CloudFlare is a service that sits between the Internet and its customers’ web servers, protecting them and speeding them up.
(If you, like me, always are looking for stuff that can be abused or misused, you have probably already guessed what comes next.)
I soon understood that this was CloudFlare functionality and not local to the website. I then immediately sent this mail to CloudFlare:
Some days ago I submitted a vulnerability report to a site that I think is one of your customers. I don’t want to disclose their name at the moment as I am not sure they are OK with that. They might have been in contact with you already, since the problem seems to be with functionality you provide.
When these URLs are accessed, your server nicely gets the page in the r parameter and returns it in the response. Multiple instances of r in the same URL are also possible.
This could be exploited by someone who wants to access another URL, but anonymously (except for your logs of course), or to access pages “in your name”, making you look bad.
It could also be exploited in another, more serious way. If you happen to have any internal web servers that are not visible to the internet (for example a bug tracker), they might be visible to the computer hosting cloudflare.com. Then this vuln could be used to fetch files from the internal servers (given that the attacker knows the URLs or brute forces them).
A search on Google for this type of URL shows that many sites has the same problem. I don’t know if you can fix it in one place or if you have to roll it out to all these sites.
If you have any questions, please let me know.
In short, the vulnerability would allow an attacker to download content via a server acting as a gateway to the inside of CloudFlare, or to anywhere else.
After this, the whole experience was very smooth. John Roberts, Platform Lead at CloudFlare, who was my contact during the following discussion, was very nice about it all. CloudFlare does not have a reward program, but I was offered a T-shirt as a token of appreciation.