W97M/Suppl is a worm the was posted to several newsgroups at
September 17th, 1999. It spreads itself as an e-mail worm, with
an attachment file called Suppl.doc.
When the attached Word 97 document is opened, the macro code
executes. It copies the active document to Windows directory as
"Anthrax.ini". Then decompresses a trojanized version of
"Wsock32.dll" that is appended to the end of the document,
"Suppl.doc".
Before the system is rebooted, there is three visible files in
the Windows directory: "DLL.lzh", "DLL.tmp" and "wininit.ini".
After the system has been rebooted, "DLL.tmp" replaces
"Wsock32.dll" and the original "Wsock32.dll" is renamed as
"Wsock33.dll". "DLL.lzh" (compressed ".dll") and "wininit.ini"
are deleted.
Then the worm will attach itself to every SMTP email message sent
from an infected user's machine as "Suppl.doc".
The worm DLL renamed to WSOCK32.DLL being started first tests if
it is already installed and activated. If yes it checks current
system time and if there passed more than 6 days 18 hours 59
minutes 18 seconds 964 milliseconds after its first installation
to system the payload routine is activated. The worm first gets
all available drives list with GetLogicalDriveStringsA function
and then filters out only fixed disks after performing
GetDriveType function. Then the worm scans all filtered drives
for the files having the following extensions:
DOC
XLS
TXT
RTF
DBF
ZIP
ARJ
RAR
and truncates these files like ExploreZip worm does. The worm
calls CreateFileA function on an existing file and then closes it
immediately. As a result all the affected files will be truncated
to zero bytes and will be unrecoverable.
[Analysis: Katrin Tocheva, Sami Rautiainen and Alexey Podrezov, F-Secure]
