This type of trojan secretly downloads malicious files from a remote server, then installs and executes the files.
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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 email messages. If the user executes the malware by clicking on the attachment, the trojan drops the following files on the system:
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 emails. A sample of the email message can be seen below:
It may use any of the following strings as its Subject:
Attachments may be any of the following filenames: