Your server is vulnerable if you are:
- Running Microsoft IIS 5/6 FTP service on Windows 2000.
- Allowing anonymous access with WRITE access.
This vulnerability relies on the fact that character strings within X.509 certificates are ASN.1 encoded, but software written in the C programming language typically manipulates character strings as null terminated character arrays. ASN.1 strings are stored using a form of Type-Length-Value (TLV) encoding. C strings are simply terminated by a null byte (\x00).