CVE-2021-47900: Gila CMS Remote Code Execution Vulnerability

CVE-2021-47900 Overview

CVE-2021-47900 is a critical remote code execution vulnerability affecting Gila CMS versions prior to 2.0.0. This vulnerability allows unauthenticated attackers to execute arbitrary system commands through manipulated HTTP headers. Specifically, attackers can inject PHP code containing shell_exec() functions within the User-Agent header, enabling full system command execution by sending crafted requests to the admin endpoint.

Critical Impact

Unauthenticated attackers can achieve complete system compromise through arbitrary command execution, potentially leading to data theft, malware installation, lateral movement, and full server takeover.

Affected Products

• Gila CMS versions prior to 2.0.0
• Web servers running vulnerable Gila CMS installations
• Systems with PHP environments hosting Gila CMS

Discovery Timeline

• 2026-01-27 - CVE CVE-2021-47900 published to NVD
• 2026-01-29 - Last updated in NVD database

Technical Details for CVE-2021-47900

Vulnerability Analysis

This vulnerability is classified under CWE-98 (Improper Control of Filename for Include/Require Statement in PHP Program), commonly known as PHP Remote File Inclusion. The flaw exists in how Gila CMS processes HTTP headers, specifically the User-Agent header, without proper sanitization or validation.

When a request is made to the admin endpoint, the application improperly handles the User-Agent header value, allowing an attacker to embed malicious PHP code. The injected code is then executed in the context of the web server, granting the attacker the ability to run system commands with the privileges of the web server process.

The network-accessible nature of this vulnerability, combined with no authentication requirements and no user interaction needed, makes it exceptionally dangerous for any exposed Gila CMS installation.

Root Cause

The root cause stems from insufficient input validation and sanitization of HTTP header data before it is processed by the application. Gila CMS fails to properly sanitize the User-Agent header before including or processing its contents, allowing PHP code injection. This represents a fundamental input validation failure where user-controlled data is trusted and executed without proper security controls.

Attack Vector

The attack is executed over the network by sending specially crafted HTTP requests to the vulnerable Gila CMS admin endpoint. An attacker constructs a malicious User-Agent header containing PHP code with shell_exec() or similar command execution functions. When the server processes this request, the embedded PHP code is interpreted and executed, allowing the attacker to run arbitrary system commands.

The attack flow involves:

1. Identifying a vulnerable Gila CMS installation (versions prior to 2.0.0)
2. Crafting an HTTP request with malicious PHP code in the User-Agent header
3. Sending the request to the admin endpoint
4. The server processes the header and executes the injected code
5. System commands are executed with web server privileges

For technical details on exploitation, refer to the Exploit-DB entry #49412 and the VulnCheck Gila CMS Advisory.

Detection Methods for CVE-2021-47900

Indicators of Compromise

• Unusual HTTP requests to admin endpoints with abnormal User-Agent headers containing PHP syntax such as shell_exec, system, exec, or passthru
• Web server logs showing User-Agent strings containing PHP code patterns like <?php or command execution functions
• Unexpected processes spawned by the web server user (e.g., www-data, apache, nginx)
• Creation of new files in web-accessible directories or modification of existing CMS files

Detection Strategies

• Implement Web Application Firewall (WAF) rules to detect and block HTTP headers containing PHP code patterns
• Monitor web server access logs for User-Agent headers with suspicious patterns including <?php, shell_exec(, system(, or other command execution signatures
• Deploy intrusion detection systems (IDS) with signatures specific to PHP injection attempts
• Enable SentinelOne Singularity to detect post-exploitation behaviors such as unauthorized process execution and file system modifications

Monitoring Recommendations

• Configure real-time alerting for web server processes executing shell commands or spawning child processes
• Establish baseline User-Agent patterns and alert on statistical anomalies or known malicious patterns
• Monitor outbound network connections from web servers for potential command-and-control communication
• Implement file integrity monitoring on Gila CMS installation directories

How to Mitigate CVE-2021-47900

Immediate Actions Required

• Upgrade Gila CMS to version 2.0.0 or later immediately
• If immediate upgrade is not possible, restrict network access to the admin endpoint using firewall rules or IP allowlists
• Implement a Web Application Firewall (WAF) to filter malicious HTTP headers before they reach the application
• Review web server logs for signs of exploitation attempts and investigate any suspicious activity

Patch Information

The vulnerability is addressed in Gila CMS version 2.0.0 and later. Organizations should upgrade to the latest stable release available from the official Gila CMS website or the GitHub repository. After upgrading, verify the installation and test that the vulnerability is no longer exploitable.

Workarounds

• Deploy a reverse proxy or WAF configured to sanitize or block requests containing PHP code patterns in HTTP headers
• Restrict access to the Gila CMS admin endpoint to trusted IP addresses only using firewall rules or .htaccess configurations
• Disable or remove the Gila CMS installation if it is not actively needed until a proper upgrade can be performed
• Implement server-side input validation as an additional layer of defense by configuring PHP to disable dangerous functions like shell_exec, exec, system, and passthru in php.ini

# Example: Restrict admin endpoint access in Apache .htaccess
<Files "admin/*">
    Order Deny,Allow
    Deny from all
    Allow from 192.168.1.0/24
    Allow from 10.0.0.0/8
</Files>