alt Hacker NewsThis attack is called "HTTP desync" or "request smuggling". It's often done intentionally by a client to try and spy on other clients' responses.
Every time you multiplex requests from multiple clients onto one upstream connection, you are probably vulnerable to this, because (despite its superficial simplicity) HTTP is just too complex to reliably match the requests and responses to upstream.
For example a desync can be triggered in some systems by having more than one Content-Length header, by mixing Content-Length with chunked encoding, or by passing an HTTP/2 header called Content-Length that doesn't match the actual content length.
Here's a DEF CON talk (6 years ago) on this topic: https://www.youtube.com/watch?v=w-eJM2Pc0KI
The same attack has been applied to SMTP by messing up the line endings surrounding the end-of-message delimiter, where it's called SMTP smuggling. It may also apply to other protocols.
Very true, this was likely an attack. Worth noting that mr kettle has done a defcon talk nearly every year on some variant of this attack, the most recent one titled "HTTP/1.1 must die" because he rightfully believes that switching to the binary headers of http/2 (specifically in reverse proxy connections to upstream servers) is the only way to systematically prevent these.