PrivateBin is an online pastebin where the server has zero knowledge of pasted data. In v1.5, PrivateBin introduced the YOURLS server-side proxy. The idea was to allow using the YOURLs URL shortener without running the YOURLs instance without authentication and/or exposing the authentication token to the public, allowing anyone to shorten any URL. With the proxy mechanism, anyone can shorten any URL pointing to the configured PrivateBin instance. The vulnerability allowed other URLs to be shortened, as long as they contain the PrivateBin instance, defeating the limit imposed by the proxy. This vulnerability is fixed in 1.7.4.
The authentication algorithm is sound, but the implemented mechanism can be bypassed as the result of a separate weakness that is primary to the authentication error.