Indications of Infection

The virus copies itself to the Windows directory as avserve.exe and creates a registry run key to load itself at startup

* HKEY_LOCAL_MACHINESOFTWAREMicrosoftWindows
CurrentVersionRun "avserve.exe" = C:WINDOWSavserve.exe

As the worm scans random ip addresses it listens on successive TCP ports starting at 1068. It also acts as an FTP server on TCP port 5554, and creates a remote shell on TCP port 9996.

A file named win.log is created on the root of the C: drive. This file contains the IP address of the localhost.

Copies of the worm are created in the Windows System directory as mybb_up.exe.

Examples

* c:WINDOWSsystem3211583_up.exe
* c:WINDOWSsystem3216913_up.exe
* c:WINDOWSsystem3229739_up.exe

Method of Infection

This worm spreads by exploiting a recent Microsoft vulnerability, spreading from machine to machine with no user intervention required.

This worm scans random IP addresses for exploitable systems. When one is found, the worm exploits the vulnerable system, by overflowing a buffer in LSASS.EXE. It creates a remote shell on TCP port 9996. Next it creates an FTP script named cmd.ftp on the remote host and executes it. This FTP script instructs the target victim to download and execute the worm (with the filename mybb_up.exe as aforementioned) from the infected host. The infected host accepts this FTP traffic on TCP port 5554.

The worm spawns multiple threads, some of which scan the local class A subnet, others the class B subnet, and others completely random subnets. The destination port is TCP 445

Release History:

* Version 1.0 - Released 5/2/04. Detected and removed Sasser.A and Sasser.B.

* Version 2.0 - Released 5/4/04. Added detection/removal for Sasser.C and Sasser.D.