Detect Ignore Process Interrupts in Splunk
Adversaries evade defensive mechanisms by launching processes immune to interrupt signals, preventing analyst-driven or system-triggered termination. The primary technique is nohup on Linux and macOS, which detaches a process from the controlling terminal and causes it to ignore SIGHUP—the hangup signal sent when a session ends or a terminal closes. Malware authors also call signal() or sigaction() directly to mask SIGINT, SIGTERM, SIGPIPE, SIGCHLD, and other control signals, as documented in BPFDoor (masks 7 signals) and BOLDMOVE (masks SIGCHLD, SIGHUP, SIGPIPE). On Windows, PowerShell's -ErrorAction SilentlyContinue or $ErrorActionPreference = 'SilentlyContinue' prevents script termination on errors, allowing malicious payloads to continue past failures that would otherwise halt execution. Real-world usage includes GoldMax Linux variant (nohup invocation for C2 persistence through SSH disconnection), UNC3886 (nohup /bin/support in /etc/init.d/localnet for semi-persistence across reboots), Sea Turtle running SnappyTCP via nohup, and OSX/Shlayer applying nohup to payload execution on macOS. Unlike Trap (T1546.005), this technique does not re-invoke the process after termination—it only prolongs the existing execution session through events that would otherwise end it.
MITRE ATT&CK
- Tactic
- Defense Evasion
- Technique
- T1564 Hide Artifacts
- Sub-technique
- T1564.011 Ignore Process Interrupts
- Canonical reference
- https://attack.mitre.org/techniques/T1564/011/
SPL Detection Query
(
index=linux_logs (sourcetype="linux_audit" type=EXECVE a0="nohup")
| eval DetectionType="nohup_exec", Platform="Linux"
| eval NohupTarget=coalesce(a1, "")
| eval NohupArgs=mvjoin(mvsort(mvfilter(match(mvmap(split("a2 a3 a4 a5 a6 a7", " "), "*"), ".*")), " ")
| eval BackgroundExecution=if(match(NohupTarget."-".NohupArgs, "&"), 1, 0)
| eval TempDirPayload=if(match(NohupTarget, "(/tmp/|/var/tmp/|/dev/shm/)"), 1, 0)
| eval SuspiciousNetTool=if(match(NohupTarget, "(netcat|ncat|socat|/bin/nc)"), 1, 0)
| eval InteractiveShell=if(match(coalesce(a2,"")."-".coalesce(a3,""), "(-i|-c)"), 1, 0)
| eval PersistencePath=if(match(NohupTarget, "(/etc/init\.d/|/etc/rc|/etc/cron|/etc/profile)"), 1, 0)
| eval SuspicionScore=BackgroundExecution+TempDirPayload+SuspiciousNetTool+InteractiveShell+PersistencePath
| table _time, host, auid, uid, a0, a1, a2, a3, a4, DetectionType, Platform, SuspicionScore
)
| append
[search index=linux_logs sourcetype="syslog" "nohup " NOT (process="cron" OR process="anacron" OR process="CRON")
| rex field=_raw "nohup\s+(?P<NohupTarget>\S+)(?P<NohupArgs>.*)"
| eval DetectionType="nohup_syslog", Platform="Linux/macOS"
| eval TempDirPayload=if(match(NohupTarget, "(/tmp/|/var/tmp/|/dev/shm/)"), 1, 0)
| eval SuspiciousNetTool=if(match(NohupTarget, "(netcat|ncat|socat|/bin/nc)"), 1, 0)
| eval SuspicionScore=TempDirPayload+SuspiciousNetTool
| table _time, host, process, NohupTarget, NohupArgs, DetectionType, Platform, SuspicionScore]
| append
[search index=wineventlog sourcetype="XmlWinEventLog:Microsoft-Windows-Sysmon/Operational" EventCode=1
(Image="*\\powershell.exe" OR Image="*\\pwsh.exe")
(CommandLine="*SilentlyContinue*" OR CommandLine="*-ErrorAction Ignore*" OR CommandLine="*ErrorActionPreference*")
| eval CmdLower=lower(CommandLine)
| eval DetectionType="ps_error_suppression", Platform="Windows"
| eval HasMaliciousIndicator=if(match(CmdLower, "(invoke-webrequest|iwr\s|net\.webclient|downloadstring|downloadfile|invoke-expression|iex\(|iex\s|-encodedcommand|-enc\s)"), 1, 0)
| eval HasPersistence=if(match(CmdLower, "(schtasks|sc\s+create|reg\s+add|new-service|set-mppreference)"), 1, 0)
| eval HasNetworkTool=if(match(CmdLower, "(start-bitstransfer|certutil|bitsadmin)"), 1, 0)
| eval SuspicionScore=HasMaliciousIndicator+HasPersistence+HasNetworkTool
| where SuspicionScore > 0
| table _time, host, User, Image, CommandLine, ParentImage, ParentCommandLine, DetectionType, Platform, SuspicionScore]
| sort - _timeDetects T1564.011 across Linux and Windows using a three-branch union. Branch 1 uses linux_audit sourcetype (auditd) to catch nohup via EXECVE records where a0='nohup', evaluating the target binary and arguments for suspicious indicators. Branch 2 catches nohup in general syslog output while excluding cron-originated executions (common false positive source). Branch 3 uses Sysmon EventCode=1 to detect PowerShell running with error suppression combined with malicious indicators—download cradles, encoded commands, persistence actions, and network tools. All branches assign a suspicion score for analyst prioritization.
Data Sources
Required Sourcetypes
False Positives & Tuning
- System administrators running long-duration jobs with nohup over SSH (rsync, tar, database exports, compilation jobs)
- CI/CD pipeline runners using nohup to background test processes or long-running build steps
- Monitoring agents (Datadog, New Relic, Nagios plugins) launched via cron or init scripts using nohup
- PowerShell DSC or SCCM deployment scripts using -ErrorAction SilentlyContinue to handle expected configuration drift
- Package management scripts (apt, yum hooks) that use nohup internally for non-blocking post-install steps
- Scheduled batch jobs where nohup is standard practice for session independence
Other platforms for T1564.011
Testing Methodology
Validate this detection against 4 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 1nohup Background Execution with Network Listener
Expected signal: Linux auditd EXECVE record: a0='nohup', a1='nc', a2='-lvp', a3='14444', a4='-s', a5='127.0.0.1'. A second EXECVE record for the nc process itself with ppid matching the nohup process. MDE DeviceProcessEvents: FileName='nohup' with child process FileName='nc' where InitiatingProcessFileName='nohup'. DeviceNetworkEvents: nc process creating a listening socket on port 14444.
- Test 2nohup Protecting Interpreter from Temp Directory
Expected signal: Auditd EXECVE: a0='chmod', a1='+x', a2='/tmp/df00tech_test_payload.sh' followed by a0='nohup', a1='/tmp/df00tech_test_payload.sh'. MDE DeviceFileEvents: file creation at /tmp/df00tech_test_payload.sh. DeviceProcessEvents: FileName='nohup', ProcessCommandLine contains '/tmp/' (TempDirPayload indicator). A second DeviceProcessEvents record for bash/sh with InitiatingProcessFileName='nohup' and FolderPath='/tmp/'.
- Test 3Programmatic Signal Masking via Python
Expected signal: Auditd EXECVE: a0='python3', a1='-c', a2 contains 'signal.SIG_IGN'. MDE DeviceProcessEvents: FileName='python3', ProcessCommandLine contains 'SIG_IGN' and 'signal'. /proc/<pid>/status SigIgn field will be non-zero (e.g., 0x0000000000003c06 with SIGHUP=1, SIGINT=2, SIGTERM=15, SIGPIPE=13 masked). Attempting `kill -SIGTERM <pid>` will have no effect; only `kill -9 <pid>` terminates the process.
- Test 4PowerShell Error Suppression with Download Cradle
Expected signal: Sysmon Event ID 1: Process Create — Image=powershell.exe, CommandLine contains 'ErrorActionPreference' and 'SilentlyContinue' and 'Net.WebClient' and 'DownloadString' and 'Invoke-WebRequest'. Sysmon Event ID 3: Network Connection attempts to 127.0.0.1:19999 (will fail with connection refused). PowerShell ScriptBlock Log Event ID 4104 in Microsoft-Windows-PowerShell/Operational with full script content.
References (11)
- https://attack.mitre.org/techniques/T1564/011/
- https://man7.org/linux/man-pages/man7/signal.7.html
- https://linux.die.net/man/1/nohup
- https://learn.microsoft.com/powershell/module/microsoft.powershell.core/about/about_preference_variables?view=powershell-7.3#erroractionpreference
- https://www.crowdstrike.com/blog/observations-from-the-stellarparticle-campaign/
- https://cloud.google.com/blog/topics/threat-intelligence/boldmove-north-korea-fortigate
- https://www.mandiant.com/resources/blog/fortinet-malware-ecosystem
- https://www.deepinstinct.com/blog/bpfdoor-an-active-chinese-global-surveillance-tool
- https://www.huntress.com/blog/a-brief-history-of-sea-turtle
- https://www.jamf.com/blog/shlayer-malware-abusing-gatekeeper-bypass-on-macos/
- https://github.com/redcanaryco/atomic-red-team/blob/master/atomics/T1564.011/T1564.011.md
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