# GHSA-4c35-wcg5-mm9h: `setNestedProperty` in `packages/next-intl/src/extractor/utils.tsx` walks a dotted key path and assigns the final value without b

> Prototype pollution in next-intl allows attackers to execute arbitrary code during the build process if experimental precompilation is enabled.

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 6 May 2026

## What is GHSA-4c35-wcg5-mm9h?

GHSA-4c35-wcg5-mm9h is a vulnerability in next-intl, published on 6 May 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

## 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) {
     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`:

```ts
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)` +

## Which versions are affected by GHSA-4c35-wcg5-mm9h?

2 open source packages ship an affected version, including next-intl.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `next-intl` | npm | All versions | 4.9.2 |
| `next-intl` | npm | All versions | 4.9.2 |

## How do I fix GHSA-4c35-wcg5-mm9h?

Upgrade next-intl to 4.9.2 or later.

## Is GHSA-4c35-wcg5-mm9h being exploited?

There is no known exploitation of GHSA-4c35-wcg5-mm9h in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for GHSA-4c35-wcg5-mm9h and fix it if it is affected.

What it is: GHSA-4c35-wcg5-mm9h is a vulnerability in next-intl, published on 6 May 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of GHSA-4c35-wcg5-mm9h in the wild and no public exploit code.

Affected packages and fixed versions:
- next-intl (npm): affected all versions; fixed in 4.9.2
- next-intl (npm): affected all versions; fixed in 4.9.2

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/GHSA-4c35-wcg5-mm9h/ (Markdown: https://insights.sbomify.com/GHSA-4c35-wcg5-mm9h/index.md)
```

## Sources

- OSV: https://osv.dev/vulnerability/GHSA-4c35-wcg5-mm9h

---
From sbomify Insights: https://insights.sbomify.com/GHSA-4c35-wcg5-mm9h/
Summaries are generated and may contain mistakes; the official description above is the record.