Detect Break Process Trees in Microsoft Sentinel
An adversary may attempt to evade process tree-based analysis by modifying executed malware's parent process ID (PPID). If endpoint protection software leverages the parent-child relationship for detection, breaking this relationship could result in the adversary's behavior not being associated with previous process tree activity. On Linux systems, adversaries may execute a series of Native API calls to alter malware's process tree. For example, adversaries can execute their payload without any arguments, call the fork() API call twice, then have the parent process exit. This creates a grandchild process with no parent process that is immediately adopted by the init system process (PID 1), which successfully disconnects the execution of the adversary's payload from its previous process tree. Another example is using the daemon syscall to detach from the current parent process and run in the background.
MITRE ATT&CK
- Tactic
- Defense Evasion
- Technique
- T1036 Masquerading
- Sub-technique
- T1036.009 Break Process Trees
- Canonical reference
- https://attack.mitre.org/techniques/T1036/009/
KQL Detection Query
Syslog
| where Timestamp > ago(24h)
| where Facility == "authpriv" or Facility == "auth" or Facility == "daemon"
| where SyslogMessage has_any ("fork", "daemon", "setsid", "double fork")
| project Timestamp, Computer, Facility, SyslogMessage;
let OrphanedProcesses = Syslog
| where Timestamp > ago(24h)
| where ProcessName !in ("systemd", "init", "cron", "atd", "sshd", "dockerd", "containerd")
| where SyslogMessage has "PPID=1" or SyslogMessage has "ppid=1"
| project Timestamp, Computer, ProcessName, SyslogMessage;
let AuditFork = Syslog
| where Timestamp > ago(24h)
| where Facility == "authpriv"
| where SyslogMessage has "type=SYSCALL" and SyslogMessage has_any ("syscall=57", "syscall=58", "syscall=56")
| project Timestamp, Computer, SyslogMessage;
union OrphanedProcesses, AuditFork
| sort by Timestamp descDetects potential process tree breaking on Linux systems by monitoring for processes adopted by init/systemd (PPID=1) that are not expected system daemons, and by tracking fork/clone/vfork syscalls via auditd. The detection monitors syslog for orphaned processes and audit logs for the specific syscall sequence (fork, clone, vfork — syscall numbers 57, 58, 56 on x86_64) used by malware like BPFDoor and XorDdos to break process tree lineage.
Data Sources
Required Tables
False Positives & Tuning
- Legitimate Linux daemons that intentionally double-fork during startup (e.g., Apache httpd, nginx, postfix, named) — these are expected to run with PPID=1
- System administration scripts that use 'nohup' or 'disown' to detach long-running background processes from the controlling terminal
- Container runtimes (Docker, containerd, CRI-O) that fork child processes which naturally become orphaned when the parent container process exits
- Cron jobs and at-scheduled tasks that fork child processes — the cron daemon itself runs with PPID=1
Other platforms for T1036.009
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 1Double Fork Process Tree Break
Expected signal: Auditd SYSCALL records for fork/clone syscalls (57/56) from bash. The resulting 'sleep 120' process will show PPID=1 in process listings. Syslog may record the orphaned process adoption.
- Test 2Daemon Syscall via Python
Expected signal: Auditd SYSCALL records for fork (57) and setsid (112) syscalls from python3. The grandchild python3 process will show PPID=1 and a new session ID (SID) in process status. /proc/<pid>/status will show PPid=1.
- Test 3Nohup Background Process Detachment
Expected signal: Auditd SYSCALL records for fork/clone from bash, followed by the nohup process. The sleep process may show PPID=1 after the parent shell exits. Process creation event from Sysmon for Linux (if deployed) shows nohup spawning sleep.
References (7)
- https://attack.mitre.org/techniques/T1036/009/
- https://0xjet.github.io/3OHA/2022/04/11/post.html
- https://sandflysecurity.com/blog/bpfdoor-an-evasive-linux-backdoor-technical-analysis/
- https://www.microsoft.com/en-us/security/blog/2022/05/19/rise-in-xorddos-a-deeper-look-at-the-stealthy-ddos-malware-targeting-linux-devices/
- https://man7.org/linux/man-pages/man2/fork.2.html
- https://man7.org/linux/man-pages/man3/daemon.3.html
- https://github.com/redcanaryco/atomic-red-team/blob/master/atomics/T1036.009/T1036.009.md
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