Consider a web page with URL α on web server A containing a link to a web page with URL β on web server B. If a web page visitor clicks on the link to β, many browsers send along with the request to server B for the web page β the referring webpage α. This information is sent in form of a HTTP request header.
Example Request with Referer Header
To illustrate this behaviour, let us assume you browse e.g. an adult-content web page β called https://newborn.xx/videos/some_video and click on a link to α that is https://blog.riemann.cc/digitalisation/2018/06/27/http-referer-and-data-protection/. Then, the HTTP request generated by your browser to download and display the webpage may contain the following fields:
Host: blog.riemann.cc User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Firefox/60.0 Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8 Accept-Language: en-GB,en;q=0.8,de-DE;q=0.5,de;q=0.3 Referer: https://newborn.xx/videos/some_video DNT: 1
- A web server may serve more than one website. Hence, a host must be given to specify the requested webpage β.
- The request contains information on the employed browser. Some servers may respond with a different, simplified webpage for e.g. outdated browsers.
- The request contains information on response formats the browser could understand and display.
- The request contains information on the preferred languages of the response. This setting reflects the language setting of the browser or operating system.
- Referer [sic]
- It is actually a spelling mistake of the word referrer. The referring web page containing the link is provided.
- DNT (do not track)
- Modern browsers notify the web servers if the visitor wishes to not be tracked.
Impact on Privacy
If the request for β to the web server B contains the Referer field, then B learns which web page the visitor has visited before, i.e. https://newborn.xx/videos/some_video in the example above. Furthermore, B learns that this referring web page contains the link to β.
The impact on the visitor privacy is mentioned in the standard RFC 7231 Section 5.5.2:
The Referer field has the potential to reveal information about the request context or browsing history of the user, which is a privacy concern if the referring resource’s identifier reveals personal information (such as an account name) or a resource that is supposed to be confidential (such as behind a firewall or internal to a secured service). Most general-purpose user agents do not send the Referer header field when the referring resource is a local “file” or “data” URI. A user agent MUST NOT send a Referer header field in an unsecured HTTP request if the referring page was received with a secure protocol.
Also the documentation portal of Firefox has a notice on its web page to the Referer field.
The Referer header has the potential to reveal information about the browsing history of the user, which is a privacy concern.
According to the recommendation of the RFC standard, Firefox does not send a Referer field when:
- the referring resource is a local “file” or “data” URI,
- an unsecured HTTP request is used and the referring page was received with a secure protocol (HTTPS).
That means, if e.g. https://newborn.xx/videos/some_video contains a link to another HTTPS web page, the Referer field is sent.
Good Data Protection Practice
Not all, if not most, visitors do not understand that their browser sends a Referer field revealing a small part of their browsing history. In consequence, those visitors cannot give an informed consent. I remember how I installed my browser to identify a potential contractual legal basis.
My Linux operating system comes with Firefox pre-installed. During the installation of Linux, I confirm varying free software licenses that are all somewhat compatible to each other in one go (one click).
Firefox is provided under the terms of the Mozilla Public License. It contains an explicit Disclaimer of Warranty.
Covered Software is provided under this License on an “as is” basis, without warranty of any kind, either expressed, implied, or statutory, including, without limitation, warranties that the Covered Software is free of defects, merchantable, fit for a particular purpose or non-infringing. The entire risk as to the quality and performance of the Covered Software is with You. Should any Covered Software prove defective in any respect, You (not any Contributor) assume the cost of any necessary servicing, repair, or correction. […]
It is yet to be answered if a) the referer information constitutes personal data and b) the sharing of it (potentially among different parties on different continents) is covered on the basis of the browser license.
Independently of the answer, I think a much better practice would be to allow visitors to switch this sharing by their browser on and off. From a data minimisation point of view, no sharing should occur by default.
Interestingly, a bug has been filed in 2013 already:
(Regression) Data leak: HTTP-Referer sent when it should not be sent
This bug makes though a different assumption on when a Referer field should be sent. Consequently, I created a new bug report:
Data leak: HTTP-Referer sent without consent
Let us see what happens.
While we wait for a comfortable user interface to disable the referer field, one can change the configuration of Firefox manually. I found the solution at a Mozilla Support page.
- In a new tab, type or paste
about:configin the address bar and press Enter/Return. Click the button promising to be careful.
- In the search box above the list, type or paste
refererand pause while the list is filtered
- Double-click the
network.http.referer.XOriginPolicypreference and enter the desired value:
|0||no restrictions (default)|
|1||base domain must match (send from a.example.com to b.example.com)|
|2||full host name must match (only b.example.com to b.example.com)|