Win32/Sasser threat description - Microsoft Security Intelligence

Threat behavior

When Win32/Sasser runs on a computer, it copies itself to the %WINDOWS% folder. In most cases, it adds a value to the registry key HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\Run. This value causes the worm to start when Windows is started.

Win32/Sasser acts as an FTP server listening on TCP port 5554. For each connection made on this port, the worm sends a copy of itself to that connected host using the file name <random number>_up.exe.

The worm generates random IP addresses using a certain logic and then sends the exploit shell code to these IP addresses on TCP port 445. If the exploit is successful, a command line shell listens on a TCP port of the remote infected machine. To complete the infection, the worm executes a remote shell script that instructs the newly infected machine to connect to the infecting host and download and execute the worm through FTP. The worm records the count of successful infections to a file on the C: drive.

Win32/Sasser also attempts to abort any unexpected system shutdown by calling AbortSystemShutdown every several seconds in a continuous loop.

Later variants of the worm may drop a variant of Netsky worm. Later variants may not infect Windows 2000 because they import IcmpSendEcho from IPHlpAPI.dll, which is not present in Windows 2000.

Prevention

Take the following steps to help prevent infection on your computer:

Enable a firewall on your computer.
Get the latest computer updates for all your installed software.
Use up-to-date antivirus software.
Limit user privileges on the computer.
Use caution when opening attachments and accepting file transfers.
Use caution when clicking on links to Web pages.
Avoid downloading pirated software.
Protect yourself against social engineering attacks.
Use strong passwords.

Enable a firewall on your computer

Use a third-party firewall product or turn on the Microsoft Windows Internet Connection Firewall.

Get the latest computer updates

Updates help protect your computer from viruses, worms, and other threats as they are discovered. It is important to install updates for all the software that is installed in your computer. These are usually available from vendor Web sites.

You can use the Automatic Updates feature in Windows to automatically download future Microsoft security updates while your computer is on and connected to the Internet.

Use up-to-date antivirus software

Most antivirus software can detect and prevent infection by known malicious software. To help protect you from infection, you should always run antivirus software, such as Microsoft Security Essentials, that is updated with the latest signature files. For more information, see http://www.microsoft.com/security/antivirus/av.aspx.

Limit user privileges on the computer

Starting with Windows Vista and Windows 7, Microsoft introduced User Account Control (UAC), which, when enabled, allowed users to run with least user privileges. This scenario limits the possibility of attacks by malware and other threats that require administrative privileges to run.

You can configure UAC in your computer to meet your preferences:

Use caution when opening attachments and accepting file transfers

Exercise caution with e-mail and attachments received from unknown sources, or received unexpectedly from known sources. Use extreme caution when accepting file transfers from known or unknown sources.

Use caution when clicking on links to Web pages

Exercise caution with links to Web pages that you receive from unknown sources, especially if the links are to a Web page that you are not familiar with, unsure of the destination of, or suspicious of. Malicious software may be installed in your computer simply by visiting a Web page with harmful content.

Avoid downloading pirated software

Threats may also be bundled with software and files that are available for download on various torrent sites. Downloading "cracked" or "pirated" software from these sites carries not only the risk of being infected with malware, but is also illegal. For more information, see 'The risks of obtaining and using pirated software'.

Protect yourself from social engineering attacks

While attackers may attempt to exploit vulnerabilities in hardware or software to compromise a computer, they also attempt to exploit vulnerabilities in human behavior to do the same. When an attacker attempts to take advantage of human behavior to persuade the affected user to perform an action of the attacker's choice, it is known as 'social engineering'. Essentially, social engineering is an attack against the human interface of the targeted computer. For more information, see 'What is social engineering?'.

Use strong passwords

Attackers may try to gain access to your Windows account by guessing your password. It is therefore important that you use a strong password – one that cannot be easily guessed by an attacker. A strong password is one that has at least 8 characters, and combines letters, numbers, and symbols. For more information, see http://www.microsoft.com/protect/yourself/password/create.mspx.

Win32/Sasser

Summary

What to do now

Recovering from recurring infections on a network

Removing this threat

Technical information