A critical sandbox escape vulnerability has been discovered in SandboxJS, a popular JavaScript library used to run untrusted code safely. Tracked as CVE-2026-25881 with a CVSS score of 8.3, the flaw allows attackers to achieve remote code execution (RCE) on host systems.
This vulnerability exploits a weakness in SandboxJS’s protective mechanism, specifically its “isGlobal” flag, which is intended to prevent sandboxed code from modifying host system objects.
In this article, you’ll learn:
- How the vulnerability works, including prototype pollution and RCE
- Proof-of-concept attack chains and potential impacts
- Recommended mitigation strategies for developers and security teams
Understanding the SandboxJS Vulnerability
Technical Details
SandboxJS uses the isGlobal flag to prevent sandboxed code from altering host objects. However, prototype references like Map.prototype or Set.prototype can bypass these protections when stored in arrays:
- Sandboxed code places prototype references in an array:
const m = [Map.prototype][0]; - Extraction from the array strips the isGlobal taint, exposing the prototype:
// valueOrProp() removes protection - Attacker modifies the prototype:
m.cmd = 'id'; - Host sees the polluted prototype:
new Map().cmd === 'id'; // Persistent across host - Remote code execution occurs when host code uses the property in sensitive operations:
execSync(obj.cmd);
This process, known as prototype pollution, allows attackers to inject arbitrary properties into core JavaScript objects, which then persist across the entire host application.
Proof-of-Concept Demonstrations
Security researcher k14uz released working PoCs showcasing three attack scenarios:
- Simple Pollution: Add a malicious property to
Map.prototypeand observe it in all new Map objects. - Function Overwrites: Overwrite built-in functions to manipulate host behavior.
- Remote Code Execution: Execute system commands like
idto reveal host information.
Attack Chain Overview:
| Action | Description |
|---|---|
| Create Array | Place prototype reference in array |
| Extract Reference | Pull prototype out, losing protection flag |
| Modify Prototype | Add malicious properties or overwrite existing ones |
| Trigger Usage | Host code uses polluted property, leading to RCE |
Any application using SandboxJS to run untrusted JavaScript is potentially vulnerable to full host compromise.
Potential Impacts
- Breaking Sandbox Isolation: Attackers can escape the intended safe environment.
- Host Code Manipulation: Polluted prototypes can modify execution flow in sensitive operations.
- Remote Code Execution: Depending on host application behavior, attackers can execute system commands, read files, or exfiltrate data.
- Persistent Threats: Prototype pollution persists across objects, creating long-lasting vulnerabilities.
Recommended Mitigation
Immediate Actions
- Upgrade SandboxJS to version 0.8.31, which:
- Preserves protection flags across array operations
- Blocks writes to built-in prototypes
Additional Security Measures
- Freeze Built-in Prototypes:
Object.freeze(Map.prototype); Object.freeze(Set.prototype); - Audit Host Applications: Identify sensitive operations that may use user-controlled object properties.
- Defense-in-Depth: Run untrusted code in isolated environments and limit access to host resources.
- Code Reviews: Ensure sandbox configurations prevent exposure of global or core objects.
Expert Insights
Key Takeaways:
- Prototype pollution is a serious threat in JavaScript sandboxes, allowing attackers to escalate privileges to the host.
- Even widely used libraries like SandboxJS require continuous security auditing and timely patching.
- Developers must combine patching, prototype freezing, and careful auditing to defend against host takeover attacks.
Strategic Recommendation: Treat all untrusted code execution as a high-risk operation, implement multiple layers of sandboxing, and continuously monitor for prototype pollution vectors.
FAQs
What is CVE-2026-25881?
A critical sandbox escape in SandboxJS that allows attackers to modify host JavaScript prototypes, enabling remote code execution.
How does the vulnerability work?
Sandboxed code places prototype references in arrays, stripping protection flags, and allows attackers to inject malicious properties into host objects.
Which SandboxJS versions are affected?
All versions up to 0.8.30. The vulnerability is patched in 0.8.31.
How can developers protect applications?
Upgrade SandboxJS, freeze built-in prototypes, audit host code, and enforce strong sandbox isolation for untrusted scripts.
Conclusion
The SandboxJS CVE-2026-25881 vulnerability underscores the challenges of securing JavaScript sandboxes. Prototype pollution allows attackers to manipulate host execution, escape isolation, and potentially gain full system access.
Developers and security teams must upgrade libraries, audit applications, and implement layered defenses to prevent remote host takeovers.
Next Step: Upgrade SandboxJS to 0.8.31, freeze built-in prototypes, and review untrusted code execution pathways to secure your applications.
