Detect Trap in Splunk
Adversaries may establish persistence by executing malicious content triggered by an interrupt signal using the trap command. The trap command is used in Unix/Linux shell scripting to specify commands that will execute when the shell receives a particular signal. Adversaries can abuse the trap command to establish persistence that executes when the shell or system receives specific signals such as EXIT (when the shell exits), ERR (on error), or signal numbers like SIGINT (2), SIGHUP (1), or SIGTERM (15). The trap command can be placed in shell initialization files to execute malicious commands whenever a user session ends or encounters an error.
MITRE ATT&CK
- Tactic
- Privilege Escalation Persistence
- Technique
- T1546 Event Triggered Execution
- Sub-technique
- T1546.005 Trap
- Canonical reference
- https://attack.mitre.org/techniques/T1546/005/
SPL Detection Query
index=syslog (sourcetype=linux_secure OR sourcetype=syslog)
| eval IsTrapCmd=if(match(_raw, "\\btrap\\b.*(EXIT|SIGTERM|SIGHUP|SIGINT|ERR|\\b[0-9]+\\b)"), 1, 0)
| eval HasSuspiciousPayload=if(match(_raw, "(curl|wget|nc\s|ncat|/dev/tcp|bash\s+-i|python[23]?\s+-c|perl\s+-e|base64|eval)"), 1, 0)
| where IsTrapCmd=1 AND HasSuspiciousPayload=1
| rex field=_raw "CMD\s*=\s*(?P<CMD>.+)"
| rex field=CMD "trap\s+(?P<TrapPayload>.+)"
| table _time, host, user, CMD, TrapPayload
| sort - _timeDetects suspicious trap commands in Linux system logs. Requires auditd with execve syscall logging to capture command arguments. Flags trap invocations that contain download tools (curl, wget), netcat, /dev/tcp reverse shell patterns, or script interpreters with inline code execution flags. The combination of trap targeting an exit/error signal with a network payload is the primary detection signal.
Data Sources
Required Sourcetypes
False Positives & Tuning
- Shell scripts using trap for legitimate cleanup on EXIT
- Database scripts using trap to ensure proper connection cleanup
- CI/CD pipeline scripts using trap ERR for error handling
- System administration scripts using trap for graceful service shutdown
Other platforms for T1546.005
Testing Methodology
Validate this detection against 3 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 1Establish EXIT Trap in Interactive Shell
Expected signal: Process creation event for bash with command line containing 'trap' and 'EXIT'. The curl invocation in the trap payload. When the shell exits, a child process for curl is spawned by the shell process.
- Test 2Add SIGTERM Trap to Shell Config
Expected signal: File modification event for ~/.bashrc. Process creation for bash writing the trap via echo. The trap payload in the file contains wget and chmod+execute pattern — high severity indicator.
- Test 3Trap ERR Signal for Stealthy Execution
Expected signal: Process creation for bash -c with trap ERR in command. The ls command fails with non-zero exit (directory doesn't exist), triggering the ERR trap. Child process (echo) spawned from the bash session. File creation event for /tmp/trap_test.txt.
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