Unverified Sign in with Apple callback parameter can override verified ID token claims

Summary

Acceptance of Extraneous Untrusted Data With Trusted Data vulnerability in the Sign in with Apple strategy in pow-auth Assent allows an unauthenticated remote attacker to impersonate another user via the user parameter of the authorization callback.

Apple posts the user's name to the callback endpoint in a user form parameter that the browser submits and the client controls. Assent.Strategy.Apple.callback/2 in lib/assent/strategies/apple.ex decodes that parameter, and fetch_user/2 merges it over the claims of the verified ID token, so the unverified map wins. An attacker who completes a Sign in with Apple login with their own Apple ID and edits the user parameter can replace sub, email, and any other claim. Applications that look up, link, or authorize accounts by the returned subject or email then act on the attacker's values.

This issue affects assent: from 0.1.25 before 0.3.2.

Details

1. Unverified input. Apple's form_post response mode delivers code, id_token, state, and on the first authorization a user parameter that holds a JSON object with the user's name and email. The browser submits this parameter and Apple does not sign it, so the client controls its content.

2. Merge over verified claims. Assent.Strategy.Apple.callback/2 decodes the user parameter with Helpers.decode_json/2 and stores the map in the config under :user. After Assent.Strategy.OIDC.fetch_user/2 has validated the ID token, Assent.Strategy.Apple.fetch_user/2 calls Map.merge/2 with the verified claims first and the decoded parameter second. Keys from the parameter overwrite keys from the token, including sub and email.

3. Result. normalize/2 reads sub and email from the merged map and sets email_verified to true, so the application receives attacker-chosen identity fields marked as verified.

4. Fix. Version 0.3.2 ignores the parameter by default. With include_unverified_name: true, only given_name and family_name are taken from it, and the verified claims are merged over them.

Proof of concept

1. Start the Sign in with Apple flow of an application that uses Assent.Strategy.Apple.
2. Authenticate at Apple with an Apple ID the attacker controls.
3. Intercept the form_post request to the application's callback URL and set the user parameter to a JSON object that carries the victim's values, such as {"sub": "<victim subject>", "email": "victim@example.com"}.
4. The application calls Assent.Strategy.Apple.callback/2. It validates the attacker's ID token and returns a user map whose sub and email are the victim's values, with email_verified set to true.

Impact

An attacker with any Apple ID can sign in to an application that uses the Apple strategy as any other user whose Apple subject or email they know. Account lookup, account linking, and email-based access control then act on the attacker-chosen identity.

Workarounds

Remove the user parameter from the callback parameters before you pass them to Assent.Strategy.Apple.callback/2, for example with Map.delete(params, "user"). Without the parameter, decode_user_params/2 yields an empty map and the verified ID token claims pass through unchanged. The user's name is then unavailable, because Apple delivers it only through this parameter.