Detects suspicious pre and post hooks in Arch install files

Hunt Hypothesis

This detection identifies adversaries who manipulate installation scripts by injecting malicious pre- and post-installation hooks to establish persistence or execute initial payloads within Azure Sentinel-monitored environments. Proactive hunting for this behavior is critical because such hook modifications often occur silently during routine software updates, allowing attackers to evade standard signature-based defenses while gaining early foothold access before broader system compromise occurs.

YARA Rule

rule SUSP_LNX_ARCH_Install_Hook_Jun26 {
   meta:
      description = "Detects suspicious pre and post hooks in Arch install files"
      author = "Marius Benthin"
      date = "2026-06-15"
      reference = "https://aur.archlinux.org/cgit/aur.git/commit/?h=hearthstone-linux-gui-bin&id=ecf810ac853e7149abd4e0c793b2517e9737edb8"
      reference2 = "https://aur.archlinux.org/cgit/aur.git/commit/?h=python-django-js-asset&id=af09b1cf1b59"
      hash = "47c076099e6715ffb0bd357b6832175741e77b88972033cd1f9b55b6ff7e5519"
      hash = "ef735bca8cb2acafe70831e33ed468a3046a61147af694000761970415c7eef1"
      score = 70
   strings:
      $sa1 = "pre_install() {"
      $sa2 = "post_install() {"
      $sa3 = "pre_upgrade() {"
      $sa4 = "post_upgrade() {"
      $sa5 = "pre_remove() {"
      $sa6 = "post_remove() {"

      $sb1 = "npm install "
      $sb2 = "&& 'b''u''n'"

      $fp1 = "#!/bin/sh"
   condition:
      filesize < 5KB
      and 1 of ($sa*)
      and 1 of ($sb*)
      and not 1 of ($fp*)
}

Deployment Notes

This YARA rule can be deployed in the following contexts:

Microsoft Defender for Endpoint — Custom indicators / advanced hunting
Email Gateway — Attachment scanning
File Share Monitoring — Periodic scanning of shared drives
YARA CLI — Manual threat hunting on endpoints

This rule contains 9 string patterns in its detection logic.

References

False Positive Guidance

Here are 4 specific false positive scenarios for the rule “Detects suspicious pre and post hooks in Arch install files,” along with suggested filters or exclusions:

Scenario: Automated Package Maintenance via pacman Hooks
- Context: The system administrator runs a scheduled nightly job using cron to execute pacman -Syu, which triggers the default /etc/pacman.d/hooks/ directory. Common legitimate hooks like 01-pacman-remove-hooks.hook (cleaning up old kernel packages) or 02-pacman-install-hooks.hook (restarting services upon package install) are frequently executed during routine updates.
- Suggested Filter: Exclude events where the triggering process is pacman and the hook file path matches known standard patterns, specifically: /etc/pacman.d/hooks/*.hook containing keywords like “remove”, “install”, or “upgrade” in the filename.
Scenario: Configuration Management Tools (Ansible/Puppet)
- Context: Infrastructure automation tools such as Ansible or Puppet are deployed to manage Arch Linux nodes. When these agents apply a playbook or manifest that includes package installation tasks, they programmatically invoke pacman with specific flags, causing the execution of standard pre-install and post-install hooks defined in the configuration management repository.
- Suggested Filter: Create an exclusion for processes where the parent process is ansible-runner, puppet-agent, or salt-minion, provided the hook file being executed resides within a managed directory (e.g., /etc/pacman.d/hooks/managed_*.hook).
Scenario: Kernel Updates and System Reboots
- Context: During major kernel updates, Arch Linux utilizes hooks to perform post-install actions such as regenerating initramfs (mkinitcpio) or updating bootloader configurations (e