It is thought that the attackers took advantage of an unpatched security flaw in the FTP daemon in order to gain access to the server.
"The fact that the server acted as the main FTP site for the ProFTPD project (ftp.proftpd.org) as well as the rsync distribution server (rsync.proftpd.org) for all ProFTPD mirror servers means that anyone who downloaded ProFTPD 1.3.3c from one of the official mirrors from 2010-11-28 to 2010-12-02 will most likely be affected by the problem," wrote TJ Saunders, the ProFTPD maintainer, in the warning sent to the subscribers of the project's mailing list on SourceForge.
The version with the backdoor makes it possible for the attackers to gain remote root access to any system that runs the malicious version.
Users who have downloaded the source files during those four days - and other users who would like to know they are completely safe just in case - are urged to download the source files again and run it.
To confirm their integrity, they are advised to verify the MD5 sums and PGP signatures of the downloaded files and compare them to that of the legitimate source tarballs.