vm2: Host-returned Promise rejection can bypass vm2's unhandled-rejection hardening and terminate the host process

Story map

Why it is Amber

• CVSS 8.6 critical
• Public exploit or proof of concept referenced
• Remotely exploitable without logging in
• On your watchlist: npm

Affected packages

References

• https://github.com/advisories/GHSA-gjq8-xm47-88rc
• https://github.com/patriksimek/vm2/security/advisories/GHSA-gjq8-xm47-88rc
• https://nvd.nist.gov/vuln/detail/CVE-2026-92954
• https://github.com/patriksimek/vm2/commit/5bb37f8b3f0498675a71cfa4df2b408d8ef7d4fb
• https://github.com/patriksimek/vm2/releases/tag/v3.11.8
• https://www.vulncheck.com/advisories/vm2-3.10.0-through-3.11.5-denial-of-service-via-host-promise
• https://github.com/advisories/GHSA-gjq8-xm47-88rc

Original description

## Summary vm2 current head (`v3.11.5`, commit `7a1f5100b96f48d34e0fe104ab37c0acc5944f92`) can still be used to terminate the host Node.js process when sandbox code calls a host-realm function that returns a rejected host Promise and then ignores the returned value. This is an incomplete-fix variant of the `GHSA-hw58-p9xv-2mjh` unhandled rejection hardening. The `localPromise` constructor now catches and consumes sandbox-created unhandled rejections, but host Promises returned across the bridge are not marked handled at the bridge boundary. If the sandbox does not attach `.catch()` or `.then(..., onRejected)`, Node's default unhandled rejection behavior terminates the host process. ## Technical Details `lib/setup-sandbox.js` hardens sandbox-created Promises by wrapping the executor and attaching a benign swallow tail: ```js apply(globalPromisePrototypeThen, this, [undefined, localPromiseSwallow]); ``` That only applies to `localPromise` instances created inside the sandbox. Host-returned Promises cross the membrane through the bridge apply path. The bridge wraps callbacks when sandbox code later calls `.then`, `.catch`, or `.finally` on a host Promise: ```js bridge.setHostPromiseSanitizers(e => handleException(from(e)), from); ``` However, if sandbox code ignores the returned host Promise, no host-side rejection handler is attached. The original host Promise remains unhandled and Node terminates the host process under the default unhandled-rejection behavior. `NodeVM` provides an in-repository example of this primitive through the special `events` builtin wrapper. `lib/builtin.js` passes host `EventEmitter.once` into the sandbox: ```js once: EventEmitter.once, ``` Then `lib/events.js` re-exports it: ```js if (host.once) module.exports.once = host.once; ``` Calling `events.once(ee, 'message')` and then emitting `error` on `ee` returns a rejected host Promise through that wrapper. If ignored by sandbox code, it terminates the host process. ## Impact An attacker who can run code in a vm2 sandbox can terminate the host Node.js process when the embedder exposes a host Promise-returning API, or when a `NodeVM` permits the `events` builtin. For web services, queues, notebook workers, plugin hosts, and multi-tenant code execution systems, a single small request can terminate the worker process. Restart policies do not fully mitigate the issue because the payload can be replayed after each restart. ## Affected Package/Versions Confirmed affected on Node.js `v25.8.0`: - `v3.10.0` - `v3.10.1` - `v3.10.2` - `v3.10.3` - `v3.10.4` - `v3.10.5` - `v3.11.0` - `v3.11.1` - `v3.11.2` - `v3.11.3` - `v3.11.4` - `v3.11.5` / current head `7a1f5100b96f48d34e0fe104ab37c0acc5944f92` The final PoV was also reproduced on current head with Node.js `v16.20.2`, `v18.20.8`, `v20.20.2`, `v22.22.3`, `v24.16.0`, and `v25.9.0` using `npx node@<major>`. The local workstation default Node.js `v25.8.0` also reproduces. ## Configuration Required The general VM PoV requires an embedder-exposed host function that can return a rejected host Promise: ```js const vm = new VM({ sandbox: { hostReject: () => Promise.reject(new Error('host-boom')), }, }); ``` The NodeVM variant requires `events` in the builtin allowlist: ```js new NodeVM({ require: { external: false, builtin: ['events'] } }); ``` `events.once()` is exposed by vm2's special `events` builtin wrapper. Node's official API documents `events.once()` as returning a Promise that rejects when the watched emitter emits `error` while waiting for another event. ## Controls - If sandbox code attaches `.catch(() => {})` to the returned host Promise, the process survives. - If sandbox code creates and ignores a sandbox-native rejected Promise, the process survives on current head. This confirms the `GHSA-hw58` localPromise hardening is active. - If sandbox code attaches `.catch(() => {})` to the `events.once()` Promise, the `NodeVM` process survives. - If `NodeVM` does not allow the `events` builtin, the `events.once()` variant does not run and the process survives. ## Disclosure Policy Fit vm2's security policy asks reporters not to open public issues and to submit This report is intended for that private route and includes the requested reproduction steps, affected versions, environment/configuration details, and impact. The affected range is within the supported `3.x` line. ## Local Proof of Concept Run from the `oss-zero-day-harness` directory: ```fish node submission-bundle/vm2-pov-test-host-promise-return-unhandled-rejection-dos/pov-host-promise-return-unhandled-rejection-dos.js ``` The crash-safe PoV executes each case in a child process. A vulnerable result has `status: 1` for the positive cases and `status: 0` for controls. Minimal VM positive case: ```js const { VM } = require('vm2'); const vm = new VM({ sandbox: { hostReject: () => Promise.reject(new Error('host-boom')), }, }); vm.run('hostReject(); 1'); setTimeout(() => console.log('survived'), 150); ``` Observed on current head: ```text Error: host-boom at hostReject (...) ``` The process exits before printing `survived`. Minimal NodeVM builtin variant: ```js const { NodeVM } = require('vm2'); const vm = new NodeVM({ require: { external: false, builtin: ['events'] }, }); vm.run(` const events = require('events'); const ee = new events.EventEmitter(); events.once(ee, 'message'); ee.emit('error', new Error('event-boom')); module.exports = 'returned'; `, 'events-pov.js'); setTimeout(() => console.log('survived'), 150); ``` Observed on current head: ```text node:internal/process/promises:332 triggerUncaughtException(err, true /* fromPromise */); Error: event-boom ``` The process exits with status `1`. ## Mitigation Applications can reduce exposure by installing a process-level `unhandledRejection` handler that swallows vm2-originated rejections, as the README recommends for related async rejection caveats. That is an application workaroun