OAuth Misconfiguration: Hijack User Accounts Without Abusing Redirects

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A critical OAuth misconfiguration allows attackers to hijack user accounts without manipulating redirect URIs. This vulnerability occurs when an application fails to validate the token issuer, enabling one-click account takeover.

You Should Know:

1. Understanding the Vulnerability

  • OAuth relies on token validation to ensure security.
  • If an app doesn’t verify the `iss` (issuer) claim in the JWT token, attackers can forge tokens from malicious identity providers.

2. Exploitation Steps

1. Set Up a Malicious Identity Provider:

  • Clone a legitimate OAuth provider (e.g., Google, Facebook).
  • Modify the token issuer (iss) to match the target application’s expected value.

2. Craft a Malicious Token:

{
"iss": "https://legitimate-provider.com",
"sub": "attacker-id",
"aud": "target-app-client-id",
"exp": 9999999999,
"iat": 1234567890
}

3. Bypass Validation:

  • If the app doesn’t check the issuer, the forged token is accepted.

4. Trigger One-Click Attack:

  • Send a phishing link with a pre-authorized malicious token.
  • Victim clicks, and their session is hijacked.

3. Detection & Mitigation

  • Check Token Issuer: 
    import jwt 
def validate_token(token): 
decoded = jwt.decode(token, options={"verify_signature": False}) 
if decoded["iss"] != "https://trusted-issuer.com": 
raise Exception("Invalid issuer!")

  • Enforce Strict Redirect URI Validation: 

    Example: OAuth2 Proxy check 
oauth2-proxy --validate-redirects --allowed-redirect-domains="trusted.com"

  • Use OpenID Configuration: 

    curl https://accounts.google.com/.well-known/openid-configuration

4. Linux & Windows Commands for Testing

  • Check JWT Tokens: 

    echo "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9..." | jwt decode -

  • Test OAuth Flows with curl: 

    curl -X POST "https://oauth-provider.com/token" -d "client_id=CLIENT_ID&grant_type=authorization_code&code=CODE"

  • Windows PowerShell OAuth Check: 

    Invoke-WebRequest -Uri "https://api.example.com/userinfo" -Headers @{"Authorization"="Bearer FAKE_TOKEN"}

What Undercode Say

This misconfiguration highlights the dangers of weak OAuth validation. Always enforce:
– Issuer Verification
– Strict Redirect URI Checks
– Token Signature Validation

Attackers evolve, but proper validation can prevent one-click takeovers.

Expected Output:

  • A logged-in session hijacked via malicious OAuth token.
  • Debug logs showing invalid issuer acceptance.

Prediction

OAuth 2.1 will enforce stricter issuer validation by default, reducing such misconfigurations.

Reference: Medium

References:

Reported By: Ahmed Basiony – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅

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