W97M/Suppl is a worm the was posted to several newsgroups at September 17th, 1999. It spreads itself as an email worm, with an attachment file called Suppl.doc.Suppl contains a destructive payload.
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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.