vm2: Sandboxed code can read and write host-realm memory via Node's shared Buffer pool

Summary

Sandboxed code is able to disclose host memory used by small allocations by Buffer.from, Buffer.concat.

Details

vm2 exposes Buffer object to sandboxed code by default. Small [Buffer allocations](https://nodejs.org/api/buffer.html#static-method-bufferallocunsafesize) (for example, Buffer.allocUnsafe(), Buffer.from(array), Buffer.from(string), and Buffer.concat()) use the same Buffer pool, which is shared with the sandbox. This allows sandboxed code to disclose host memory used by the functions listed above.

PoC

Tested against vm2@3.11.5 in the node REPL.

Buffer.from('host-memory-should-not-leak-to-sandbox')
new (require('vm2').VM)().run(`Buffer.from(Buffer.from([0]).buffer, 0, Buffer.from([0]).buffer.byteLength).toString('ascii')`)

<img width="1044" height="104" alt="Screenshot 2026-07-23 at 17 54 15" src="https://github.com/user-attachments/assets/987b860c-6702-4818-bfaa-c77919c777e5" />

Impact

Since sandbox acquires an ArrayBuffer that is used by the host, it can disclose sensitive data going through functions mentioned above and even write to these buffers, which can lead to sensitive data exposure and potentially denial-of-service.