CVE-2026-25881: SandboxJS Sandbox Escape RCE Vulnerability

CVE-2026-25881 Overview

SandboxJS is a JavaScript sandboxing library designed to safely execute untrusted code. Prior to version 0.8.31, a critical sandbox escape vulnerability exists that allows sandboxed code to mutate host built-in prototypes by laundering the isGlobal protection flag through array literal intermediaries. When a global prototype reference (e.g., Map.prototype, Set.prototype) is placed into an array and then retrieved, the isGlobal taint tracking is stripped, permitting direct prototype mutation from within the sandboxed environment.

Critical Impact

This vulnerability enables persistent host-side prototype pollution and may lead to Remote Code Execution (RCE) in applications that use polluted properties in sensitive sinks, such as execSync(obj.cmd).

Affected Products

• SandboxJS versions prior to 0.8.31
• Node.js applications utilizing vulnerable SandboxJS versions
• Web applications implementing JavaScript sandboxing with affected library versions

Discovery Timeline

• 2026-02-09 - CVE CVE-2026-25881 published to NVD
• 2026-02-10 - Last updated in NVD database

Technical Details for CVE-2026-25881

Vulnerability Analysis

This vulnerability is classified under CWE-1321 (Improper Neutralization of Special Elements used in a Prototype Pollution Attack). The core issue lies in how SandboxJS tracks and maintains taint information for global prototype references when they pass through array literal intermediaries.

The sandboxing mechanism uses an isGlobal flag to identify and protect global built-in prototypes from modification by sandboxed code. However, when these protected references are stored in an array and subsequently retrieved, the taint tracking mechanism fails to preserve the protection flag. This creates a bypass vector where attackers can launder global prototype references through arrays to strip their protection status.

The attack requires network access and involves high complexity to successfully exploit. When successful, the scope changes from the sandboxed environment to the host environment, potentially compromising confidentiality, integrity, and availability of the host application.

Root Cause

The root cause is a flaw in the taint propagation logic within SandboxJS's prototype protection mechanism. The library correctly identifies and marks global prototypes with the isGlobal flag when directly accessed. However, the taint tracking does not properly propagate when these references are used as array elements. Array operations effectively "wash" the taint information, allowing the protected reference to be extracted without its protective marker, thereby permitting direct mutation of host-side prototypes.

Attack Vector

The attack exploits the discrepancy between direct prototype access (which is properly protected) and indirect access through array intermediaries (which bypasses protection). An attacker executes sandboxed code that:

1. Obtains a reference to a global prototype (e.g., Map.prototype)
2. Places this reference into an array literal
3. Retrieves the reference from the array, which now lacks the isGlobal taint
4. Directly mutates the prototype, polluting the host environment

This prototype pollution persists beyond the sandbox execution and affects all subsequent code in the host environment. When polluted properties are accessed in sensitive operations like command execution functions, it can lead to arbitrary code execution on the host system.

Detection Methods for CVE-2026-25881

Indicators of Compromise

• Unexpected properties appearing on JavaScript built-in prototypes such as Object.prototype, Array.prototype, Map.prototype, or Set.prototype
• Sandboxed code attempting to create arrays containing prototype references
• Application behavior anomalies suggesting prototype chain manipulation
• Unexpected command execution or system calls originating from prototype property access

Detection Strategies

• Implement runtime monitoring for prototype modifications using Object.freeze() on critical prototypes before sandbox execution
• Deploy application-level logging to track all sandbox inputs and outputs for suspicious patterns
• Use integrity checks on built-in prototypes before and after sandbox code execution
• Monitor for patterns where global prototype references are being stored in array structures

Monitoring Recommendations

• Enable verbose logging for SandboxJS operations in development and staging environments
• Implement prototype integrity verification as part of application health checks
• Monitor application logs for errors related to unexpected prototype properties
• Set up alerts for unusual patterns in sandbox execution that involve array manipulation of built-in objects

How to Mitigate CVE-2026-25881

Immediate Actions Required

• Upgrade SandboxJS to version 0.8.31 or later immediately
• Audit all applications using SandboxJS to identify potentially affected deployments
• Review any untrusted code that has been executed through the sandbox for malicious patterns
• Implement additional runtime prototype integrity checks as a defense-in-depth measure

Patch Information

The vulnerability has been fixed in SandboxJS version 0.8.31. The patch addresses the taint propagation issue by ensuring the isGlobal protection flag is maintained when prototype references pass through array intermediaries. Organizations should update to this version immediately.

For detailed technical information about the fix, see the GitHub Commit and the GitHub Security Advisory GHSA-ww7g-4gwx-m7wj.

Workarounds

• Freeze critical built-in prototypes using Object.freeze() before executing any sandboxed code
• Implement additional validation layers that check prototype integrity after sandbox execution
• Restrict sandbox capabilities to prevent access to global prototype references where possible
• Consider alternative sandboxing solutions until the patch can be applied

# Update SandboxJS to the patched version
npm update sandboxjs@0.8.31

# Or specify exact version in package.json
npm install sandboxjs@0.8.31 --save-exact