Detecting Open Redirect Vulnerability in API Endpoints

Introduction

Imagine clicking on what seems like a legitimate link, only to find yourself on a malicious website designed to steal your information. Sounds scary, right? This scenario is often made possible by an open redirect vulnerability—a common yet underestimated security flaw. In this blog, we’ll demystify this vulnerability, walk you through real-life examples, and share tips on how to protect your APIs from exploitation.

What is an Open Redirect Vulnerability?

Let’s break it down. An open redirect happens when an application takes a user’s input (like a URL) and redirects them without checking if the destination is safe. Here’s a quick example:

GET /redirect?url=https://example.com

If the url parameter isn’t validated, an attacker could change it to something malicious:

GET /redirect?url=https://malicious.com

Now, unsuspecting users who follow the link will end up on a malicious site, possibly exposing their sensitive information.

How Open Redirects Work

Open redirects occur when a developer mistrusts attacker-controlled input to redirect to another site, usually via a URL parameter, HTML <meta> refresh tags, or the DOM window location property.

Example 1: URL Parameter

Many websites intentionally redirect users to other sites by placing a destination URL as a parameter in an original URL. The application uses this parameter to tell the browser to send a GET request to the destination URL. For example:

https://www.google.com/?redirect_to=https://www.gmail.com

If an attacker changes the URL to the following:

https://www.google.com/?redirect_to=https://www.attacker.com

And the application doesn’t validate the redirect_to parameter, users can be redirected to the malicious domain.

Example 2: HTML Meta Tags

Another common scenario involves <meta> tags:

<meta http-equiv="refresh" content="0; url=https://www.google.com/">

Attackers can use this redirect behavior to inject their own redirect URLs by exploiting vulnerabilities that allow control over the tag’s content attribute.

Example 3: JavaScript Manipulation

Attackers can modify the window’s location property via JavaScript to trigger redirects:

window.location = 'https://www.google.com';
window.location.href = 'https://www.google.com';
window.location.replace('https://www.google.com');

These methods are often used in cross-site scripting (XSS) attacks or by exploiting applications that intentionally allow users to define redirect URLs.

Shopify Login Open Redirect Case Study

Let’s dive into a real-world example to better understand the risk. Shopify, a popular e-commerce platform, had a vulnerability in its login functionality. Here’s how it worked:

The login endpoint accepted a parameter (checkout_url) that attackers could manipulate. For example:

https://mystore.myshopify.com/account/login?checkout_url=attacker.com

By tweaking the checkout_url, attackers could redirect users to malicious domains. Imagine thinking you’re logging into your favorite Shopify store, but instead, you land on a site that looks identical but is designed to steal your login credentials. Shopify addressed this issue after it was reported in 2015, rewarding the researcher with a $500 bounty.

What Can We Learn from This?

1. Always validate user input, especially parameters that control redirection.
2. Use strict whitelists to define safe domains.
3. Stay cautious of creative tricks, like using special characters to manipulate URLs.

Why Should You Care About Open Redirects?

At first glance, open redirects might not seem like a big deal. But they can have serious consequences:

• Phishing Attacks: Imagine a user clicks on a link from your site and ends up on a fake login page. Trust is instantly broken.
• Credential Theft: Fake pages can harvest usernames and passwords.
• Brand Damage: Frequent exploitation can tarnish your company’s reputation.
• Traffic Hijacking: Redirects can manipulate SEO rankings or steal your website traffic.

How to Spot Open Redirect Vulnerabilities in APIs

1. Manual Testing

• Dig Into Redirect Parameters: Look for parameters like url, next, or return.
• Test Malicious Inputs: Replace legitimate URLs with harmful ones and observe what happens.
• Pay Attention to Responses: Watch for redirects with status codes like “302 Found” and Location headers.

2. Automated Scanning

• Use tools like Cyprox.io’s API Scanner to quickly find vulnerabilities.
• Build scripts that test redirect parameters against a set of malicious URLs.

3. Code Review

• Dive into the logic behind redirection functions and modules. Are they validating input properly?

How to Prevent Open Redirect Vulnerabilities

1. Validate Input

• Whitelist Trusted URLs: Don’t redirect to any domain outside your control.
• Block External URLs: Restrict redirection to your application’s domains or subdomains.

2. Secure the Output

• Encode user inputs to prevent URL manipulation.

3. Monitor Logs

• Regularly check logs for suspicious redirect patterns.

4. Educate Your Team

• Teach developers the importance of secure coding practices, especially around user input.

Wrapping It Up

Open redirects can be sneaky, allowing attackers to redirect unsuspecting users to malicious websites. These vulnerabilities often rely on trust, tricking people into believing they're visiting a legitimate site when, in reality, they’re not. Spotting them requires a sharp eye and a bit of curiosity. Sometimes the clues are obvious, like parameters named redirect_to, domain_name, or checkout_url, as we've seen in earlier examples. But other times, they’re hidden behind less obvious names, like r or u.

The key to uncovering these vulnerabilities lies in thoroughly testing these parameters. Don’t hesitate to get creative—try adding special characters, like a period, to see if part of the URL is hardcoded or vulnerable to manipulation. This persistence can make all the difference.

Take the HackerOne interstitial redirect as an example. It highlights how important it is to recognize the tools and services websites rely on when you’re hunting for vulnerabilities. Staying observant and persistent, while clearly demonstrating the issue, can help you convince a company of the problem’s seriousness and earn that well-deserved bounty. After all, success in this field is often about spotting what others overlook.

No one wants their platform to be a launchpad for phishing attacks or malicious redirects. By understanding and addressing open redirect vulnerabilities, you can protect your users and your brand’s reputation. Take the Shopify case as a lesson—simple mistakes can lead to big consequences.

Want to safeguard your APIs? Try Cyprox.io’s API Scanner today and take your security to the next level. Remember, a secure platform isn’t just good for users—it’s good for business too!

Resources:

• https://www.hackerone.com/reports/101962/
• https://cwe.mitre.org/data/definitions/601.html