FTP Sharing and Vulnerabilities

Although FTP is widely used, there are a number of vulnerabilities that should be addressed to ensure security. FTP authentication is sent as cleartext, making it easy for someone with a packet sniffer to view usernames and passwords. Because hackers and malicious software could be used to obtain this information quite easily, when traffic doesn't need to cross firewalls or routers on a network, it is important to block ports 20 and 21.

Port 21 is the control port for FTP, while port 20 is the data port. FTP uses port 21 to begin a session, accessing the port over TCP to provide a username and password. Because FTP doesn't use encryption, this information is sent using cleartext, allowing anyone using a packet sniffer to capture the packet and view this information.To avoid such attacks, encryption should be used whenever possible to prevent protocol analyzers from being used to access this data.

It is important to be careful with user accounts and their permissions on FTP servers. If users will only be downloading files and don't require individual accounts, then a server could be configured to allow anonymous access. In doing so, anyone could login to the account without a password, or by using their e-mail address as a password. Not only does this make it easier to distribute files to users, but it also removes the need to worry about authentication information being transmitted using cleartext. If certain users also need to upload files, then individual user accounts are wise to implement, as this will provide limitations over who can put files on your server. In all cases however, it is advisable to limit permissions and privileges to the FTP server as much as possible, and never give anyone more access than absolutely necessary.

If FTP servers are going to be accessed by the public, it is important to isolate it from the rest of the network, so that if security is compromised the attacker won't be able to access servers and workstations on your internal network. By placing FTP servers on a perimeter network, the server is separated from the internal network, preventing such attacks from occurring.

When configuring FTP servers, it is also important to design the directory structure carefully and ensure that users don't have more access than necessary.The root directory of the FTP server is where FTP clients will connect to by default, so these should not contain any confidential data or system files. In addition to this, you should limit the ability to write to directories, preventing users from uploading files to a directory that may be malicious. Regardless of whether you provided write access on purpose, you should review the FTP directories on a regular basis to ensure that no unexpected files have been added to the server.

Another aspect of FTP that opens the system up to security problems is the third-party mechanism included in the FTP specification known as proxy FTP. It is used to allow an FTP client to have the server transfer the files to a third computer, which can expedite file transfers over slow connections. However, it also makes the system vulnerable to something called a "bounce attack."

Bounce attacks are outlined in RFC 2577, and involves attackers scanning other computers through an FTP server. Because the scan is run against other computers through the FTP server, it appears at face value that the FTP server is actually running the scans.This attack is initiated by a hacker who first uploads files to the FTP server.Then they send an FTP "PORT" command to the FTP server, using the IP address and port number of the victim machine, and instruct the server to send the files to the victim machine.This can be used, for example, to transfer an upload file containing SMTP commands so as to forge mail on the third-party machine without making a direct connection. It will be hard to track down the perpetrator because the file was transferred through an intermediary (the FTP server).

Packet Sniffing FTP Transmissions

As mentioned earlier in this section, FTP traffic is sent in cleartext so that credentials, when used for an FTP connection, can easily be captured via MITM attacks, eavesdropping, or sniffing. Sniffing is a type of passive attack that allows hackers to eavesdrop on the network, capture passwords, and use them for a possible password cracking attack.