next-intl has prototype pollution with `experimental.messages.precompile` via attacker-controlled translation catalog

## Summary `setNestedProperty` in `packages/next-intl/src/extractor/utils.tsx` walks a dotted key path and assigns the final value without blocking the reserved keys `__proto__`, `constructor`, or `prototype`. When the next-intl Next.js plugin is configured with `experimental.messages` and `messages.precompile: true`, a JSON translation catalog containing a top‑level `__proto__` key causes `setNestedProperty(result, '__proto__.isAdmin', compiledMessage)` to assign onto `Object.prototype`, polluting every object in the running build process. ## Details Root cause — `packages/next-intl/src/extractor/utils.tsx:13-34`: ```ts export function setNestedProperty( obj: Record<string, any>, keyPath: string, value: any ): void { const keys = keyPath.split('.'); let current = obj; for (let i = 0; i < keys.length - 1; i++) { const key = keys[i]; if ( !(key in current) || typeof current[key] !== 'object' || current[key] === null ) { current[key] = {}; } current = current[key]; } current[keys[keys.length - 1]] = value; } ``` The existence check `!(key in current)` uses the `in` operator, which walks the prototype chain. For `key === '__proto__'`, `'__proto__' in {}` is `true` (it's inherited from `Object.prototype`) and `typeof current['__proto__'] === 'object'` (it *is* `Object.prototype`). The guard therefore never re-initializes `current[key]`, and `current = current['__proto__']` redirects all subsequent writes onto `Object.prototype`. The final assignment `current[keys[keys.length-1]] = value` sets `Object.prototype[<attacker key>] = <attacker value>`. Build-time data flow: 1. `packages/next-intl/src/plugin/catalog/catalogLoader.tsx:55-83` — the webpack/turbopack loader receives the catalog file `source` and, if `options.messages.precompile` is enabled, calls `codec.decode(source, {locale})`. 2. `packages/next-intl/src/extractor/format/codecs/JSONCodec.tsx:9-18` — `decode` runs `JSON.parse(source)`. V8 installs `__proto__` as an **own data property** on the result when the JSON key is literally `"__proto__"` (bypassing the normal `Object.prototype.__proto__` setter that would otherwise reassign the prototype). 3. `JSONCodec.tsx:33-53` — `traverseMessages` iterates `Object.keys(obj)`, which for a JSON‑parsed object includes the own `__proto__` key. It reads `obj.__proto__` (returns the attacker’s nested object, not `Object.prototype`, because it's an own property), recurses into it, and emits message id `__proto__.isAdmin`. 4. `catalogLoader.tsx:71` — `precompileMessages(decoded, cache)`. 5. `catalogLoader.tsx:89-131` — for each message, calls `setNestedProperty(result, message.id, compiledMessage)`. With `message.id === '__proto__.isAdmin'`, `setNestedProperty` walks into `Object.prototype` and assigns `Object.prototype.isAdmin = compiledMessage`. The same sink is also reachable via `JSONCodec.encode` (`JSONCodec.tsx:20-26`) and `POCodec` (`packages/next-intl/src/extractor/format/codecs/POCodec.tsx:87`) during extraction, both of which feed attacker-influenced `message.id` values into `setNestedProperty` — but those paths require control of source-code identifiers, which is a weaker attack vector than the build-time catalog path above. After pollution, every subsequent object access during the remainder of the Next.js build pipeline (webpack, turbopack, babel, next-intl’s own logic) inherits the attacker-controlled properties. This is a classic gadget-chain precondition for corrupting build-tool internals and tampering with generated bundles, since many build tools use patterns like `if (obj.someFlag)` or `options[key] ?? default` that are sensitive to polluted prototypes. Trust boundary note: next-intl’s message catalogs are realistically attacker-influenced in practice. Translation files are routinely round-tripped through external TMS systems (Crowdin, Lokalise, Transifex), accepted via community locale PRs, or pulled from third-party translation packages — any of which can carry a crafted `__proto__` key unnoticed, since JSON translation diffs are usually merged with minimal scrutiny. ## PoC Prerequisites: a Next.js project using next-intl ≤ 4.9.1 with the Next.js plugin configured: ```ts // next.config.ts import createNextIntlPlugin from 'next-intl/plugin'; const withNextIntl = createNextIntlPlugin({ experimental: { messages: { path: './messages', format: 'json', locales: 'infer', precompile: true } } }); export default withNextIntl({}); ``` 1. Drop a malicious catalog at `messages/en.json`: ```json { "Greeting": "Hello", "__proto__": { "isAdmin": "polluted" } } ``` 2. Run `next build` (or `next dev`). The `catalogLoader` will invoke `JSONCodec.decode` → `traverseMessages` → `precompileMessages` → `setNestedProperty`. 3. Minimal reproduction of the sink itself (verified locally against the v4.9.1 source): ```js function setNestedProperty(obj, keyPath, value) { con

• github.com/amannn/next-intl/security/advisories/GHSA-4c35-wcg5-mm9h (opens in a new tab)
• github.com/amannn/next-intl (opens in a new tab)

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