Cloudflare is deprecating the __cfduid cookie. Starting on 10 May 2021, we will stop adding a “Set-Cookie” header on all HTTP responses. The last __cfduid cookies will expire 30 days after that.
We never used the __cfduid cookie for any purpose other than providing critical performance and security services on behalf of our customers. Although, we must admit, calling it something with “uid” in it really made it sound like it was some sort of user ID. It wasn't. Cloudflare never tracks end users across sites or sells their personal data. However, we didn't want there to be any questions about our cookie use, and we don’t want any customer to think they need a cookie banner because of what we do.
So why did we use the __cfduid cookie before, and why can we remove it now?
The primary use of the cookie is for detecting bots on the web. Malicious bots may disrupt a service that has been explicitly requested by an end user (through DDoS attacks) or compromise the security of a user's account (e.g. through brute force password cracking or credential stuffing, among others). We use many signals to build machine learning models that can detect automated bot traffic. The presence and age of the cfduid cookie was just one signal in our models. So for our customers who benefit from our bot management products, the cfduid cookie is a tool that allows them to provide a service explicitly requested by the end user.
The value of the cfduid cookie is derived from a one-way MD5 hash of the cookie's IP address, date/time, user agent, hostname, and referring website -- which means we can’t tie a cookie to a specific person. Still, as a privacy-first company, we thought: Can we find a better way to detect bots that doesn’t rely on collecting end user IP addresses?
For the past few weeks, we’ve been experimenting to see if it’s possible to run our bot detection algorithms without using this cookie. We’ve learned that it will be possible for us to transition away from using this cookie to detect bots. We’re giving notice of deprecation now to give our customers time to transition, while our bot management team works to ensure there’s no decline in quality of our bot detection algorithms after removing this cookie. (Note that some Bot Management customers will still require the use of a different cookie after April 1.)
While this is a small change, we’re excited about any opportunity to make the web simpler, faster, and more private.