T1574.006 - Dynamic Linker Hijacking#
Adversaries may execute their own malicious payloads by hijacking environment variables the dynamic linker uses to load shared libraries. During the execution preparation phase of a program, the dynamic linker loads specified absolute paths of shared libraries from environment variables and files, such as LD_PRELOAD on Linux or DYLD_INSERT_LIBRARIES on macOS. Libraries specified in environment variables are loaded first, taking precedence over system libraries with the same function name.(Citation: Man LD.SO)(Citation: TLDP Shared Libraries)(Citation: Apple Doco Archive Dynamic Libraries) These variables are often used by developers to debug binaries without needing to recompile, deconflict mapped symbols, and implement custom functions without changing the original library.(Citation: Baeldung LD_PRELOAD)
On Linux and macOS, hijacking dynamic linker variables may grant access to the victim process’s memory, system/network resources, and possibly elevated privileges. This method may also evade detection from security products since the execution is masked under a legitimate process. Adversaries can set environment variables via the command line using the export command, setenv function, or putenv function. Adversaries can also leverage Dynamic Linker Hijacking to export variables in a shell or set variables programmatically using higher level syntax such Python’s os.environ.
On Linux, adversaries may set LD_PRELOAD to point to malicious libraries that match the name of legitimate libraries which are requested by a victim program, causing the operating system to load the adversary’s malicious code upon execution of the victim program. LD_PRELOAD can be set via the environment variable or /etc/ld.so.preload file.(Citation: Man LD.SO)(Citation: TLDP Shared Libraries) Libraries specified by LD_PRELOAD are loaded and mapped into memory by dlopen() and mmap() respectively.(Citation: Code Injection on Linux and macOS)(Citation: Uninformed Needle) (Citation: Phrack halfdead 1997)(Citation: Brown Exploiting Linkers)
On macOS this behavior is conceptually the same as on Linux, differing only in how the macOS dynamic libraries (dyld) is implemented at a lower level. Adversaries can set the DYLD_INSERT_LIBRARIES environment variable to point to malicious libraries containing names of legitimate libraries or functions requested by a victim program.(Citation: TheEvilBit DYLD_INSERT_LIBRARIES)(Citation: Timac DYLD_INSERT_LIBRARIES)(Citation: Gabilondo DYLD_INSERT_LIBRARIES Catalina Bypass)
Atomic Tests#
Atomic Test #3 - Dylib Injection via DYLD_INSERT_LIBRARIES#
injects a dylib that opens calculator via env variable
Supported Platforms: macos
Elevation Required (e.g. root or admin)
Dependencies: Run with bash!#
Description: Compile the dylib from (#{source_file}). Destination is #{dylib_file}#
Check Prereq Commands:#
gcc -dynamiclib PathToAtomicsFolder/T1574.006/src/MacOS/T1574.006.c -o /tmp/T1574006MOS.dylib
Get Prereq Commands:#
gcc -dynamiclib PathToAtomicsFolder/T1574.006/src/MacOS/T1574.006.c -o /tmp/T1574006MOS.dylib
Invoke-AtomicTest T1574.006 -TestNumbers 3 -GetPreReqs
Attack Commands: Run with bash#
DYLD_INSERT_LIBRARIES=/tmp/T1574006MOS.dylib /Applications/Firefox.app/Contents/MacOS/firefox
Invoke-AtomicTest T1574.006 -TestNumbers 3
Cleanup:#
kill `pgrep Calculator`
kill `pgrep firefox`
Invoke-AtomicTest T1574.006 -TestNumbers 3 -Cleanup
Detection#
Monitor for changes to environment variables and files associated with loading shared libraries such as LD_PRELOAD and DYLD_INSERT_LIBRARIES, as well as the commands to implement these changes.
Monitor processes for unusual activity (e.g., a process that does not use the network begins to do so). Track library metadata, such as a hash, and compare libraries that are loaded at process execution time against previous executions to detect differences that do not correlate with patching or updates.