What it does. Scan Linux systems for persistence mechanisms including crontab/systemd entries, LD_PRELOAD injection, shell profile modifications (.bashrc, .profile), and SSH authorized_keys backdoors, then correlate findings with auditd logs into an installation timeline. Use during incident response or threat hunting to detect or confirm how an adversary maintained access to a compromised Linux host. Part of mukul975/Anthropic-Cybersecurity-Skills (817 security skills) (mukul975/Anthropic-Cybersecurity-Skills).

Install

• npx skills add mukul975/Anthropic-Cybersecurity-Skills --skill analyzing-persistence-mechanisms-in-linux, or copy the skill folder into ~/.claude/skills/analyzing-persistence-mechanisms-in-linux/.
• Raw file: curl -sL https://raw.githubusercontent.com/mukul975/Anthropic-Cybersecurity-Skills/HEAD/skills/analyzing-persistence-mechanisms-in-linux/SKILL.md

SKILL.md (verbatim)

name: analyzing-persistence-mechanisms-in-linux
description: Scan Linux systems for persistence mechanisms including crontab/systemd entries, LD_PRELOAD injection, shell profile modifications (.bashrc, .profile), and SSH authorized_keys backdoors, then correlate findings with auditd logs into an installation timeline. Use during incident response or threat hunting to detect or confirm how an adversary maintained access to a compromised Linux host.
domain: cybersecurity
subdomain: threat-hunting
tags:
- linux-persistence
- crontab
- systemd
- ld-preload
- auditd
- threat-hunting
- incident-response
mitre_attack:
- T1053.003
- T1543.002
- T1574.006
- T1546.004
- T1098.004
version: '1.0'
author: mahipal
license: Apache-2.0
d3fend_techniques:
- Executable Denylisting
- Execution Isolation
- File Metadata Consistency Validation
- Process Termination
- Content Format Conversion
nist_csf:
- DE.CM-01
- DE.AE-02
- DE.AE-07
- ID.RA-05

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

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

Other files in this skill

• LICENSE
• references/api-reference.md
• scripts/agent.py

references/api-reference.md (verbatim)

Linux Persistence Mechanisms Detection API Reference

Crontab Inspection Commands

# List current user crontab
crontab -l

# List crontab for a specific user (requires root)
crontab -l -u username

# List all system cron jobs
ls -la /etc/cron.d/ /etc/cron.daily/ /etc/cron.hourly/ /etc/cron.weekly/ /etc/cron.monthly/
cat /etc/crontab

# Find recently modified cron files
find /var/spool/cron/ /etc/cron* -mtime -7 -type f 2>/dev/null

Systemd Unit Audit Commands

# List all enabled services
systemctl list-unit-files --type=service --state=enabled

# List all active timers
systemctl list-timers --all

# Show service details
systemctl cat suspicious.service

# Find non-package-managed unit files
find /etc/systemd/system/ -name '*.service' -exec sh -c \
  'dpkg -S "$1" 2>/dev/null || echo "UNMANAGED: $1"' _ {} \;

# Check for user-level systemd units
find /home -path '*/.config/systemd/user/*.service' 2>/dev/null

LD_PRELOAD Detection

# Check ld.so.preload file
cat /etc/ld.so.preload 2>/dev/null

# Check environment for LD_PRELOAD
env | grep LD_PRELOAD
cat /proc/*/environ 2>/dev/null | tr '\0' '\n' | grep LD_PRELOAD

# Check running processes for injected libraries
for pid in /proc/[0-9]*; do
  grep -l LD_PRELOAD "$pid/environ" 2>/dev/null && echo "PID: $(basename $pid)"
done

Auditd Rules for Persistence Monitoring

# Monitor crontab modifications
-w /etc/crontab -p wa -k cron_modification
-w /etc/cron.d/ -p wa -k cron_modification
-w /var/spool/cron/ -p wa -k cron_modification

# Monitor systemd unit changes
-w /etc/systemd/system/ -p wa -k systemd_modification

# Monitor ld.so.preload
-w /etc/ld.so.preload -p wa -k ld_preload_modification

# Monitor shell profiles
-w /etc/profile -p wa -k profile_modification
-w /etc/profile.d/ -p wa -k profile_modification

# Monitor authorized_keys
-w /root/.ssh/authorized_keys -p wa -k ssh_key_modification

# Search audit logs for persistence events
ausearch -k cron_modification --start today
ausearch -k systemd_modification -i

SSH Authorized Keys Audit

# Find all authorized_keys files
find / -name authorized_keys -type f 2>/dev/null

# Check for command restrictions in keys
grep 'command=' /home/*/.ssh/authorized_keys /root/.ssh/authorized_keys 2>/dev/null

MITRE ATT&CK Techniques

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