Linux.DoS.tfn2k.tfn

SUMMARY

TFN and TFN2K are not viruses, but attack tools that can be used to perform a distributed DoS attack. TFN and TFN2K can perform various attacks such as UDP flood attacks (similar to Trinoo), ICMP flood attack (similar to Smurf), and TCP SYN flood attacks.

Threat Assessment

Wild

• Wild Level: Low
• Number of Infections: 0 - 49
• Number of Sites: 0 - 2
• Geographical Distribution: Low
• Threat Containment: Easy
• Removal: Easy

Damage

• Damage Level: Low

Distribution

• Distribution Level: Low

TECHNICAL DETAILS

When a TCP connection is made from system to system, it exchanges the following sequence of messages before making the connection:

1. The client sends SYN message to a server.
2. The server sends a SYN-ACK message back to client.
3. The client sends an ACK message to confirm and the connection is opened.

Once the connection is made, service-specific data can be exchanged between client and server. During these initial communications, each SYN/ACK request is tracked in a table since multiple machines may want to connect to the server. If too many half connections are made, it may cause an overflow in the table and the server will not be able to accept new connections.

TCP SYN flood attacks are performed by sending a SYN with a spoofed IP, for which the server will wait for a SYN-ACK that will never arrive. These half connections will eventually time out. However, if you can pump enough half connections before the server is waiting for the time-out, it will cause the server to not accept new connections, resulting in a denial of service.

It may be possible to detect these attacks by looking for SYN_RECEIVED state using tools such as NetStat. You may also want to install a filter on the router to check if external packets actually have external IPs and also check to see if internal IPs have internal IP addresses.

TFN2K is a newer version of TFN that supports Windows NT and UNIX platforms. It also has some new features that make detection more difficult, including sending decoy information to avoid being traced.

Recommendations

Symantec Security Response encourages all users and administrators to adhere to the following basic security "best practices":

• Turn off and remove unneeded services. By default, many operating systems install auxiliary services that are not critical, such as an FTP server, telnet, and a Web server. These services are avenues of attack. If they are removed, blended threats have less avenues of attack and you have fewer services to maintain through patch updates.
• If a blended threat exploits one or more network services, disable, or block access to, those services until a patch is applied.
• Always keep your patch levels up-to-date, especially on computers that host public services and are accessible through the firewall, such as HTTP, FTP, mail, and DNS services (for example, all Windows-based computers should have the current Service Pack installed.). Additionally, please apply any security updates that are mentioned in this writeup, in trusted Security Bulletins, or on vendor Web sites.
• Enforce a password policy. Complex passwords make it difficult to crack password files on compromised computers. This helps to prevent or limit damage when a computer is compromised.
• Configure your email server to block or remove email that contains file attachments that are commonly used to spread viruses, such as .vbs, .bat, .exe, .pif and .scr files.
• Isolate infected computers quickly to prevent further compromising your organization. Perform a forensic analysis and restore the computers using trusted media.
• Train employees not to open attachments unless they are expecting them. Also, do not execute software that is downloaded from the Internet unless it has been scanned for viruses. Simply visiting a compromised Web site can cause infection if certain browser vulnerabilities are not patched.