Web Shell - file a.php

Hunt Hypothesis

This detection identifies potential web shell deployments where adversaries create or modify an ‘a.php’ file to establish a persistent foothold for remote command execution and lateral movement within the environment. Proactive hunting is essential in Azure Sentinel because attackers often utilize generic filenames like ‘a.php’ to evade signature-based detections, requiring behavioral analysis to uncover hidden malicious activity before it escalates into a broader compromise.

YARA Rule

rule webshell_PHP_a {
	meta:
		description = "Web Shell - file a.php"
		license = "Detection Rule License 1.1 https://github.com/Neo23x0/signature-base/blob/master/LICENSE"
		author = "Florian Roth (Nextron Systems)"
		date = "2014/01/28"
		score = 70
		hash = "e3b461f7464d81f5022419d87315a90d"
		id = "393e738a-b4c2-5630-a55f-c3caee4ff75e"
	strings:
		$s1 = "echo \"<option value=\\\"\". strrev(substr(strstr(strrev($work_dir), \"/\""
		$s2 = "echo \"<option value=\\\"$work_dir\\\" selected>Current Directory</option>"
		$s4 = "<input name=\"submit_btn\" type=\"submit\" value=\"Execute Command\"></p> " fullword
	condition:
		2 of them
}

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 3 string patterns in its detection logic.

• Source Rule

False Positive Guidance

Here are 4 specific false positive scenarios for the “Web Shell - file a.php” detection rule, including suggested filters and exclusions:

• Scenario: Automated Backup or Migration Tool Deployment

  • Context: Enterprise backup solutions like Veeam or migration scripts using PowerShell often deploy temporary PHP scripts named a.php to specific directories (e.g., /var/www/html/maintenance/) to verify connectivity or initiate data transfer before the main job runs. These files are created by service accounts, not interactive users.
  • Suggested Filter: Exclude file creation events where the parent process is a known backup agent (e.g., Veeam.Backup.Service.exe or powershell.exe) and the source user account belongs to the Backup-Admins group.

• Scenario: Scheduled CMS Maintenance Tasks

  • Context: Content Management Systems like WordPress or Drupal, managed via cron jobs on Linux servers, frequently generate a temporary file named a.php during routine maintenance windows (e.g., database optimization or cache clearing). This often occurs at 2:00 AM UTC when no human interaction is expected.
  • Suggested Filter: Create an exclusion for events occurring between 01:00 and 04:00 local time on the web server, specifically targeting files located in standard maintenance directories (e.g., */wp-content/maintenance/ or */public_html/scripts/).

• Scenario: DevOps CI/CD Pipeline Artifact Upload

  • Context: During a deployment cycle managed by tools like Jenkins, GitLab CI, or Azure DevOps, the pipeline may upload a generic test script named a.php to the staging environment to validate server health before promoting the full application build. This is triggered by the SYSTEM or `