When libcurl sends a request to a server via a HTTP proxy, it
copies the entire URL into the request and sends if off.
If the given URL contains line feeds and carriage returns those will
be sent along to the proxy too, which allows the program to for
example send a separate HTTP request injected embedded in the URL.
Many programs allow some kind of external sources to set the URL or
provide partial pieces for the URL to ask for, and if the URL as
received from the user is not stripped good enough this flaw allows
malicious users to do additional requests in a way that was not
intended, or just to insert request headers into the request that
the program didn't intend.
We are not aware of any exploit of this flaw.