Late last week, the developers of a popular open source FTP server, ProFTPD, warned that they’ve accidentally been distributing a booby-trapped version of their FTP server’s source code. If you downloaded and installed ProFTPD from a legitimate distribution server between November 28 and December 1, you likely have a backdoor on your system.
In an ironic twist, the attackers actually leveraged a zero day vulnerability in ProFTPD in order to place this maliciously modified source code onto the official ProFTPD distribution servers. The modified ProFTPD 1.3.3c source code contains a backdoor, which gives any remote attacker who knows the “secret password,” complete root access to your FTP server. If you install this modified source, it’s extremely easy for attackers to access the backdoor — in fact, someone has already released a metasploit module to help you do it.
Hopefully, you haven’t installed ProFTPD 1.3.3c in the last week, but if you have, I highly recommend you remove it and download the latest version. More importantly, make sure to use the PGP and MD5 signatures that the ProFTPD team provides to ensure that you have a legitimate copy of unmodified source code. Furthermore, you may want to restrict access to your ProFTPD server using firewall or ACL rules. Unfortunately, the ProFTPD team hasn’t mentioned much about the zero day exploit the attackers leveraged to install this backdoor, so we can only assume that ProFTPD 1.3.3c is still vulnerable to this mysterious flaw. Some experts have suggested that the attackers may have used the unpatched ProFTPD flaw highlighted in last month’s Phrack magazine in this attack. Whether it was the Phrack zero day or some other one, the latest version of ProFTPD is vulnerable, so use it with caution. —Corey Nachreiner, CISSP