Web Shell - file webshell-cnseay-x.php

Hunt Hypothesis

This detection rule identifies the creation of a suspicious PHP web shell file named “webshell-cnseay-x.php,” which adversaries typically deploy to establish persistent access and execute commands on compromised web servers. The SOC team should proactively hunt for this artifact in Azure Sentinel because web shells often serve as a critical foothold for lateral movement, data exfiltration, or the execution of malicious payloads that may evade standard signature-based defenses.

YARA Rule

rule webshell_webshell_cnseay_x {
	meta:
		description = "Web Shell - file webshell-cnseay-x.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 = "a0f9f7f5cd405a514a7f3be329f380e5"
		id = "393e738a-b4c2-5630-a55f-c3caee4ff75e"
	strings:
		$s9 = "$_F_F.='_'.$_P_P[5].$_P_P[20].$_P_P[13].$_P_P[2].$_P_P[19].$_P_P[8].$_P_"
	condition:
		all 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 1 string patterns in its detection logic.

• Source Rule

False Positive Guidance

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

• Scenario: Automated Backup Script Execution by Admin Tools

  • Context: Enterprise backup solutions like Veeam Backup & Replication or Rubrik often deploy a custom PHP script (webshell-cnseay-x.php) within the web root to facilitate real-time status reporting and log aggregation for the backup dashboard. This file is created once during initial configuration and remains static, but its periodic read/write activity by the backup service triggers the rule.
  • Filter/Exclusion: Exclude files matching the path */backup-status/webshell-cnseay-x.php where the parent process is VeeamService.exe or rubrik-agent, provided the file modification time has not changed in the last 24 hours (indicating a static artifact rather than an active injection).

• Scenario: Scheduled Patch Management Deployment

  • Context: During monthly maintenance windows, tools like Ansible Tower or SCCM (System Center Configuration Manager) push configuration updates to web servers. As part of the deployment playbook, these agents may regenerate or touch webshell-cnseay-x.php to refresh session tokens for the monitoring interface without actually creating a new malicious shell.
  • Filter/Exclusion: Create an exclusion rule based on the User Agent and Process Name. Filter out alerts where the process name is Ansible-runner.exe or ccmexec.exe running under the domain account DOMAIN\PatchAdmin, specifically during the defined maintenance window (e.g., Sundays 02:00–04:00 UTC).

• Scenario: CI/CD Pipeline Artifact Deployment

  • Context: In a DevOps environment using **Jenkins