CVE-2026-34208: Nyariv SandboxJS Auth Bypass Vulnerability

CVE-2026-34208 Overview

CVE-2026-34208 is a critical sandbox escape vulnerability in SandboxJS, a JavaScript sandboxing library used to isolate untrusted code execution. Prior to version 0.8.36, the library's protection mechanism against direct assignment to global objects can be bypassed through an exposed callable constructor path. This allows attackers to write arbitrary properties into host global objects and persist malicious mutations across sandbox instances within the same process.

Critical Impact

Attackers can escape the JavaScript sandbox completely, gaining the ability to modify host global objects and persist malicious code across all sandbox instances in the same Node.js process.

Affected Products

• nyariv sandboxjs versions prior to 0.8.36
• Node.js applications using vulnerable SandboxJS versions
• Any application relying on SandboxJS for untrusted code isolation

Discovery Timeline

• 2026-04-06 - CVE CVE-2026-34208 published to NVD
• 2026-04-09 - Last updated in NVD database

Technical Details for CVE-2026-34208

Vulnerability Analysis

This vulnerability represents a protection mechanism failure (CWE-693) in SandboxJS's sandbox isolation implementation. SandboxJS is designed to execute untrusted JavaScript code safely by preventing modifications to global objects like Math, Array, and other built-in constructors. While the library successfully blocks direct assignment patterns such as Math.random = maliciousFunction, it fails to account for an alternative code path that achieves the same result.

The core issue lies in how JavaScript's prototype chain and constructor functions interact within the sandbox environment. The this.constructor reference resolves to the internal SandboxGlobal function, and because Function.prototype.call is permitted within the sandbox, attackers can leverage this combination to bypass the protection entirely.

Root Cause

The root cause is an incomplete protection mechanism that blocks direct property assignments but leaves the constructor call path exposed. When sandboxed code accesses this.constructor, it obtains a reference to the SandboxGlobal function. By invoking this.constructor.call(target, attackerObject), the attacker can effectively write arbitrary properties to host global objects. This architectural oversight allows the sandbox boundary to be crossed, violating the fundamental security assumption that sandboxed code cannot affect the host environment.

Attack Vector

The attack is network-exploitable with no user interaction required. An attacker who can supply JavaScript code to be executed within a SandboxJS sandbox can exploit this vulnerability to:

1. Access the internal SandboxGlobal constructor via this.constructor
2. Use Function.prototype.call to invoke the constructor with attacker-controlled arguments
3. Write arbitrary properties to host global objects outside the sandbox
4. Persist these mutations across multiple sandbox instances sharing the same process

The exploitation path this.constructor.call(target, attackerObject) allows property injection into any accessible global object. Because these mutations persist across sandbox instances within the same Node.js process, a single successful exploitation can compromise all subsequent sandbox executions.

Detection Methods for CVE-2026-34208

Indicators of Compromise

• Unexpected modifications to global JavaScript objects such as Math, Array, Object, or Function prototypes
• Anomalous behavior in sandbox instances that suggests cross-instance contamination
• JavaScript runtime errors or unexpected function behavior after executing sandboxed code
• Evidence of property injection on built-in constructors that were not present in the original application code

Detection Strategies

• Implement runtime integrity checks on critical global objects before and after sandbox execution
• Monitor for access patterns involving this.constructor.call within sandboxed code
• Deploy application-level logging to track sandbox execution and detect anomalous mutation patterns
• Use static analysis tools to identify potentially malicious code patterns in input destined for sandbox execution

Monitoring Recommendations

• Audit all Node.js applications to identify usage of SandboxJS versions prior to 0.8.36
• Implement dependency scanning in CI/CD pipelines to flag vulnerable package versions
• Enable verbose logging for sandbox execution to capture constructor access patterns
• Monitor application behavior for signs of global object tampering or unexpected state persistence

How to Mitigate CVE-2026-34208

Immediate Actions Required

• Upgrade SandboxJS to version 0.8.36 or later immediately
• Audit all applications using SandboxJS to ensure they are running the patched version
• Review recent sandbox executions for potential exploitation attempts
• Consider temporarily disabling sandbox functionality if immediate upgrade is not possible

Patch Information

The vulnerability is fixed in SandboxJS version 0.8.36. The patch addresses the constructor path bypass by properly restricting access to the internal SandboxGlobal function and preventing its invocation through Function.prototype.call. Organizations should update their package.json dependencies and run npm update or yarn upgrade to pull the patched version. For detailed information about the fix, refer to the GitHub Security Advisory.

Workarounds

• If upgrading is not immediately possible, implement additional input validation to reject code containing this.constructor patterns
• Consider running each sandbox execution in an isolated child process to prevent cross-instance contamination
• Apply a wrapper layer that freezes critical global objects before sandbox execution using Object.freeze()
• Evaluate alternative sandboxing solutions if the risk is unacceptable for your threat model

# Upgrade SandboxJS to patched version
npm update sandboxjs@0.8.36

# Verify installed version
npm list sandboxjs

# For yarn users
yarn upgrade sandboxjs@0.8.36