Event Triggered Execution: LC_LOAD_DYLIB Addition

Adversaries may establish persistence by executing malicious content triggered by the execution of tainted binaries. Mach-O binaries have a series of headers that are used to perform certain operations when a binary is loaded. The LC_LOAD_DYLIB header in a Mach-O binary tells macOS and OS X which dynamic libraries (dylibs) to load during execution time. These can be added ad-hoc to the compiled binary as long as adjustments are made to the rest of the fields and dependencies. [1] There are tools available to perform these changes.

Adversaries may modify Mach-O binary headers to load and execute malicious dylibs every time the binary is executed. Although any changes will invalidate digital signatures on binaries because the binary is being modified, this can be remediated by simply removing the LC_CODE_SIGNATURE command from the binary so that the signature isn’t checked at load time. [2]

ID: T1546.006
Sub-technique of:  T1546
Tactics: Privilege Escalation, Persistence
Platforms: macOS
Permissions Required: User
Data Sources: Binary file metadata, File monitoring, Process command-line parameters, Process monitoring
Version: 1.0
Created: 24 January 2020
Last Modified: 24 March 2020

Mitigations

Mitigation Description
Audit

Binaries can also be baselined for what dynamic libraries they require, and if an app requires a new dynamic library that wasn\u2019t included as part of an update, it should be investigated.

Code Signing

Enforce that all binaries be signed by the correct Apple Developer IDs.

Execution Prevention

Allow applications via known hashes.

Detection

Monitor processes for those that may be used to modify binary headers. Monitor file systems for changes to application binaries and invalid checksums/signatures. Changes to binaries that do not line up with application updates or patches are also extremely suspicious.

References