Trojan-Downloader:W32/Oficla identifies a family of malware that are distributed as attachments to fake email messages.
On installation, the trojan-downloader connects to a remote server and downloads additional files (which are mostly malicious) onto the system. The specific files downloaded will vary depending on the variant.
A representative example of Oficla variant is:
Oficla variants are distributed as executable or zipped files attached to misleading email messages. Some of the most common messages used to deliver this trojan involve fake offers for iTunes Gift certificates or for Amazon.com orders, UPS invoice other attachments are disguised as resumes.
The text in the email message entices the unsuspecting user to open the attached executable; Oficla's actual malicious code is packed (using a custome packer) in a DLL file secretly embedded in the executable file.
Once the attached executable file is launched by the user, it will extract the DLL file into the Windows temporary folder.
The DLL file is saved, usually with the 1.tmp filename:
During installation, the 1.tmp file will be executed and injected to a normal process. Subsequently, when running under the normal process it will create a file in the Windows system folder.
The created file uses a somewhat non-standrd extension file, for example:
Note: The filename and extension name may change from variant to variant.
In order for it to automatically start on restart, it modifies the registry:
- HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon Shell = Explorer.exe
- HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon Shell = Explorer.exe rundll32.exe [Oficla_dll file] ["parameter"]
Example of a modified registry entry:
- HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon Shell = Explorer.exe rundll32.exe fjof.sto vffwd