This type of trojan secretly downloads malicious files from a remote server, then installs and executes the files.
Once detected, the F-Secure security product will automatically disinfect the suspect file by either deleting it or renaming it.
Trojan-Downloader:W32/Small.DAM is a variant from the Small malware family that loads a malicious service on the system.
The malware arrives on the system as an infected file attachment to spam e-mail messages. If the user executes the malware by clicking on the attachment, the trojan drops the following files on the system:
- %SysDir%\wincom32.sys - Kernel mode driver component
- %SysDir%\peers.ini - Initialization file component
It also installs itself as a service with the name "wincom32" by creating the following registry keys:
The kernel mode driver is an advanced payload injector that carries a user-mode PE executable file. When the driver executes, it changes the services.exe process context and allocates new memory for the payload.
It then copies the PE executable from kernel memory to the address space of services.exe and prepares the image for execution. Finally, it queues an Asynchronous Procedure Call (APC) for services.exe to execute the payload in its context.
On execution, the service causes the system to send UDP packets from source port 4000 to IP addresses and destination ports taken from the "peers" section of the decoded initialization file. The initialization file also maintains a "blacklist" section.
Additional information about Small.DAM's activities can be found on our weblog.
Small.DAM arrives on the system as an attachment file to spam e-mails. A sample of the e-mail message can be seen below:
It may use any of the following strings as its Subject:
- 230 dead as storm batters Europe.
- A killer at 11, he's free at 21 and...
- British Muslims Genocide
- Naked teens attack home director.
- U.S. Secretary of State Condoleezza Rice has kicked German Chancellor Angela Merkel
Attachments may be any of the following filenames:
- Full Clip.exe
- Full Story.exe
- Read More.exe