Unmasking the Danger: Open Redirection via Referer Header

In the world of web security, even small problems can lead to big issues. One such problem is open redirection, especially through the Referer header. Recently, while searching for bugs, I found this vulnerability and saw how dangerous it can be. Attackers can use this flaw to send users to harmful websites, which can lead to phishing attacks, malware infections, or data theft. This blog will explain how this type of vulnerability works, the risks it brings, and ways to protect against it.

When users are redirected without their knowledge, it’s usually through links that look normal but are actually harmful. By changing the Referer header, attackers can trick users into clicking on links that redirect them to unsafe sites. This can damage the trust and security of web applications. We’ll explore how attackers exploit this vulnerability, what threats it poses to users and systems, and what steps developers and security experts can take to prevent it. Simple methods like checking and cleaning inputs or using strict rules for handling URLs can help keep your systems safe from these kinds of attacks.

Open redirection occurs when a web application accepts a user-controlled input that specifies a URL to which the application will redirect. If the input is not properly validated, attackers can manipulate it to redirect users to malicious sites. This can lead to phishing attacks, theft of credentials, and other security breaches.

The Referer header in an HTTP request tells the server the URL of the page that linked to the resource being requested. While helpful for tracking and analytics, improper handling of this header can turn it into a vector for open redirection attacks.

My Bug Hunting Experience: During a recent bug hunting session, I discovered that a web application was vulnerable to open redirection via the Referer header. Here’s how the vulnerability manifested and how I demonstrated the exploit::

1. Initial Setup: I found that the application used the Referer header to determine the redirection URL. This could occur during profile updates, login processes, or any URL where redirection is involved.

2. Manipulating the Referer Header: By crafting a request with a malicious URL in the Referer header, I could control the redirection. For instance, changing the Referer to https://malicious-site.com instead of a legitimate URL.

3. Executing the Attack: Once the server received the manipulated request, it redirected me to the attacker’s specified URL. This demonstrated the vulnerability clearly, as the user was sent to a potentially harmful site.

Step 1: Log In and Intercept the Request: Log in to the application and navigate to the profile page. Change the name of the user and submit the form. Intercept the POST request using Burp Suite. The request will include the Referer header with the URL https://redacted.com/user/profile

Step 2: Analyze the Response: In the server response, observe the 302 Not Found status, indicating a redirection. The response will have a Location header pointing to the redirect URL.

Step 3: Modify the Referer Header: Change the Referer header to a malicious URL “Referer: https://webhook.site/d1dd1fac-3b47-4e32-b518-54fe7edccdb6” Forward the modified request to the server. Notice that the response’s Location header now includes the malicious URL from the Referer header.

Step 4: Observe the Redirection: The server redirects to the malicious URL specified in the Referer header.

Step 5: Verify the Request Capture: On the backend of the malicious site (webhook.site), verify that the request was captured. The captured request will show that the redirection originated from app.redacted.com

The potential consequences of this vulnerability are severe:

Phishing Attacks: Users may be deceived into entering sensitive information on fraudulent websites.Malware Distribution: Users could be redirected to sites hosting harmful content.Security Bypasses: Attackers might use this vulnerability to bypass security policies or filters.Validate and Sanitize Inputs: Always validate and sanitize any user inputs, especially URLs. Implement strict whitelisting to ensure only allowed domains can be used for redirection.Use Relative URLs: When possible, use relative URLs for internal redirections to avoid the risks associated with user-provided URLs.Implement Redirect Notices: Inform users when they are being redirected and require their confirmation before proceeding to the new location.Monitor and Log Redirects: Track and log all redirection activities to detect and analyze any unusual patterns.Security Testing: Include regular tests for open redirection vulnerabilities in your security assessments to identify and mitigate risks early.

Open redirection vulnerabilities, especially those exploiting the Referer header, represent a subtle yet serious threat in web security. My recent bug hunting experience highlighted how easily these vulnerabilities can be overlooked and exploited. By implementing robust validation, careful input handling, and proactive security measures, you can protect your application and its users from these risks.

https://cheatsheetseries.owasp.org/cheatsheets/Unvalidated_Redirects_and_Forwards_Cheat_Sheet.html
https://hackerone.com/reports/5199

Sulman Farooq S is a passionate cybersecurity enthusiast with over two years of experience in penetration testing. He specializes in network, web application, mobile application, and API penetration testing. Follow me on Medium for more insights into cybersecurity and vulnerability assessments.

Stay vigilant and keep your applications secure! Happy (ethical) hacking! 😊😊😊