Hunting for Open Redirect Vulnerabilities: A Beginner to Expert Guide

Hello, fellow security enthusiasts! Today, I’m excited to take you through the process of discovering open redirect vulnerabilities. We’ll start with the basics and work our way to advanced techniques that will level up your bug-hunting skills.

An open redirect vulnerability occurs when a web application allows users to be redirected to an external URL without properly validating it. This might sound harmless, but open redirects can be dangerous, especially when used in phishing attacks to make malicious URLs look more trustworthy. They can also aid attackers in bypassing security restrictions or performing social engineering attacks. The consequences of an open redirect can be severe, leading to account takeover, cross-site scripting (XSS), phishing scams, information disclosure, and other malicious activities that exploit unsuspecting users.

When looking for open redirects, we focus on URLs with redirect-like parameters. Manually inspecting URLs is a time-consuming task, so let’s look at how to automate the process with tools to help us identify URLs that might be vulnerable.

Here’s a quick overview of how to gather potential URLs for testing:

Now that we have our list of URLs, let’s explore a range of techniques to uncover open redirect vulnerabilities, from beginner-friendly methods to advanced tactics.

1. Basic Parameter Testing

For beginners, the simplest way to test open redirects is to manually replace URL parameters with an external domain. Here’s a quick example:

Modify it to:

If the application doesn’t validate this change and redirects you to https://evil.com, congratulations—you’ve found an open redirect!

2. Testing for Alternate Parameter Names

Developers don’t always use the same parameter names, so try common alternatives like target, dest, goto, callback, to, etc. Here’s an example of how you might find a variation:

3. Encoding Tricks

Encoding the target URL can sometimes bypass validation mechanisms. Here are a few examples:

Try modifying parameters with these encodings to see if they redirect to the encoded URL.

4. Using Bypasses like //

If the application blocks URLs with http or https, you might be able to trick it by using // instead:

Browsers interpret //evil.com as an external URL if no protocol is specified. This simple trick often bypasses input filters.

5. Exploring Base64 Encoding

Many applications use Base64 encoding to mask redirect URLs. If you spot Base64 strings in redirect parameters, decode them to reveal the true URL and replace it with an encoded malicious URL.

For example:

Decode aHR0cHM6Ly9nb29nbGUuY29t, and if it’s a valid redirect URL, replace it with an encoded malicious link.

6. Using Tool-Based Parameter Testing with QSReplace

A tool like QSReplace is handy for replacing parameters in bulk. It allows you to test multiple URLs at once by replacing any parameter with a specified URL:

If any of these URLs redirect to http://evil.com, you may have found a vulnerability.

7. Identifying JavaScript-Based Redirects with LinkFinder

JavaScript files sometimes contain redirect logic that isn’t immediately visible from the main URL. LinkFinder is a tool that extracts URLs and parameters from JavaScript files. You might find hidden redirect parameters within client-side code.

8. Fuzzing Parameters with FFUF

Using a fuzzing tool like ffuf can help you find non-standard redirect parameters. FFUF will brute-force parameter names and can quickly uncover less obvious redirect functionalities.

8. Fuzzing Parameters with FFUF

Using a fuzzing tool like ffuf can help you find non-standard redirect parameters. FFUF will brute-force parameter names and can quickly uncover less obvious redirect functionalities.

9. Examining the DOM for Client-Side Redirects

Sometimes redirects are implemented entirely in the DOM. Use the browser’s developer tools to search for keywords like window.location, document.location, or redirect. These can indicate a client-side redirect that may be vulnerable to manipulation.

10. Exploring Custom Payloads for Edge Case Testing

Advanced open redirect detection involves creating payloads that exploit URL parsing quirks. For example: