Referrer-policy header cannot be recognized

Sooraj V Nair
Published on
07 May 2024
4 min read

Referrer header is a request header from where the traffic originated in a site. A referrer policy header controls which referrer information should be included with the request.

It is an HTTP header that controls how much referrer information (the URL of the page that linked to the resource being requested) is passed along when navigating from one page to another.

However, if a browser encounters a Referrer Policy header that it cannot recognize, it typically defaults to a safer option like “strict-origin-when-cross-origin” or “no-referrer-when-downgrade.

If there is no adequate prevention in place the URL and even sensitive information contained in the URL will be leaked to the cross site. The lack of Referrer Policy header might affect privacy of the users and sites itself.

This behavior ensures that sensitive referrer information isn’t inadvertently leaked, especially when navigating to cross-origin resources. If a browser cannot recognize the specific policy specified in the header, it will fall back to a safer default to maintain user privacy and security

In this blog we will look at the impacts of such referrer policy and how it can be prevented.

Impacts of referrer-policy that cannot be recognized

When a referrer policy header cannot be recognized by a browser, it typically defaults to a safer option to ensure user privacy and security. Here are some potential impacts of this behavior:

1. Limited control

Website owners may not have full control over how referrer information is handled when navigating from one site to another. This can make it challenging to implement specific referrer policies tailored to their needs.

2. Reduced tracking

If the browser defaults to a more restrictive referrer policy, it may limit the amount of referrer information passed along when navigating to external sites. This can impact analytics and tracking systems that rely on referrer data to measure user behavior and traffic sources.

3. Compatibility issues

In some cases, a website’s functionality or third-party services may depend on the proper recognition of a specific referrer policy header. If the browser defaults to a different policy, it could lead to compatibility issues and unexpected behavior.

4. Security concerns

Without proper recognition of the intended referrer policy, there is a risk that sensitive referrer information could be leaked unintentionally. Browsers typically default to safer options to mitigate this risk, but there is still a possibility of data exposure, especially when navigating to cross-origin resources.

5. Development challenges

Web developers may face challenges in ensuring consistent behavior across different browsers and versions, especially if they rely on specific referrer policies to enforce security measures or track user interactions.

In short, while the defaulting to safer options by browsers helps protect user privacy and security, it can also introduce challenges for website owners and developers in managing referrer information and ensuring compatibility with their systems and services.

How can you prevent the Referrer-Policy header which cannot be recognized?

To prevent the issue of the Referrer Policy header not being recognized, you can take the following steps:

1. Use standardized referrer-policies

Stick to the standardized referrer policies widely recognized and supported by modern browsers. Common policies include “no-referrer”, “no-referrer-when-downgrade”, “same-origin”, “origin”, “strict-origin”, “strict-origin-when-cross-origin”, and “unsafe-url”.

Using these standard policies increases the likelihood that browsers will recognize and properly interpret the header.

2. Check browser compatibility

Before implementing a referrer policy, verify its compatibility with different browsers and versions. Ensure that the policy you choose is supported across major browsers to minimize the risk of unrecognized headers.

3. Fallback mechanism

Implement a fallback mechanism in your web application or server configuration. If a browser encounters a referrer policy header it cannot recognize, you can specify a default policy that is widely supported and aligns with your privacy and security requirements.

4. Testing and monitoring

Regularly test your web application in various browsers to ensure that the referrer policy headers are being recognized correctly. Monitor for any compatibility issues or unexpected behavior and adjust your implementation as needed.

5. Stay informed

Keep abreast of updates and changes to browser specifications and standards related to referrer policies. By staying informed about browser behavior and best practices, you can adapt your implementation to maintain compatibility and security.

6. Provide feedback

If you encounter issues with unrecognized referrer policy headers in specific browsers, consider providing feedback to browser vendors or participating in relevant developer communities. Your feedback can help improve browser compatibility and standards compliance over time.

By following these steps, you can reduce the likelihood of encountering issues with unrecognized referrer policy headers and ensure consistent behavior across different browsers and platforms.

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Written by
Sooraj V Nair
Sooraj V Nair
Cyber Security Engineer
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