All the information is there: it's a
Vulnerability that allows an attacker to bypass
authentication.
For example, say such a vulnerability exists in an FTP server. By sending special prepared input an attacker could make the server think that the user is allowed to connect, even though no credentials are know. The attacker could then manipulate files on the server. If this happens to be a download server for software, he could even inject further vulnerabilities into the software provided there.
