Name :	Email-Worm:W32/Brontok.N
Category:	Malware
Type:	Email-Worm
Platform:	W32

This type of worm is embedded in an e-mail attachment, and spreads using the infected computer's e-mailing networks.

Allow F-Secure Anti-Virus to disinfect the relevant files.

For more general information on disinfection, please see Removal Instructions.

Email-Worm:W32/Brontok.N is a complex e-mail worm that also disables anti-virus software, creates multiple copies of itself on a local hard disk, and takes measures to make its removal difficult.

Brontok.N was found at the end of March 2006.

Installation

After the worm's file is started, it copies itself with different names to different folders on a local hard drive. The file names can be semi-randomly generated or they can be any of the following:

Some of the worm's files have hidden, system, and read-only attributes. The worm can create its files with COM, EXE, and PIF extensions. Brontok worm creates multiple launch points for the copied files. Those include startup Registry keys as well as scheduled jobs. For example:

Additionally, the worm creates a few text files on a local hard disk. The file named baca bro !!!.txt that is created in the root of the system drive has the following text:

The c.bron.tok.txt file contains the following text:

The worm keeps several copies of itself in memory.

Propagation

The worm sends messages with the following subjects:

The message body text can be one of the following:

The attachment name is:

To collect e-mail addresses to spread itself, the worm looks on drives from C: to Z for address in files with the following extensions:

The discovered addresses are checked against a rather large list of strings. If part of a discovered e-mail address matches an entry in that list, that e-mail address is discarded.

The worm creates folders named Spread.Mail.Bro and Spread.Sent.Bro in one of its hidden subfolders. The first folder contains the list of e-mail addresses that the worm harvested from the infected computer. Each e-mail address is represented by a file that has the same text as the c.bron.tok.txt file (see above).

When the worm sends an e-mail to an address, the corresponding file is moved to the second folder. This is done to avoid sending the worm to the same address multiple times.

Payload

In addition to its propagation efforts, the worm has a rather old-fashioned effect. It can show the following texts in the command prompt window:

The effect is shown after the worm terminates applications that have the following text strings in their window titles:

The worm disables Task Manager, Registry Editor, and terminates applications that have the following text strings in their window titles:

The worm modifies Windows HOSTS file to block access to the following domains: