Threat description



Silver is a dangerous worm spreading through the Internet and IRC channels, as well as infecting files on a local network. The worm itself is a Windows application written in Delphi. It is about 90Kb long (the worm also may be compressed by a PE EXE compression tool, so the actual file size can be less than original).


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Technical Details

The worm tries two different methods to send infected e-mails from infected computers. First of all, it looks for Eudora e-mailer installed in a system. If it is present, the worm scans Eudora outgoing email database (OUT.MBX file), gets e-mail addresses from there and sends infected e-mails with the attached worm's copy to these addresses. The worm's messages look like this:

Subject: concerning last week ...  Text:    Please review the enclosed and get back with me ASAP.           Double click the Icon to open it.  Attach:  c:\silver.exe  

Then the worm tries to access an installed e-mail system not depending on the brand. The worm uses MAPI functions to do this: it connects to the installed e-mail system, gets messages from there, reads e-mail addresses and uses them to send its copies. In this case the messages look like this:

Subject: Re: now this is a nice pic :-)  Text:    Thought you might be interested in seeing her  Attach:  naked.jpg.exe  

To affect IRC clients the worm looks for C:\MIRC, C:\MIRC32, C:\PIRCH98 directories and overwrites IRC scripts in there with a program that sends a copy of the worm to each user who enters an infected IRC channel.

The mIRC script has also additional features. When a user sends a message to IRC channel that contains a word 'silverrat', the worm replies to that user with 'I have the Silver Rat virus' message (so the worm reports about infected computers). If the 'pyrealrat' text is found in a channel, the script opens the C: drive on affected machine as file server (that gives access to all data on the C: drive to a hacker).

To infect remote computers on the network, the worm scans all drives from C: to Z: and looks for WINDOWS directory. If there is one, the worm copies itself there and registers itself in Windows. The worm adds its execution string to auto-run section in WIN.INI file, or to Registry depending on Windows version (Win9x or WinNT). This means that the worm is able to infect remote computers in case their drives are shared for reading/writing.

To install itself into the system, the worm copies itself to directories with these names:

to Windows dir:        SILVER.EXE, SILVER.VXD, NAKED.JPG.EXE, NAKED.JPG.SCR  to C: drive root dir:  SILVER.EXE  

The worm then registers itself in auto-run fields in the system registry:

HKCU\Software\Microsoft\Windows\CurrentVersion\Run  HKLM\Software\Microsoft\Windows\CurrentVersion\Run  HKLM\Software\Microsoft\Windows\CurrentVersion\RunServices  HKU\Software\Microsoft\Windows\CurrentVersion\Run  

All these fields contain the instruction:

 "Silver Rat" = WinDir\silver.exe  

where "WinDir" is the name of the Windows directory.

As a result the worm copy is executed four times on each Windows startup. Furthermore, the worm affects more registry keys to run itself more times (and to send more infected emails as a result).

Windows applications are linked with filename extensions by special records in the system registry. These records point to application that is run to process files with specified extension. When a file is opened, Windows gets its extension and then refers to system registry to get the name of application that processes files of that type.

The worm uses that Windows feature and modifies more than 100 such registry keys - it replaces original reference to applications with a reference to its own copy (SILVER.VXD). The worm does that for three different keys per application:

\shell\open\command  \shell\edit\command  \Shell\play\command  

The patched registry keys looks like follows:

HKCR\AIFFFILE\shell\open\command = "C:\WINDOWS\silver.vxd 33157 "%1" %"  HKCR\AIFFFILE\shell\play\command = "C:\WINDOWS\silver.vxd 53157 "%1" %"  HKCR\ASFFILE\shell\open\command = "C:\WINDOWS\silver.vxd 379157 "%1" %"  

where digits in the line are IDs to run the host file (see below).

The list of affected applications (registry keys that link filename extension with application) is rather large:

accesshtmlfile      iqyfile regedit         fonfile  accessthmltemplate  IVFfile regfile         GatewayFile  AIFFFILEjpegfileSHCmdFile       htafile  AllaireTemplate     JSFile  SoundRec        icsfile  anifile ldap    tgafile         mhtmlfile  artfile mailto  txtfile         MMS  aspfile mic     VBSFile         MMST  AudioCD MIDFile wab_auto_file   MMSU  aufile  money   Winamp.File     NSM  AVIFile MOVFile WinRAR          MSBD  Briefcase           MPEGFILEWinRAR.ZIP      motiffile  cdafile MPlayer WinZip          Msi.Package  Chat    mscfile wrifile         Msi.Patch  CSSfile msee    WSFFile         ofc.Document  curfile msgfile x-internet-signup           ofx.Document  Drive   MSProgramGroup      xbmfile         pjpegfile  DrWatsonLog         Net2PhoneApp        xmlfile         PNM  Excel.Workspace     NetscapeMarkup      xnkfile         qwb.Document  ftp     news    xslfile         rtsp  giffile nntp    m3ufile         scpfile  helpfileNotes.Link          ASFFile         scriptletfile  hlpfile ossfile ASXFile         SSM  htfile  outlook BeHostFile      ThemeFile  htmlfilePBrush  ChannelFile     TIFImage.Document  http    pcxfile chm.file        ttffile  https   pngfile CMCDWangImage.Document  icofile powerpointhtmlfile  Connection Manager Profile  Whiteboard  icquser ramfile eybfile         WIFImage.Document  inifile RealMedia File      fndfile         WSHFile  

The worm stores original keys in the another registry key:

HKLM\Software\Silver Rat  

This key contains the list of all keys that were replaced as it was shown above. This list is used by the worm to run original application: the worm gets application name and command line from that "backup" list, and spawns it.

Such method of affecting system registry is very dangerous. In the case that the worm copy is removed from the system, Windows cannot pass files to applications that are listed above. As a result, Windows stays mostly nonfunctional after that. In a case that a file from affected list is opened, it reports an error message that the associated SILVER.VXD cannot be found.

The worm pays special attention to system backup files and gets rid of them to prevent restoring the registry files from backup. The worm corrupts (overwrites first 5K of each file with trash data) and deletes the files to do this:

USER.DA0 and SYSTEM.DA0 in Windows directory  SYSTEM.1ST in root directory of C: drive  

The worm has a payload routine that is run in a case of "uninstalling". The worm creates the "uninstall" key in system registry:

HKLM\Software\Microsoft\Windows\CurrentVersion\Uninstall\Silver Rat   DisplayName     = "Silver Rat Virus"   UninstallString = "c:\silver.exe /uninstall"  

As a result, the worm record is visible in ControlPanel/AddRemovePrograms window as "Silver Rat Virus". If the "Remove" button is pressed, the worm displays the message box:

Blood  "I have to return some videos" - American Psycho  

and fills the header line in the Recycle Bin window with garbage.

The worm looks for active anti-virus applications and terminates them by their names:

AVP Monitor  Norton AntiVirus Auto-Protect  Norton AntiVirus v5.0  VShieldWin_Class  NAI_VS_STAT  McAfee VirusScan Scheduler  ZoneAlarm  WRQ NAMApp Class  

It also looks for anti-virus files (databases) and deletes them:


The worm also tries to affect VBS files but fails because of a bug.

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