Worm:W32/Yaha.E spreads as an attachment with different names in e-mail messages with different subjects and bodies, the message composition routine is really complex.
The worm's file is a Windows PE (portable executable) file about 27 kb long. The file is packed with UPX file compressor and UPX text strings in the body of the worm are replaced with dots probably by the worm's author.
Worm's files usually have random data (garbage) in the end, the length and content of that area is different in different samples. The worm has most of its text strings encrypted with a simple crypto algorithm.
Yaha.E worm became widespread on 20th of June 2002. The worm was reported from several different countries.
The worm usually installs itself with random name to C:\Recycler or C:\Recycled folder or, if these folders are not available, into Windows directory.
The worm then modifies default EXE file startup key to make itself run every time an EXE program is started.
Also one of the threads of the worm continuously refreshes this key to prevent its modification by computer user or by disinfection utility. The same thread tries to kill Windows Task Manager process.
The worm creates a file named KITKAT in a temporary folder and writes its BASE64-encoded file data there. That file will be different from the original worm sample as the worm adds random data to its end (from MUNCH file that it generates and then deletes). The length of random data area differs every time. This technique was supposed to fool some anti-virus scanners that use CRC to detect standalone malware.
The worm doesn't start when launched from Internet Explorer cache folder (folder containing CONTENT.IE5 string). If the worm is started from MSTASKMON.EXE file, it adds execution string for this file into WIN.INI to be started every time Windows runs. The worm starts itself as a service process, so its task is not visible in Task Manager under Windows 9x systems.
When the worm's file is started on a clean system and its file has SCR extension, it may display a messagebox or/and a videoeffect. The effect will look like that (the screen will be shaking):
One of worm's threads from time to time tries to connect to a 'www.pak.gov.pk' website (possibly related to the Pakistanian government). If the worm is widespread, this can cause a DoS (Denial of Service) attack on that webserver.
The worm looks for and terminates the processes that have the following strings in their names:
So the worm not only kills tasks of anti-virus and security software, it also kills tasks of Klez and Sircam worms. The worm has different process killing routines for different types of operating systems. The worm doesn't allow programs with the above mentioned strings in their names to start on an infected system. The worm also looks for and terminates the Windows Task Manager process.
The worm creates the TXT file with random name in Windows directory with the following text:
The worm loads Windows Address Book into memory, reads .NET Messenger data, Yahoo! Messenger profiles, ICQ data files, HTML, DOC and TXT files in cache folders, desktop and personal folders and stores e-mail addresses it could find there into a DLL file with a random name in Windows folder.
The worm uses a set of pre-defined smtp server names, message bodies, subjects, attachment names and extensions and selects them randomly to compose an e-mail message that it will send to all found e-mail addresses. It can send different types of e-mails, similar to Klez worms e-mails. The worm may or may not include IFrame exploit into the message that it sends out.
The subject of infected e-mail can be one of the following messages or a combination of them:
Also the worm can compose the subject from 2 or more different parts with or without 'Fw:' prefix:
For example the subject of an infected message can be 'Cool stuff to see' or 'FW: Nice Screensaver for you !!'. There can be 'Fw:' prefix to the subject.
The infected message body can contain one of the following strings or blocks of strings:
Then there can follow a fake undeliverable message report or a fake screensaver subscription message. In case the worm sends a fake bounced message, it looks like that:
Then there goes an EML file attachment with random name that contains the worm's sample and usually IFrame exploit to make the attachment run automatically on unpatched e-mail clients.
In case the worm spreads itself with a fake screensaver subscription message, it looks like that:
In this case the worm spreads itself with one of the below given names and only SCR extension:
The worm also spreads itself as an attachment with double extension and with one of the following names or with a random name:
The first extension of the attachment can be:
The second (last) extension of the infected attachment can be:
For example the worm's attachment name can be GOLDFISH.MPG.PIF or BIODATA.DOC.SCR. The attachment content type that the worm uses can be one of the following:
Propagation (Local Area Network)
The worm has network spreading capabilities. One of the threads constantly looks for open shares and searches directories with the following names there:
When the worm finds a directory with any of the above mentioned names, it tries to find WIN.INI file in that directory. If it can find such a file, the worm copies itself as MSTASKMON.EXE file into the same directory and modifies WIN.INI file on remote system to start itself there after next reboot.
It should be noted that this functionality will only work under Windows 9x systems as WIN.INI file is not used by NT-based systems. But the worm can be activated on remote NT-based system if a user runs MSTASKMON.EXE file there.