Detect Setuid and Setgid in Splunk
Adversaries abuse the setuid (SUID) and setgid (SGID) permission bits on Linux and macOS to execute code in another user's context, typically root. When a file with SUID is executed, it runs as the file owner rather than the executing user. Adversaries can set SUID on their malware to enable future privilege escalation, or exploit existing SUID binaries listed on GTFOBins. Keydnap malware added setuid to binaries; Exaramel for Linux used a setuid binary for privilege escalation. The find command is commonly used by attackers to discover exploitable SUID/SGID binaries.
MITRE ATT&CK
- Technique
- T1548 Abuse Elevation Control Mechanism
- Sub-technique
- T1548.001 Setuid and Setgid
- Canonical reference
- https://attack.mitre.org/techniques/T1548/001/
SPL Detection Query
index=linux_logs (sourcetype="linux_secure" OR sourcetype="syslog" OR sourcetype="auditd")
| eval detection_type=case(
sourcetype="auditd" AND match(_raw, "(?i)(chmod|fchmod)") AND
match(_raw, "(?i)(4[0-7]{3}|u\+s|2[0-7]{3}|g\+s|6[0-7]{3})"),
"Auditd_Chmod_SUID_SGID",
sourcetype="syslog" AND match(_raw, "(?i)chmod.*([+]s|4[0-7]{3}|2[0-7]{3})"),
"Syslog_SUID_Set",
match(_raw, "(?i)find.*(-perm.*(\+4000|\-4000|/4000|\+2000|\-2000|setuid|setgid))"),
"Find_SUID_Discovery",
true(), null()
)
| where isnotnull(detection_type)
| eval user=coalesce(user, extractSlow("uid=(\\d+)", _raw))
| table _time, host, user, detection_type, _raw
| sort - _timeDetects setuid/setgid abuse via Linux audit logs (auditd) and syslog. Auditd chmod/fchmod syscall events with SUID/SGID bit patterns (4xxx, 2xxx, u+s, g+s) are the primary signal. Syslog chmod entries with +s or numeric SUID patterns are a secondary signal. Find commands searching for -perm with SUID/SGID patterns indicate attacker discovery phase.
Data Sources
Required Sourcetypes
False Positives & Tuning
- Package manager post-install scripts setting SUID on system utilities
- System administrators setting SUID on authorized binaries
- Security audit scripts enumerating SUID files
Other platforms for T1548.001
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 1Set SUID Bit on a Test Binary
Expected signal: Syslog/auditd: chmod syscall on /tmp/df00tech-suid-test with mode 04xxx. Process creation event for chmod with u+s argument. Sysmon for Linux (if deployed): FileModify event for /tmp/df00tech-suid-test.
- Test 2Discover SUID Binaries on the System
Expected signal: Process creation event for find with -perm /4000 argument. Syslog entry for find execution.
- Test 3Set SGID Bit on Test File
Expected signal: Auditd: chmod syscall with mode=02755 for /tmp/df00tech-sgid-test. Process creation for chmod command.
References (5)
- https://attack.mitre.org/techniques/T1548/001/
- https://gtfobins.github.io/#+suid
- https://www.welivesecurity.com/2016/07/06/new-osxkeydnap-malware-hungry-credentials/
- http://man7.org/linux/man-pages/man2/setuid.2.html
- https://github.com/redcanaryco/atomic-red-team/blob/master/atomics/T1548.001/T1548.001.md
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