CVE-2026-1774: CASL Ability Prototype Pollution Flaw

CVE-2026-1774 Overview

CASL Ability, versions 2.4.0 through 6.7.4, contains a prototype pollution vulnerability that allows attackers to modify the behavior of JavaScript objects through manipulation of the Object prototype. This vulnerability affects applications using the CASL authorization library, potentially enabling attackers to bypass security controls, achieve remote code execution, or cause denial of service conditions.

Critical Impact

Prototype pollution in CASL Ability can allow unauthenticated attackers to manipulate application behavior, bypass authorization checks, or achieve remote code execution through the network without user interaction.

Affected Products

• CASL Ability versions 2.4.0 through 6.7.4

Discovery Timeline

• 2026-02-10 - CVE CVE-2026-1774 published to NVD
• 2026-02-11 - Last updated in NVD database

Technical Details for CVE-2026-1774

Vulnerability Analysis

This prototype pollution vulnerability exists in the CASL Ability authorization library, which is widely used in Node.js applications for implementing attribute-based access control (ABAC). Prototype pollution occurs when an attacker can inject properties into JavaScript's base Object prototype, causing those properties to be inherited by all objects in the application. In the context of CASL Ability, this can have devastating consequences as the library is responsible for authorization decisions.

The vulnerability is particularly severe because CASL Ability operates at the authorization layer of applications. By polluting the prototype chain, an attacker could potentially manipulate permission checks, inject malicious properties that affect ability definitions, or alter the behavior of condition matching used in authorization rules. This could lead to complete bypass of access controls implemented using CASL.

For additional technical background on prototype pollution attacks, refer to the Mozilla Developer Guide and the MITRE CWE Definition for CWE-1321.

Root Cause

The vulnerability stems from improper handling of user-controlled input during object property assignment operations within CASL Ability. When processing ability definitions or conditions, the library fails to properly sanitize input that could contain special properties like __proto__, constructor, or prototype. This allows an attacker to inject properties into the Object prototype through carefully crafted input, which then affects all objects in the application's JavaScript runtime.

Attack Vector

The attack is conducted over the network, requiring no authentication or user interaction. An attacker can exploit this vulnerability by sending malicious JSON payloads containing prototype pollution gadgets to endpoints that process CASL ability definitions or permission checks.

The exploitation mechanism typically involves:

1. Identifying an application endpoint that accepts user input processed by CASL Ability
2. Crafting a malicious payload containing __proto__ or constructor.prototype properties
3. Sending the payload to the target application
4. The malicious properties are merged into the Object prototype
5. Subsequent authorization checks or application logic inherit the polluted properties

Depending on the application's codebase and how CASL is implemented, successful exploitation can lead to authentication bypass, privilege escalation, remote code execution, or denial of service. See the CERT Vulnerability Report for additional details.

Detection Methods for CVE-2026-1774

Indicators of Compromise

• Unusual JSON payloads containing __proto__, constructor, or prototype properties in HTTP request bodies or query parameters
• Unexpected application behavior following authorization checks that previously functioned correctly
• Log entries showing manipulation of object properties in authorization-related code paths
• Runtime errors or exceptions related to unexpected object property access in CASL-related modules

Detection Strategies

• Implement Web Application Firewall (WAF) rules to detect and block requests containing prototype pollution payloads such as __proto__ or constructor.prototype in JSON bodies
• Deploy application-level logging to capture and alert on suspicious property access patterns during CASL ability evaluation
• Use static code analysis tools to identify vulnerable code patterns where user input is merged with objects without sanitization
• Leverage SentinelOne Singularity XDR to monitor Node.js application behavior for signs of prototype pollution exploitation

Monitoring Recommendations

• Monitor application logs for anomalies in authorization decisions or unexpected permission grants
• Set up alerts for HTTP requests containing known prototype pollution patterns targeting your CASL-enabled endpoints
• Track dependency versions across your application portfolio to identify instances of vulnerable CASL Ability versions
• Implement runtime application self-protection (RASP) to detect prototype manipulation attempts in real-time

How to Mitigate CVE-2026-1774

Immediate Actions Required

• Upgrade CASL Ability to a version newer than 6.7.4 that contains the security fix
• Audit all endpoints that process user input through CASL Ability for potential prototype pollution attack surfaces
• Implement input validation to reject payloads containing __proto__, constructor, or prototype properties
• Review application logs for signs of exploitation attempts

Patch Information

Organizations should update CASL Ability to the latest patched version immediately. The vulnerability affects versions 2.4.0 through 6.7.4. Consult the GitHub CASL Ability Package repository for the latest security updates and release information.

Workarounds

• Freeze the Object prototype at application startup using Object.freeze(Object.prototype) to prevent modifications, though this may cause compatibility issues with some libraries
• Implement a custom JSON parser or middleware that strips dangerous properties (__proto__, constructor, prototype) from incoming requests before they reach CASL processing
• Use Object.create(null) for objects that receive untrusted input to create prototype-less objects that cannot be polluted
• Consider using the secure-json-parse npm package as a drop-in replacement for JSON parsing to prevent prototype pollution

# Configuration example
# Update CASL Ability to the latest patched version
npm update @casl/ability

# Or install a specific patched version
npm install @casl/ability@latest

# Verify installed version
npm list @casl/ability

# Audit your project for known vulnerabilities
npm audit