CVE-2026-4239: Lagom WHMCS Template RCE Vulnerability

CVE-2026-4239 Overview

A prototype pollution vulnerability has been identified in Lagom WHMCS Template versions up to 2.3.7. The vulnerability exists within the Datatables component, where improper handling of user-controlled input allows attackers to modify the prototype of JavaScript objects. This type of vulnerability (CWE-94: Improper Control of Generation of Code) can lead to property injection attacks that may affect the behavior of the entire application.

The flaw enables remote attackers to manipulate object prototype attributes, potentially leading to denial of service, authentication bypass, or in some scenarios, remote code execution depending on how the application uses the polluted properties. The exploit has been publicly disclosed, and the vendor was contacted about this vulnerability but did not respond.

Critical Impact

Remote attackers can exploit prototype pollution in the Datatables component to inject arbitrary properties into JavaScript object prototypes, potentially compromising application integrity and security controls.

Affected Products

• Lagom WHMCS Template versions up to 2.3.7
• Datatables component within Lagom WHMCS Template

Discovery Timeline

• 2026-03-16 - CVE-2026-4239 published to NVD
• 2026-03-16 - Last updated in NVD database

Technical Details for CVE-2026-4239

Vulnerability Analysis

Prototype pollution is a JavaScript-specific vulnerability that occurs when an attacker can inject properties into an object's prototype. In the case of CVE-2026-4239, the Datatables component in Lagom WHMCS Template fails to properly sanitize user input before using it to set object properties. This allows attackers to traverse the prototype chain using special properties like __proto__, constructor, or prototype.

When successful, this attack pollutes the base Object.prototype, meaning all objects in the application inherit the malicious properties. The impact depends on how these polluted properties are subsequently used by the application logic—potentially affecting security checks, configuration options, or triggering unexpected code paths.

Root Cause

The root cause of this vulnerability is improper input validation within the Datatables component. The code fails to sanitize or reject property keys that could be used to access and modify object prototypes. When processing user-controlled data (likely through configuration objects or data structures), the application allows property assignments without checking for prototype-related keys like __proto__ or constructor.prototype.

Attack Vector

The attack can be launched remotely over the network. An authenticated attacker with low privileges can exploit this vulnerability through user interaction with the vulnerable Datatables component. The attacker crafts malicious input containing prototype pollution payloads, which are then processed by the vulnerable code path, resulting in modification of the JavaScript Object prototype.

The vulnerability mechanism involves injecting properties through nested object assignments. When the Datatables component merges or processes user-supplied configuration objects, attackers can include special keys that traverse to the Object prototype. For detailed technical analysis and proof-of-concept code, refer to the GitHub PoC Repository and the PoC JavaScript file.

Detection Methods for CVE-2026-4239

Indicators of Compromise

• Unusual JavaScript errors or unexpected object behavior in WHMCS application logs
• Client-side console errors indicating undefined or unexpected object properties
• Authentication or authorization anomalies that cannot be explained by normal application behavior
• Unexpected modifications to application configuration or behavior without administrative changes

Detection Strategies

• Monitor application logs for unusual Datatables component errors or exceptions
• Implement Content Security Policy (CSP) violations monitoring for unexpected script behavior
• Deploy Web Application Firewall (WAF) rules to detect prototype pollution payloads containing __proto__, constructor, or prototype in request parameters
• Review client-side JavaScript execution for anomalous object property access patterns

Monitoring Recommendations

• Enable verbose logging for the WHMCS application and Datatables component interactions
• Monitor for requests containing suspicious JSON payloads with nested __proto__ or constructor keys
• Implement runtime application self-protection (RASP) to detect prototype modification attempts
• Set up alerts for unusual authentication bypass attempts or privilege escalation events

How to Mitigate CVE-2026-4239

Immediate Actions Required

• Disable or restrict access to the affected Datatables functionality until a patch is available
• Implement input validation at the server level to reject requests containing prototype pollution payloads
• Review and audit any custom code that interacts with the Lagom WHMCS Template Datatables component
• Apply Web Application Firewall rules to block requests containing __proto__, constructor.prototype, or similar payloads

Patch Information

No official patch is currently available from the vendor. The vendor was contacted about this disclosure but did not respond. Organizations using Lagom WHMCS Template should monitor for vendor updates and consider alternative solutions if no patch is released.

For additional vulnerability details, see VulDB Entry #351181.

Workarounds

• Implement server-side input sanitization to strip or reject prototype-related property keys before processing
• Use Object.freeze(Object.prototype) in the application initialization to prevent prototype modifications (note: may break legitimate functionality)
• Deploy a reverse proxy or WAF with rules specifically targeting prototype pollution attack patterns
• Consider replacing the vulnerable Datatables component with a hardened alternative that properly sanitizes object property assignments

# Example WAF rule for ModSecurity to block prototype pollution attempts
SecRule ARGS "@rx (__proto__|constructor\s*\.\s*prototype)" \
    "id:100001,\
    phase:2,\
    deny,\
    status:403,\
    msg:'Prototype Pollution Attempt Detected',\
    severity:'CRITICAL'"