Mongoose is a MongoDB object modeling tool designed to work in an asynchronous environment. Prior to 6.13.12, 7.8.13, 8.24.7, and 9.11.4, Mongoose can descend into inherited properties while loading database documents in init(), setting dotted paths in $__set, and merging update documents in utils.mergeClone. Attacker-controlled keys such as toString or valueOf can therefore cause Mongoose to write into shared built-in functions instead of an own property. A crafted document, Mixed path, non-strict document or subdocument, findOneAndReplace() replacement, chained update, or Query.prototype.merge() source can overwrite Object.prototype.toString.call, causing later queries on unrelated models to fail until the process restarts. Separately, when an attacker controls the dotted path passed to doc.set(path, value) for a subdocument configured with strict: false, descent through $__set can reach $__schema and alter shared validators or schema options for later documents. Mongoose already blocks __proto__, constructor, and prototype, so this issue overwrites properties on reachable shared objects rather than adding new properties directly to Object.prototype. This issue is fixed in versions 6.13.12, 7.8.13, 8.24.7, and 9.11.4.

Affected: npm: mongoose

### Summary

Three Mongoose code paths copy data into objects by property name without checking whether the property is the object's own or inherited from its prototype:

1. Loading a document from the database (`init()`, also used by `Model.hydrate()`) 2. Setting a dotted path on a document (`$__set`, used by `new Model(obj)`, `doc.set()` and `doc.$set()`) 3. Merging update documents (`utils.mergeClone`, used by `findOneAndReplace()`, chained update calls and `Query.prototype.merge()`)

When a key names an inherited property, such as `toString` or `valueOf`, or a document internal such as `$__schema`, Mongoose writes into the shared object behind that property. The data comes from the attacker.

- **Denial of service.** For example, `{ "toString": { "call": 1 } }` overwrites `Object.prototype.toString.call`. Mongoose and most other libraries rely on `Object.prototype.toString.call()`. After the overwrite, every later query in the process throws `TypeError: _toString.call is not a function` until the process restarts. This includes queries on unrelated models. - **Changing shared schema state.** Through a subdocument with `strict: false`, a path such as `profile.$__schema.paths.email.validators` reaches the subdocument's schema, which every document shares. Setting it to `[]` removes the `required` validator for every document in the process.

Mongoose already blocked `__proto__`, `constructor` and `prototype`, so these bugs cannot add properties to `Object.prototype` itself…