In Germany we noticed a new massive wave of “Rechnung” malware spammed mails continue today with a special scam inside.
The messages received today have diverse subject lines (“Abbuchung”, “Lastschrift”, “Amtsgericht”). The email bodies are also differently written, however have the same meaning – the information that money have been debit directly from the user’s account.
The malware comes attached in “Rechnung.zip” archive as “zertifikat.ssl” (WORM_AUTORUN.PB). Additionally to this the archive includes another file “Rechnung.txt.lnk”. Note that the file has double extensions, unlike zertifikat.ssl. Due to the default Windows Explorer configuration the extensions of known files are kept hidden so that mostly this file Rechnung.txt.lnk is displayed as Rechnung.txt in the archive as after it is extracted on disk.
Only one statement consistently exists in all email samples we’ve found, and is highlighted below:
In the said statement, the initiators point out that the recipient doesn’t have to care the “zertifikat.ssl” file since this is only the certificate for the invoice itself. The criminals try to lure users assuring that the file Rechnung.txt.lnk is the only one that needs to be double checked. Indeed… this is true! A file with the .SSL extension, like the malware file zertifikat.ssl, wouldn’t execute on simple double click, but it would when a .LNK file connecting to it is opened, which in this case is “Rechnung.txt.lnk”.
Similar to Autorun.inf and .PIF files, LNK files execute automatically the path inside their code:
To make sure that the SSL file is properly executed, this particular “Rechnung.txt.lnk” file calls the system’s commandline c:WindowsSystem32cmd.exe to execute the zertifikat.ssl from the current directory. The execution through LNK files is not a special trick. It is one of the usual functionalities and features of Windows operating system, without them our life would be like Internet without Web 2.0.
When an experienced user try to open the LNK file even with an editor, he will be confused by seeing the contents of the file zertifikat.ssl. Actually to view the original file, the user needs to rename it first by using the command line (cmd.exe).
Users are advised to stay vigilant. The optical illusions in Windows operating system are considered to be features – not bad at all if they wouldn’t be exploited by criminals.