CVE-2026-25587: SandboxJS RCE Vulnerability

CVE-2026-25587 Overview

SandboxJS is a JavaScript sandboxing library designed to securely execute untrusted code in an isolated environment. Prior to version 0.8.29, a critical vulnerability exists that allows attackers to escape the sandbox through prototype pollution. Because Map is included in SAFE_PROTOTYPES, its prototype can be obtained via Map.prototype. By overwriting Map.prototype.has, an attacker can bypass sandbox restrictions and achieve arbitrary code execution outside the sandboxed environment. This vulnerability is classified as CWE-94 (Improper Control of Generation of Code - Code Injection).

Critical Impact

This sandbox escape vulnerability allows attackers to execute arbitrary code outside of the sandboxed environment, completely bypassing the security isolation that SandboxJS is designed to provide. Applications relying on SandboxJS for security boundaries are at risk of full system compromise.

Affected Products

• SandboxJS versions prior to 0.8.29
• Applications utilizing SandboxJS for JavaScript code sandboxing
• Node.js and browser environments running vulnerable SandboxJS versions

Discovery Timeline

• 2026-02-06 - CVE CVE-2026-25587 published to NVD
• 2026-02-06 - Last updated in NVD database

Technical Details for CVE-2026-25587

Vulnerability Analysis

This sandbox escape vulnerability stems from an oversight in how SandboxJS handles safe prototypes. The library maintains a list of SAFE_PROTOTYPES that are considered trusted and accessible within the sandbox. However, the Map object was incorrectly included in this list, allowing sandboxed code to access Map.prototype directly.

The critical flaw lies in the ability to overwrite built-in methods on Map.prototype. When an attacker overwrites Map.prototype.has with malicious code, the sandbox's internal mechanisms that rely on Map.has() for security checks become compromised. This allows the attacker to manipulate the sandbox's control flow and escape the isolated execution context.

The vulnerability enables network-based attacks with no user interaction required, allowing complete compromise of confidentiality, integrity, and availability while affecting resources beyond the vulnerable component's security scope.

Root Cause

The root cause is the inclusion of Map in the SAFE_PROTOTYPES list without adequate protection against prototype method overwrites. The sandbox failed to properly freeze or protect the prototypes of objects it considered safe, allowing attackers to poison these prototypes with malicious functions that execute when the sandbox internally uses these methods.

Attack Vector

An attacker can exploit this vulnerability by submitting malicious JavaScript code to any application that uses SandboxJS for code isolation. The attack requires no authentication or user interaction and can be executed remotely over the network. The attacker crafts code that:

1. Accesses Map.prototype through the exposed safe prototype
2. Overwrites Map.prototype.has with a malicious function
3. Triggers the sandbox's internal use of Map.has(), causing the malicious code to execute with elevated privileges outside the sandbox

The security patch introduces additional protections by importing LocalScope and reservedWords to harden the sandbox against such bypass attempts:

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

Source: GitHub Commit Update

 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 Update

Detection Methods for CVE-2026-25587

Indicators of Compromise

• Unexpected code execution outside of sandbox boundaries in application logs
• Modifications to Map.prototype or other built-in JavaScript prototypes in sandboxed code
• Unusual process spawning or network connections originating from sandboxed execution contexts
• Error messages or stack traces indicating prototype chain manipulation

Detection Strategies

• Monitor JavaScript execution logs for attempts to access or modify Map.prototype.has or similar prototype methods
• Implement runtime integrity checks for built-in JavaScript object prototypes
• Deploy application-level logging to capture all code submitted to SandboxJS for sandbox execution
• Use SentinelOne's Singularity platform to detect anomalous code execution patterns indicative of sandbox escape attempts

Monitoring Recommendations

• Enable verbose logging in applications using SandboxJS to capture sandbox execution details
• Set up alerts for any modifications to JavaScript built-in prototypes in production environments
• Regularly audit code submitted to sandbox environments for known escape patterns
• Implement network monitoring to detect unexpected outbound connections from sandboxed processes

How to Mitigate CVE-2026-25587

Immediate Actions Required

• Upgrade SandboxJS to version 0.8.29 or later immediately
• Audit all applications using SandboxJS to identify vulnerable deployments
• Review logs for any evidence of exploitation attempts prior to patching
• Consider temporarily disabling sandbox functionality if immediate patching is not possible

Patch Information

The vulnerability has been fixed in SandboxJS version 0.8.29. The security patch hardens the sandbox against code execution bypass by implementing additional protections for prototype handling. The fix introduces LocalScope and reservedWords imports to properly validate and restrict access to potentially dangerous prototype chains. For complete technical details, refer to the GitHub Security Advisory GHSA-66h4-qj4x-38xp and the security patch commit.

Workarounds

• Implement additional validation layers around code submitted to SandboxJS
• Use Object.freeze() on critical prototypes before initializing the sandbox environment
• Deploy network segmentation to limit the impact of potential sandbox escapes
• Consider alternative sandboxing solutions until patching is complete

# Update SandboxJS to patched version
npm update sandboxjs@0.8.29

# Verify installed version
npm list sandboxjs

# For yarn users
yarn upgrade sandboxjs@0.8.29