Web Shell - file cihshell_fix.php

Hunt Hypothesis

This detection identifies potential web shell deployments where adversaries establish a persistent backdoor named cihshell_fix.php to execute commands and maintain access within the Azure environment. Proactive hunting for this specific artifact is critical because web shells often serve as an initial foothold that enables attackers to pivot laterally, exfiltrate sensitive data, or escalate privileges before triggering broader alerting mechanisms.

YARA Rule

rule webshell_cihshell_fix {
	meta:
		description = "Web Shell - file cihshell_fix.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 = "3823ac218032549b86ee7c26f10c4cb5"
		id = "393e738a-b4c2-5630-a55f-c3caee4ff75e"
	strings:
		$s7 = "<tr style='background:#242424;' ><td style='padding:10px;'><form action='' encty"
		$s8 = "if (isset($_POST['mysqlw_host'])){$dbhost = $_POST['mysqlw_host'];} else {$dbhos"
	condition:
		1 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 5 specific false positive scenarios for the Web Shell - file cihshell_fix.php detection rule, including suggested filters and exclusions:

• Scheduled Maintenance Script Execution by DevOps Team

  • Scenario: The DevOps team utilizes a nightly maintenance job (e.g., running via cron or Azure Automation) that deploys a temporary patching script named cihshell_fix.php to the web root (/var/www/html/maintenance) to fix legacy database connection issues. This file is uploaded by the CI/CD pipeline (Jenkins/GitLab CI) and removed automatically after execution, but the rule triggers on its creation.
  • Filter/Exclusion: Exclude files created within the maintenance directory path (*/maintenance/cihshell_fix.php) or filter out events where the source process is the specific CI/CD agent (e.g., process_name: "jenkins-agent" or user: "svc-deploy-bot").

• Third-Party Security Scanner Deployment

  • Scenario: An external vulnerability management tool, such as Tenable Nessus or Qualys, performs a web application scan. As part of its active testing phase, the scanner injects a temporary probe file named cihshell_fix.php into the public-facing directory to test server response headers and authentication flows before deleting it immediately after the scan completes.
  • Filter/Exclusion: Create an exclusion for files created by known security service accounts (e.g., user: "svc-nessus" or source_ip: 10.20.50.x) during defined maintenance windows (e.g., 02:00–04:00 UTC).

• Automated CMS Plugin Update

  • Scenario: The enterprise uses a Content Management System like WordPress or **Drupal