CVE-2026-25521: Locutus Prototype Pollution Vulnerability

CVE-2026-25521 Overview

A critical prototype pollution vulnerability has been identified in Locutus, a JavaScript library that brings standard libraries from other programming languages to JavaScript for educational purposes. This vulnerability exists in versions from 2.0.12 to before 2.0.39 and allows attackers to pollute Object.prototype via crafted input using String.prototype, bypassing previous security mitigations.

Critical Impact

This prototype pollution vulnerability enables attackers to modify JavaScript object prototypes, potentially leading to remote code execution, denial of service, or property injection attacks across the entire application context.

Affected Products

• Locutus versions 2.0.12 through 2.0.38
• Applications using vulnerable Locutus versions for stdlib functionality
• Node.js and browser-based JavaScript applications integrating Locutus

Discovery Timeline

• February 4, 2026 - CVE-2026-25521 published to NVD
• February 5, 2026 - Last updated in NVD database

Technical Details for CVE-2026-25521

Vulnerability Analysis

This vulnerability is classified under CWE-1321 (Improper Neutralization of Special Elements in Data Query Logic), which specifically addresses prototype pollution issues in JavaScript applications. Prototype pollution occurs when an attacker is able to inject properties into existing JavaScript language construct prototypes, such as Object.prototype.

In the case of Locutus, a previous security fix attempted to mitigate prototype pollution by implementing checks to detect forbidden keys in user input. However, this mitigation was incomplete. Attackers discovered that by leveraging String.prototype as an alternative attack vector, they could still successfully pollute Object.prototype with arbitrary properties.

The local attack vector indicates that exploitation requires the attacker to have some form of local access or ability to provide crafted input to the vulnerable functions within the application context.

Root Cause

The root cause stems from insufficient input validation in the Locutus library. While developers implemented checks to block direct prototype pollution attempts through forbidden key detection, they failed to account for alternative pollution pathways through String.prototype. This oversight demonstrates the complexity of fully mitigating prototype pollution vulnerabilities in JavaScript, where multiple prototype chains can serve as attack vectors.

Attack Vector

The attack leverages JavaScript's prototype chain inheritance mechanism. By crafting malicious input that targets String.prototype instead of directly attempting to modify Object.prototype, attackers can bypass the existing security controls. Since String.prototype ultimately inherits from Object.prototype, properties injected via this vector propagate throughout the prototype chain.

The vulnerability is exploitable locally, meaning an attacker must be able to pass crafted input to the vulnerable functions. This could occur through:

• Direct function calls with malicious parameters
• User-controlled data processed by the library
• Configuration objects parsed by vulnerable code paths

Detailed technical information about the exploitation mechanism can be found in the GitHub Security Advisory.

Detection Methods for CVE-2026-25521

Indicators of Compromise

• Unexpected properties appearing on Object.prototype or String.prototype in runtime debugging
• Application behavior anomalies caused by polluted prototype properties affecting object operations
• Error logs indicating unusual property access patterns or type coercion failures
• JavaScript runtime exceptions related to prototype chain traversal

Detection Strategies

• Implement dependency scanning to identify Locutus versions between 2.0.12 and 2.0.38 in your package manifests
• Use Software Composition Analysis (SCA) tools to flag vulnerable library versions in your codebase
• Monitor JavaScript runtime environments for prototype modifications using Object.freeze(Object.prototype) in development
• Conduct code reviews focusing on areas where user input flows into Locutus library functions

Monitoring Recommendations

• Enable verbose logging for JavaScript applications to capture unusual object property modifications
• Implement runtime integrity checks that validate prototype chain state periodically
• Deploy application-level monitoring to detect behavioral anomalies indicative of prototype pollution exploitation
• Utilize SentinelOne's application control and behavioral analysis capabilities to detect suspicious JavaScript execution patterns

How to Mitigate CVE-2026-25521

Immediate Actions Required

• Upgrade Locutus to version 2.0.39 or later immediately to apply the security patch
• Audit all applications using Locutus to identify instances running vulnerable versions
• Review application code for locations where user-controlled input is passed to Locutus functions
• Consider implementing input validation at the application layer as defense-in-depth

Patch Information

The vulnerability has been patched in Locutus version 2.0.39. The fix addresses the bypass technique through String.prototype and strengthens the prototype pollution protections. The patch commit is available at GitHub Code Commit.

Organizations should update their package.json dependencies to require locutus >= 2.0.39 and run dependency updates across all environments.

Workarounds

• Freeze critical prototypes using Object.freeze(Object.prototype) and Object.freeze(String.prototype) if application compatibility allows
• Implement strict input validation to sanitize user-controlled data before passing to Locutus functions
• Use Object.create(null) for objects that will receive untrusted data to create prototype-less objects
• Consider replacing Locutus with alternative implementations if immediate patching is not feasible

# Update Locutus to patched version
npm update locutus@2.0.39

# Alternatively, specify minimum version in package.json
# "locutus": ">=2.0.39"

# Verify installed version
npm list locutus