Feathersjs is a framework for creating web APIs and real-time applications with TypeScript or JavaScript. In 5.0.44 and earlier, the _.merge(target, source) utility exported by @feathersjs/commons recursively merges source into target by iterating Object.keys(source). When source was produced by JSON.parse and contains a __proto__, constructor, or prototype key, that key is returned as an own-enumerable property; the recursive merge then resolves target['__proto__'] to Object.prototype and writes attacker-supplied properties onto it, polluting the prototype for all plain objects in the process for the lifetime of the Node process. This issue is fixed in version 5.0.45.

Affected: npm: @feathersjs/commons

### Impact

The `_.merge(target, source)` utility exported by `@feathersjs/commons` recursively merges `source` into `target` by iterating `Object.keys(source)`. When `source` was produced by `JSON.parse` and contains a `__proto__` (or `constructor` / `prototype`) key, that key is returned as an own-enumerable property. The recursive merge then resolves `target['__proto__']` to `Object.prototype` and writes the attacker-supplied properties onto it, polluting the prototype for all plain objects in the process for the lifetime of the Node process.

**Scope of real-world risk is limited.** No first-party Feathers package routes input — trusted or untrusted — through `commons._.merge`. The `@feathersjs/authentication` package, which does merge request-influenced data, uses `lodash/merge` (prototype-pollution-safe since 4.17.12), not this utility. Exploitation therefore requires a downstream plugin or application to pass JSON-parsed, attacker-controlled input directly through the exported `_.merge`.

### Patches

Fixed in `@feathersjs/commons@5.0.45`. The fix skips `__proto__`, `constructor`, and `prototype` keys during iteration — the standard remediation used by lodash and others.

### Workarounds

Avoid passing JSON-parsed untrusted input through `commons._.merge`. Freezing `Object.prototype` or validating/sanitizing keys upstream also mitigates.

### Credit

Reported responsibly by Andrew Ridings (@ridingsa).