Win32/Valla threat description - Microsoft Security Intelligence

Threat behavior

Win32/Valla.2048 is a virus that appends itself to executable files on an infected computer.

When run, Win32/Valla.2048 recursively scans the following folders:

%windir% (for example, "C:\WINDOWS")
<system folder> (for example, "C:\WINDOWS\SYSTEM32")
Root directory of the system disk (for example, "C:\")
Directory Win32/Valla.2048 was run from (for example, "C:\Documents and Settings\<username>\Desktop")
Parent directory of directory Win32/Valla.2048 was run from (for example, "C:\Documents and Settings\<username>")

For each directory scanned, Win32/Valla.2048 randomly chooses ~20% of its subdirectories to scan (and recursively scans those subdirectories), and randomly chooses ~20% of its ".exe" files to infect. Win32/Valla.2048 won't scan directories whose names begin with a period (".").

Before infecting an ".exe" file, Win32/Valla.2048 checks to see whether or not the ".exe" file contains a Portable Executable section named "XOR", which Win32/Valla.2048 uses to mark files that it has infected. If such a section name is found then the file will not be re-infected. If the "XOR" section name is not found in the ".exe" file, then Win32/Valla.2048 appends 2 KB (2048 bytes) to the end of the file which includes its viral code and data. Win32/Valla.2048 updates the Portable Executable header of the ".exe" file so that when the ".exe" file is run, the viral code executes before the code of the original ".exe" file executes. It also creates a new Portable Executable section named "XOR" in the file to prevent re-infection.

The last 253 bytes of an ".exe" file infected with Win32/Valla.2048 contains a message from the virus author.

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 websites.

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 email 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 webpage 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 webpage 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 eight characters, and combines letters, numbers, and symbols. For more information, see http://www.microsoft.com/protect/yourself/password/create.mspx.

Win32/Valla

Summary

What to do now

Technical information