Hands-On Guide: Auditing & Exploiting OAuth 2.0 & OpenID Connect (OIDC) Vulnerabilities (2026)

Canonical Source: This guide was originally published on Andrax Pentester.

Hands-On Guide: Auditing & Exploiting OAuth 2.0 & OpenID Connect (OIDC) Vulnerabilities (2026 Edition)

OAuth 2.0 (RFC 6749) and OpenID Connect (OIDC) form the foundation of modern API, web, and mobile identity federation. However, implementation oversights frequently lead to catastrophic Account Takeover (ATO) and Privilege Escalation flaws.

In this field guide, we examine the primary OAuth 2.0 attack vectors and audit methodologies.

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Core Vulnerability Vectors

1. Loose Redirect URI Matching & Code Exfiltration

When the Authorization Server fails to perform exact-string matching on redirect_uri, attackers use directory traversal (/../../attacker) or subdomain hijacking to exfiltrate authorization codes:

GET /authorize?response_type=code&client_id=client_88921a&redirect_uri=https://app.victim.com/oauth/callback/../../attacker_path&scope=openid%20profile&state=xyz123 HTTP/1.1
Host: idp.enterprise.com

2. State Parameter Omission & CSRF Account Linking

Omission of state parameter verification allows attackers to force target victims to exchange attacker-controlled authorization codes, linking the victim session to the attacker's social account.

3. PKCE Downgrade Attacks (RFC 7636)

Public clients omitting code_challenge or authorization servers accepting code_challenge_method=plain expose authorization codes to interception and replay.

4. JWT Key Confusion (RS256 to HS256)

Verification libraries allowing header algorithm switching let attackers sign forged identity tokens using the server's public RSA key as an HMAC secret.

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Automated Python OAuth Security Auditor

import urllib.parse
import hashlib
import base64
import os
import requests

class OAuthAuditor:
    def __init__(self, authorize_url: str, client_id: str, valid_redirect_uri: str):
        self.authorize_url = authorize_url
        self.client_id = client_id
        self.valid_redirect_uri = valid_redirect_uri
        self.session = requests.Session()

    def audit_redirect_uri_traversal(self):
        print("[*] Testing Redirect URI Path Traversal & Wildcard Acceptance...")
        parsed_uri = urllib.parse.urlparse(self.valid_redirect_uri)
        test_uris = [
            f"{self.valid_redirect_uri}/../../evil_callback",
            f"https://attacker-{parsed_uri.netloc}/callback"
        ]

        for test_uri in test_uris:
            params = {
                "response_type": "code",
                "client_id": self.client_id,
                "redirect_uri": test_uri,
                "scope": "openid profile email",
                "state": "test_state_123"
            }
            res = self.session.get(self.authorize_url, params=params, allow_redirects=False)
            if res.status_code in [301, 302]:
                location = res.headers.get("Location", "")
                if "evil_callback" in location or "attacker" in location:
                    print(f"  [!] CRITICAL: Loose Redirect URI accepted: {test_uri}")

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Defensive Requirements (OAuth 2.1 Standard)

1. Enforce strict exact-match string comparison on redirect_uri.
2. Require SHA-256 PKCE (S256) challenges for all authorization requests.
3. Bind state tokens to encrypted, SameSite=Strict; Secure; HttpOnly session cookies.

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