Analyzing Persistence Mechanisms In Linux

Analyzing Persistence Mechanisms in Linux

Overview

Adversaries establish persistence on Linux systems through crontab jobs, systemd service/timer units, LD_PRELOAD library injection, shell profile modifications (.bashrc, .profile), SSH authorized_keys backdoors, and init script manipulation. This skill scans for all known persistence vectors, checks file timestamps and integrity, and correlates findings with auditd logs to build a timeline of persistence installation.

When to Use

• When investigating security incidents that require analyzing persistence mechanisms in linux
• When building detection rules or threat hunting queries for this domain
• When SOC analysts need structured procedures for this analysis type
• When validating security monitoring coverage for related attack techniques

Detection Gaps & Validation

• Watch rules not loaded = no events. auditd is the whole detection here; confirm with auditctl -l and that -w /etc/crontab -p wa, -w /etc/ld.so.preload, and the ~/.ssh/authorized_keys paths are actually armed. A FIM-only setup will miss in-memory and transient artifacts.
• Vectors commonly skipped: per-user crontabs in /var/spool/cron/crontabs/, systemd-run transient units (never touch disk), user units in ~/.config/systemd/user/, at jobs in /var/spool/cron/atjobs, /etc/update-motd.d/, udev rules, ~/.config/autostart, and PAM//etc/ld.so.conf.d modules.
• Don't trust mtime. Attackers touch -r to backdate files; pivot to inode ctime and auditd SYSCALL/PROCTITLE records for the real write time.
• LD_PRELOAD evasion: injection via a per-process env var (not /etc/ld.so.preload) leaves no file to scan — catch it in auditd execve environment or /proc/<pid>/environ.
• Validate the rule fires: add auditctl -w /etc/crontab -p wa -k cron_persist, append a benign line to /etc/crontab, then ausearch -k cron_persist — you should see the write with the acting UID.
• Tune false positives: package managers legitimately install systemd units and cron jobs. Baseline each finding against the package DB (dpkg -S <path> / rpm -qf <path>) and alert only on files not owned by any package.

Prerequisites

• Root or sudo access on target Linux system (or forensic image)
• auditd configured with file watch rules on persistence paths
• Python 3.8+ with standard library (os, subprocess, json)
• Optional: OSSEC/Wazuh agent for file integrity monitoring alerts

Steps

1. Scan Crontab Entries — Enumerate all user crontabs, /etc/cron.d/, /etc/cron.daily/, and anacron jobs for suspicious commands
2. Audit Systemd Units — Check /etc/systemd/system/ and ~/.config/systemd/user/ for non-package-managed service and timer units
3. Detect LD_PRELOAD Hijacking — Check /etc/ld.so.preload and LD_PRELOAD environment variable for injected shared libraries
4. Inspect Shell Profiles — Scan .bashrc, .bash_profile, .profile, /etc/profile.d/ for injected commands or reverse shells
5. Check SSH Authorized Keys — Audit all authorized_keys files for unauthorized public keys with command restrictions
6. Correlate Auditd Logs — Search auditd logs for file modification events on persistence paths to build an installation timeline
7. Generate Persistence Report — Produce a risk-scored report of all discovered persistence mechanisms

Expected Output

• JSON report of all persistence mechanisms found with risk scores
• Timeline of persistence installation from auditd correlation
• MITRE ATT&CK technique mapping (T1053, T1543, T1574, T1546)
• Remediation commands for each detected persistence mechanism