdefense.pentesteracademy.com)
[**CAP\_NET\_ADMIN**](https://man7.org/linux/man-pages/man7/capabilities.7.html) 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.
[**CAP\_SYS\_CHROOT**](https://man7.org/linux/man-pages/man7/capabilities.7.html) permits the use of the `chroot(2)` system call. This may allow escaping of any `chroot(2)` environment, using known weaknesses and escapes:
* [How to break out from various chroot solutions](https://deepsec.net/docs/Slides/2015/Chw00t\_How\_To\_Break%20Out\_from\_Various\_Chroot\_Solutions\_-\_Bucsay\_Balazs.pdf)
[**CAP\_SYS\_BOOT**](https://man7.org/linux/man-pages/man7/capabilities.7.html) 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](https://man7.org/linux/man-pages/man7/capabilities.7.html) 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.
**Most of these examples were taken from some labs of** [**https://attackdefense.pentesteracademy.com/**](https://attackdefense.pentesteracademy.com), so if you want to practice this privesc techniques I recommend these labs.
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