CVE-2026-6594: brikcss merge Prototype Pollution RCE Flaw

CVE-2026-6594 Overview

A prototype pollution vulnerability has been identified in brikcss merge up to version 1.3.0. This security flaw affects object merging functionality where manipulation of the __proto__, constructor.prototype, or prototype arguments can lead to improperly controlled modification of object prototype attributes. The vulnerability is remotely exploitable and poses risks to applications utilizing this package for object manipulation operations.

Critical Impact

Attackers can pollute JavaScript object prototypes remotely, potentially leading to denial of service, property injection, or in certain application contexts, remote code execution.

Affected Products

• brikcss merge versions up to and including 1.3.0

Discovery Timeline

• April 20, 2026 - CVE-2026-6594 published to NVD
• April 22, 2026 - Last updated in NVD database

Technical Details for CVE-2026-6594

Vulnerability Analysis

This vulnerability is classified as CWE-94 (Improper Control of Generation of Code) and manifests as a prototype pollution flaw. Prototype pollution occurs when an attacker can inject properties into JavaScript object prototypes, which then propagate to all objects inheriting from that prototype. In the case of brikcss merge, the library's object merging functionality fails to properly sanitize special properties like __proto__, constructor.prototype, and prototype during merge operations.

When user-controlled input is passed to the merge function without adequate validation, an attacker can craft malicious payloads that modify the base Object.prototype. This modification affects all JavaScript objects in the application's runtime environment, potentially altering application behavior, bypassing security checks, or enabling further exploitation chains.

Root Cause

The root cause of this vulnerability lies in insufficient input validation during the object merge process. The brikcss merge library does not implement proper checks or sanitization for dangerous properties (__proto__, constructor, prototype) when recursively merging objects. This allows attackers to traverse prototype chains and inject arbitrary properties into the base object prototype.

Attack Vector

The attack is network-accessible, requiring no privileges or user interaction. An attacker can exploit this vulnerability by supplying crafted JSON data containing prototype pollution payloads to any endpoint or function that processes user input through the vulnerable merge function.

The exploitation typically involves sending a payload with nested __proto__ or constructor.prototype properties. When the merge function processes this input, it inadvertently copies these malicious properties onto the Object prototype, affecting all objects in the application scope.

For detailed technical information and proof-of-concept examples, refer to the GitHub PoC Repository and VulDB entry #358229.

Detection Methods for CVE-2026-6594

Indicators of Compromise

• Unexpected properties appearing on JavaScript objects that were not explicitly defined
• Application behavior anomalies such as authentication bypasses or modified configuration values
• Error logs showing unexpected prototype chain modifications or property access errors

Detection Strategies

• Implement static code analysis to identify usage of brikcss merge library version 1.3.0 or earlier in your codebase
• Deploy runtime monitoring to detect prototype pollution attempts by tracking modifications to Object.prototype
• Use Software Composition Analysis (SCA) tools to identify vulnerable dependencies in your package manifests

Monitoring Recommendations

• Monitor application logs for unusual object property access patterns
• Implement integrity checks on critical object prototypes to detect unauthorized modifications
• Set up alerts for dependency vulnerability databases flagging brikcss merge

How to Mitigate CVE-2026-6594

Immediate Actions Required

• Audit your applications for usage of brikcss merge library versions up to 1.3.0
• Consider replacing brikcss merge with alternative libraries that implement prototype pollution protections
• Implement input validation to sanitize dangerous properties before passing data to merge functions

Patch Information

The vendor was contacted early about this disclosure but did not respond in any way. As of the last update, no official patch has been released. Organizations should consider migrating to alternative object merging libraries with active maintenance and prototype pollution protections.

For additional vulnerability context, consult the VulDB submission #791805 and VulDB CTI information.

Workarounds

• Freeze the Object.prototype using Object.freeze(Object.prototype) to prevent prototype modifications (note: this may break some application functionality)
• Implement a wrapper function that filters out __proto__, constructor, and prototype keys before calling the merge function
• Use Object.create(null) for creating objects that do not inherit from Object.prototype when handling untrusted input

# Example: Check for vulnerable package version in Node.js projects
npm ls @brikcss/merge
# Review if version is <= 1.3.0 and plan migration