Last Friday, Tavis Ormandy from Google’s Project Zero contacted Cloudflare to report a security problem with our edge servers. He was seeing corrupted web pages being returned by some HTTP requests run through Cloudflare. It turned out that in some unusual circumstances, which I’ll detail below, our edge servers were running past the end of a buffer and returning memory that contained private information such as HTTP cookies, authentication tokens, HTTP POST bodies, and other sensitive data. And some of that data had been cached by search engines.
For the avoidance of doubt, Cloudflare customer SSL private keys were not leaked. Cloudflare has always terminated SSL connections through an isolated instance of NGINX that was not affected by this bug.
We quickly identified the problem and turned off three minor Cloudflare features (email obfuscation, Server-side Excludes and Automatic HTTPS Rewrites) that were all using the same HTML parser chain that was causing the leakage. At that point it was no longer possible for memory to be returned in an HTTP response.
Because of the seriousness of such a bug, a cross-functional team from software engineering, infosec and operations