The __cfduid cookie is used by Cloudflare to identify individual clients behind a shared IP address and apply security settings on a per-client basis. For example, if the visitor is in a coffee shop where there are infected machines, but the specific visitor's machine is trusted (e.g. because they've completed a challenge within the Cloudflare Challenge Passage period), the cookie allows Cloudflare to identify that client and not challenge them again. It does not correspond to any user ID in the website, and does not store any personally identifiable information. It has a default expiration of 1 year.