CVE-2026-25586: SandboxJS Sandbox Escape RCE Vulnerability

CVE-2026-25586 Overview

A critical sandbox escape vulnerability exists in SandboxJS, a JavaScript sandboxing library, in versions prior to 0.8.29. The vulnerability allows attackers to bypass sandbox security controls by shadowing the hasOwnProperty method on a sandbox object. This technique disables prototype whitelist enforcement in the property-access path, permitting direct access to __proto__ and other blocked prototype properties. Successful exploitation enables host Object.prototype pollution with persistent cross-sandbox impact.

Critical Impact

This vulnerability enables complete sandbox escape with the ability to pollute the host Object.prototype, affecting all sandboxed environments and potentially compromising the entire application runtime.

Affected Products

• SandboxJS versions prior to 0.8.29
• Applications using vulnerable SandboxJS for JavaScript code sandboxing
• npm packages dependent on vulnerable SandboxJS versions

Discovery Timeline

• 2026-02-06 - CVE-2026-25586 published to NVD
• 2026-02-06 - Last updated in NVD database

Technical Details for CVE-2026-25586

Vulnerability Analysis

This vulnerability is classified as CWE-74 (Improper Neutralization of Special Elements in Output Used by a Downstream Component), commonly referred to as an injection vulnerability. The flaw exists in how SandboxJS validates property access within sandboxed JavaScript execution contexts.

The sandbox relies on prototype whitelist enforcement to prevent untrusted code from accessing dangerous properties like __proto__, constructor, and other prototype chain properties. However, the validation mechanism uses hasOwnProperty checks that can be circumvented when an attacker shadows this method on a sandbox object.

When hasOwnProperty is shadowed with a malicious implementation, the prototype whitelist enforcement fails silently, allowing the attacker to traverse the prototype chain and access blocked properties. This enables Object.prototype pollution on the host environment, which persists across sandbox instances and can affect all JavaScript execution within the application.

Root Cause

The root cause stems from insufficient hardening of the property access validation path in the SandboxJS execution engine. The library trusted the hasOwnProperty method without verifying it hadn't been tampered with or shadowed by malicious sandbox code. This architectural weakness allowed attackers to disable the security mechanism designed to prevent prototype chain access.

Attack Vector

The attack is network-exploitable with no authentication required and no user interaction needed. An attacker can craft malicious JavaScript code that shadows hasOwnProperty on a sandbox object, then leverages this to access __proto__ and pollute the host Object.prototype. The scope is changed (as indicated by the CVSS vector), meaning the vulnerability impacts resources beyond the vulnerable component—specifically, other sandbox instances and potentially the host application itself.

The attack flow involves:

1. Creating or modifying a sandbox object to shadow hasOwnProperty
2. Accessing blocked prototype properties like __proto__
3. Polluting Object.prototype with malicious properties
4. Achieving persistent impact across all sandbox instances

The security patch introduces changes to both src/SandboxExec.ts and src/parser.ts to harden the sandbox against this bypass technique:

   IOptionParams,
   IOptions,
   IScope,
+  LocalScope,
   replacementCallback,
   SandboxGlobal,
   SubscriptionSubject,

Source: GitHub Commit - SandboxJS

 import unraw from './unraw.js';
-import { CodeString, isLisp, LispType } from './utils.js';
+import { CodeString, isLisp, LispType, reservedWords } from './utils.js';
 
 export type DefineLisp<
   op extends LispType,

Source: GitHub Commit - SandboxJS

Detection Methods for CVE-2026-25586

Indicators of Compromise

• Unexpected modifications to Object.prototype in application runtime
• JavaScript code attempting to shadow hasOwnProperty within sandboxed contexts
• Access attempts to __proto__, constructor, or prototype properties from sandbox code
• Anomalous behavior in sandboxed code execution indicating prototype chain manipulation

Detection Strategies

• Monitor sandbox execution logs for attempts to redefine or shadow built-in methods like hasOwnProperty
• Implement runtime integrity checks for Object.prototype to detect pollution
• Deploy application security monitoring to detect unexpected prototype modifications
• Review npm dependency trees for vulnerable SandboxJS versions using npm audit

Monitoring Recommendations

• Enable verbose logging in SandboxJS configurations to capture property access patterns
• Set up alerts for prototype pollution detection in your application monitoring stack
• Implement Content Security Policy (CSP) headers where applicable to limit script execution
• Monitor for unusual error patterns that may indicate sandbox escape attempts

How to Mitigate CVE-2026-25586

Immediate Actions Required

• Upgrade SandboxJS to version 0.8.29 or later immediately
• Audit applications using SandboxJS for any signs of prototype pollution
• Review sandboxed code execution logs for suspicious activity
• Consider temporarily disabling sandboxed code execution if upgrade is not immediately possible

Patch Information

The vulnerability is fixed in SandboxJS version 0.8.29. The fix hardens the sandbox against code execution bypass by introducing LocalScope handling and incorporating reservedWords validation in the parser. The patch is available via the GitHub commit 67cb186c. Additional details are available in the GitHub Security Advisory GHSA-jjpw-65fv-8g48.

Workarounds

• Implement additional input validation on code submitted to the sandbox before execution
• Add runtime prototype freeze on critical objects (Object.freeze(Object.prototype)) as defense-in-depth
• Deploy monitoring for prototype pollution patterns in production environments
• Consider alternative sandboxing solutions if immediate upgrade is not feasible

# Upgrade SandboxJS to patched version
npm update sandboxjs@0.8.29

# Or install specific patched version
npm install sandboxjs@^0.8.29

# Verify installed version
npm list sandboxjs