Detect LC_MAIN Hijacking in Elastic Security
Adversaries may hijack the LC_MAIN Mach-O load command in macOS binaries to redirect initial execution flow to malicious code before returning control to the legitimate entry point. The LC_MAIN header, introduced in OS X 10.8, defines the entry point offset for a Mach-O executable. By patching this offset to point at an injected code section or cave, an attacker can execute arbitrary code under the identity of a trusted binary, bypassing application whitelisting controls that validate only the file path or name. This technique has been deprecated in the MITRE ATT&CK framework but remains relevant for forensic analysis of older macOS malware samples and legacy systems.
MITRE ATT&CK
- Tactic
- Defense Evasion
- Canonical reference
- https://attack.mitre.org/techniques/T1149/
Elastic Detection Query
process where event.type == "start" and
host.os.type == "macos" and
process.name in ("otool", "jtool", "jtool2", "vtool", "install_name_tool", "lipo") and
(
process.args : "-l" or
process.args : "--load-commands" or
process.args : "LC_MAIN" or
process.args : "LC_THREAD" or
process.args : "LC_UNIXTHREAD" or
process.args : "entryoff" or
process.args : "stacksize"
) and
(
process.command_line : "/Applications/*" or
process.command_line : "/usr/bin/*" or
process.command_line : "/usr/local/bin/*" or
process.command_line : "/usr/sbin/*" or
process.command_line : "/bin/*" or
process.command_line : "/sbin/*" or
process.command_line : "/opt/*"
)Detects Mach-O inspection tools (otool, jtool, jtool2, vtool, install_name_tool, lipo) invoked with load command inspection flags (-l, --load-commands, LC_MAIN, LC_THREAD, LC_UNIXTHREAD, entryoff, stacksize) against binaries in sensitive system paths on macOS. Indicative of LC_MAIN entry-point offset reconnaissance prior to hijacking.
Data Sources
Required Tables
False Positives & Tuning
- macOS developers legitimately use otool and install_name_tool during build and link workflows targeting binaries in /usr/local/bin or /opt/homebrew
- Security researchers and reverse engineers running jtool2 or MachOView to audit applications installed in /Applications as part of authorized vulnerability research
- System administrators using lipo or vtool to inspect universal/fat binaries when managing multi-architecture deployments on Apple Silicon transition environments
Other platforms for T1149
Testing Methodology
Validate this detection against 5 adversary techniques from Atomic Red Team. Each test below lists the behaviour to exercise and the telemetry you should expect to see. Executable commands and cleanup steps are available with Pro.
- Test 1Inspect LC_MAIN Entry Point of a System Binary
Expected signal: macOS Unified Log / ESF process event: process_name=otool, cmdline='otool -l /bin/ls', parent=bash/zsh. osquery process_open_files will show /bin/ls opened for reading by otool. No file modification events are generated by this read-only operation.
- Test 2Enumerate All Load Commands of a Sensitive Application Binary
Expected signal: ESF/stream:process event: process_name=otool, cmdline targeting /Applications/Safari.app/Contents/MacOS/Safari with -l flag. macOS FSEvent: Safari binary opened for reading with otool PID. DeviceProcessEvents (MDE): FileName=otool, ProcessCommandLine contains '-l' and '/Applications/Safari.app/Contents/MacOS/Safari'.
- Test 3Verify Code Signature Validity of a Modified Binary
Expected signal: ESF process event: process_name=codesign, cmdline contains '-v --deep --strict /bin/ls'. macOS Unified Log subsystem com.apple.security.codesigning records the verification result with target binary path and signing identity. If a binary were actually modified, this command would produce a 'code object is not signed at all' or 'a sealed resource is missing or invalid' error.
- Test 4Simulate Code Cave Discovery Using nm and size
Expected signal: ESF process events for nm and size with respective command lines targeting /usr/bin/true. Both binaries are in /usr/bin/ (a monitored sensitive path). DeviceProcessEvents: FileName in ('nm', 'size'), ProcessCommandLine contains '/usr/bin/true'. These events fire consecutively and may indicate scripted reconnaissance.
- Test 5Write a Test File to an App Bundle MacOS Directory (Simulated Binary Drop)
Expected signal: ESF/stream:file events: FileCreated for /tmp/TestApp.app/Contents/MacOS/TestApp and /tmp/TestApp.app/Contents/MacOS/TestApp.bak. DeviceFileEvents: ActionType=FileCreated, FolderPath contains '/MacOS/', InitiatingProcessFileName=bash/zsh. The /tmp/ path is not in the monitored sensitive paths by default — adjust the FolderPath filter to include /tmp/*.app/Contents/MacOS/ for this test to trigger the hunting query.
References (9)
- https://attack.mitre.org/techniques/T1149/
- https://assets.documentcloud.org/documents/2459197/bit9-carbon-black-threat-research-report-2015.pdf
- https://www.virusbulletin.com/uploads/pdf/conference/vb2014/VB2014-Wardle.pdf
- https://developer.apple.com/library/archive/documentation/DeveloperTools/Conceptual/MachOTopics/0-Introduction/introduction.html
- https://github.com/aidansteele/osx-abi-macho-file-format-reference
- https://redcanary.com/blog/mac-malware-2021/
- https://objective-see.org/blog.html
- https://github.com/redcanaryco/atomic-red-team/blob/master/atomics/T1149/T1149.md
- https://www.sentinelone.com/labs/20-common-tools-techniques-used-by-macos-threat-actors-malware/
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