Backdoor:W32/OnionDuke (both A and B variants) are DLL files dropped by Trojan-Dropper:W32/OnionDuke and used to download and execute additional malicious components on the affected system.
Depending on the settings of your F-Secure security product, it will either automatically delete, quarantine or rename the suspect file, or ask you for a desired action.
More information on the scanning and removal options available in your F-Secure product can be found in the Help Center.
You may also refer to the Knowledge Base on the F-Secure Community site for more information.
The trojan-dropper that drops the backdoors is itself delivered in a "wrapper" that is silently added to files being downloaded through a certain Tor node exit. If the modified file is downloaded and executed on an unsuspecting user's system, the wrapper silently writes to disk and then executes the original file, misleading the user into believing the download was untampered. The wrapper also silently executes a second binary file, which is Trojan-Dropper:W32/OnionDuke.A.
The dropper in turn decrypts an encrypted DLL camouflaged as an embedded GIF image file (Backdoor:W32/OnionDuke.B), then executes it. This will decrypt an embedded configuration, then attempt to connect to hardcoded command and control URLs specified within.
Based on the instructions returned, the backdoor may then download and execute additional components. Of particular note is Backdoor:W32/OnionDuke.A, which includes a different hardcoded domain for communication, and appears to also be able to use Twitter as a communication channel.
For more details, see: