Hijack Execution Flow: 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 various environment variables and files, such as LD_PRELOAD on Linux or DYLD_INSERT_LIBRARIES on macOS.^[2][3][4] Libraries specified in environment variables are loaded first, taking precedence over system libraries with the same function name.^[5][6][7] 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.^[8]

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 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. 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 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.^[9][10][11]

Hijacking dynamic linker variables may grant access to the victim process's memory, system/network resources, and possibly elevated privileges.