Detect Trap in Microsoft Sentinel
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/
KQL Detection Query
DeviceProcessEvents
| where Timestamp > ago(24h)
| where FileName in~ ("bash", "sh", "zsh", "ksh", "dash")
| where ProcessCommandLine has "trap"
| where ProcessCommandLine has_any (
"EXIT", "SIGTERM", "SIGHUP", "SIGINT", "ERR",
"0", "1", "2", "15" // signal numbers
)
| extend TrapPayload = extract(@"trap\s+['\"]?(.+?)['\"]?\s+(EXIT|SIGTERM|SIGHUP|SIGINT|ERR|[0-9]+)", 1, ProcessCommandLine)
| extend SuspiciousTrap = ProcessCommandLine has_any (
"curl", "wget", "nc", "ncat", "bash -i", "/dev/tcp",
"python", "perl", "ruby", "base64", "eval"
)
| where SuspiciousTrap
| project Timestamp, DeviceName, AccountName, FileName, ProcessCommandLine,
TrapPayload, SuspiciousTrap,
InitiatingProcessFileName, InitiatingProcessCommandLine
| sort by Timestamp descDetects suspicious use of the bash/zsh trap command with potentially malicious payloads. Monitors shell process command lines for trap invocations targeting common signals (EXIT, SIGTERM, SIGHUP, SIGINT, ERR) combined with indicators of malicious activity such as download tools, reverse shell patterns, or scripting languages. Focuses on trap commands that include network utilities, encoding functions, or execution of external interpreters.
Data Sources
Required Tables
False Positives & Tuning
- Shell scripts that use trap for legitimate cleanup operations (removing temp files on exit, resetting terminal settings on SIGINT)
- Database backup scripts that use trap EXIT to ensure connection cleanup and lock release on unexpected termination
- CI/CD pipeline scripts that use trap ERR for error handling and rollback operations
- System administration scripts that use trap SIGTERM/SIGHUP for graceful shutdown of services or daemons
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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