CVE-2026-25142: SandboxJS RCE Vulnerability

CVE-2026-25142 Overview

SandboxJS is a JavaScript sandboxing library designed to execute untrusted JavaScript code in a restricted environment. A critical vulnerability exists in versions prior to 0.8.27 where SandboxJS does not properly restrict access to the __lookupGetter__ method. This oversight allows attackers to obtain prototype references, which can be leveraged to escape the sandbox and achieve remote code execution on the host system.

Critical Impact

This sandbox escape vulnerability allows attackers to bypass security restrictions and execute arbitrary code on the underlying system, completely defeating the purpose of the sandboxing mechanism. Applications relying on SandboxJS for secure code execution are at risk of full system compromise.

Affected Products

• SandboxJS versions prior to 0.8.27
• Applications using vulnerable SandboxJS versions for untrusted code execution
• Web applications implementing client-side JavaScript sandboxing with SandboxJS

Discovery Timeline

• 2026-02-02 - CVE CVE-2026-25142 published to NVD
• 2026-02-03 - Last updated in NVD database

Technical Details for CVE-2026-25142

Vulnerability Analysis

This vulnerability (CWE-94: Improper Control of Generation of Code) represents a fundamental flaw in how SandboxJS restricts access to JavaScript's prototype chain. The __lookupGetter__ method, when left unrestricted, provides a direct path to traverse the prototype chain and access objects that should be isolated from sandboxed code.

In JavaScript, the prototype chain is the mechanism by which objects inherit properties and methods from their prototypes. Sandboxing libraries must carefully control access to this chain to prevent untrusted code from reaching dangerous objects like Function, Object, or the global scope. The failure to restrict __lookupGetter__ creates a reliable escape primitive that attackers can exploit to break out of the sandbox entirely.

The network-accessible nature of this vulnerability means any application exposing SandboxJS functionality over the network is susceptible to remote exploitation without requiring any user interaction or authentication.

Root Cause

The root cause lies in the conditional logic within the executor.ts file, specifically in how the sandbox validates property access. The original code structure allowed certain code paths to bypass the prototype chain validation when accessing properties that were not the constructor. The vulnerability exists between lines 368-398 of the executor.ts file, where the validation logic failed to properly cover all cases of prototype access.

Attack Vector

An attacker can craft malicious JavaScript code that leverages __lookupGetter__ to traverse up the prototype chain, ultimately obtaining references to unrestricted objects. From these objects, the attacker can access the Function constructor or other dangerous primitives, enabling arbitrary code execution outside the sandbox boundaries.

The attack is particularly dangerous because:

• It requires no privileges or authentication
• It can be executed remotely over the network
• No user interaction is required
• The scope extends beyond the vulnerable component, potentially impacting the entire host system

           throw new SandboxError(`Static method or property access not permitted: ${a.name}.${b}`);
         }
       }
-    } else if (b !== 'constructor') {
+    }
+    
+    if (b !== 'constructor') {
       let prot = a;
       while ((prot = Object.getPrototypeOf(prot))) {
         if (prot.hasOwnProperty(b)) {

Source: GitHub SandboxJS Commit

The patch restructures the conditional logic to ensure that prototype chain validation is always performed regardless of the preceding static method check. By changing from an else if to a separate if statement, the validation now covers all code paths properly.

Detection Methods for CVE-2026-25142

Indicators of Compromise

• Unusual JavaScript execution patterns involving __lookupGetter__ or __lookupSetter__ methods
• Attempts to access Object.prototype or Function.prototype from sandboxed contexts
• Unexpected process spawning or system calls originating from Node.js applications using SandboxJS
• Log entries showing prototype chain traversal attempts within sandbox boundaries

Detection Strategies

• Implement application-level logging to capture all code submitted to SandboxJS for execution
• Deploy runtime application self-protection (RASP) solutions to detect sandbox escape attempts
• Monitor for usage patterns involving deprecated prototype accessor methods (__lookupGetter__, __lookupSetter__, __defineGetter__, __defineSetter__)
• Review application logs for error messages related to prototype access violations

Monitoring Recommendations

• Enable verbose logging for SandboxJS execution contexts in development and staging environments
• Set up alerts for any successful execution of code containing prototype manipulation methods
• Monitor system resource usage for applications using SandboxJS to detect potential post-exploitation activity
• Implement anomaly detection for JavaScript execution patterns that differ from baseline behavior

How to Mitigate CVE-2026-25142

Immediate Actions Required

• Upgrade SandboxJS to version 0.8.27 or later immediately
• Audit all applications using SandboxJS to identify vulnerable deployments
• Implement input validation to reject code containing __lookupGetter__ patterns as a temporary measure
• Consider temporarily disabling features that execute untrusted JavaScript until patching is complete

Patch Information

The vulnerability has been fixed in SandboxJS version 0.8.27. The security patch modifies the conditional logic in src/executor.ts to ensure prototype chain validation is performed consistently across all code paths. Organizations should update their dependencies to include this fix.

For detailed patch information, refer to the GitHub Security Advisory GHSA-9p4w-fq8m-2hp7 and the security commit.

Workarounds

• If immediate patching is not possible, implement a pre-execution filter that blocks code containing __lookupGetter__, __lookupSetter__, and similar prototype accessor methods
• Deploy network-level controls to restrict access to applications using vulnerable SandboxJS versions
• Consider using alternative sandboxing solutions such as isolated VMs or Web Workers with restricted capabilities until the upgrade can be completed
• Implement Content Security Policy (CSP) headers to limit the impact of potential sandbox escapes in browser contexts

# Update SandboxJS to patched version
npm update sandboxjs@0.8.27

# Verify installed version
npm list sandboxjs

# Audit for additional vulnerabilities
npm audit