Description
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 various environment variables and files, such as <code>LD_PRELOAD</code> on Linux or <code>DYLD_INSERT_LIBRARIES</code> on macOS.(Citation: TheEvilBit DYLD_INSERT_LIBRARIES)(Citation: Timac DYLD_INSERT_LIBRARIES)(Citation: Gabilondo DYLD_INSERT_LIBRARIES Catalina Bypass) 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) Each platform's linker uses an extensive list of environment variables at different points in execution. These variables are often used by developers to debug binaries without needing to recompile, deconflict mapped symbols, and implement custom functions in the original library.(Citation: Baeldung LD_PRELOAD) Hijacking dynamic linker variables may grant access to the victim process's memory, system/network resources, and possibly elevated privileges. On Linux, adversaries may set <code>LD_PRELOAD</code> 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. For example, adversaries have used `LD_PRELOAD` to inject a malicious library into every descendant process of the `sshd` daemon, resulting in execution under a legitimate process. When the executing sub-process calls the `execve` function, for example, the malicious library’s `execve` function is executed rather than the system function `execve` contained in the system library on disk. This allows adversaries to [Hide Artifacts](https://attack.mitre.org/techniques/T1564) from detection, as hooking system functions such as `execve` and `readdir` enables malware to scrub its own artifacts from the results of commands such as `ls`, `ldd`, `iptables`, and `dmesg`.(Citation: ESET Ebury Oct 2017)(Citation: Intezer Symbiote 2022)(Citation: Elastic Security Labs Pumakit 2024) Hijacking dynamic linker variables may grant access to the victim process's memory, system/network resources, and possibly elevated privileges.
Threat-Mapped Scoring
ATT&CK Kill Chain Metadata
- Tactics: persistence, privilege-escalation, defense-evasion
- Platforms: Linux, macOS
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Detection Guidance:
Monitor for changes to environment variables and files associated with loading shared libraries such as <code>LD_PRELOAD</code> and <code>DYLD_INSERT_LIBRARIES</code>, 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.