Trojan.Agent refers to a generic Trojan malware detection. It is used for detections that are either associated with no specific malware families or not enough information is available to pinpoint the malware family.
Downloaders and droppers are helper programs for various types of malware such as Trojans and rootkits. Usually they are implemented as scripts (VB, batch) or small applications.
They don’t carry any malicious activities by themselves, but just open a way for attack by downloading/decompressing and installing the core malicious modules. To avoid detection, a dropper may also create noise around the malicious module by downloading/decompressing some harmless files.
Very often, they auto-delete themselves after the goal has been achieved.
Downloaders and droppers emerged from the idea of malware files that were able to download additional modules (i.e. Agobot, released in 2002).
An interesting example of a modern downloader is OnionDuke (discovered in 2014), carried by infected Tor nodes. It is a wrapper over legitimate software. When a user downloads software via an infected Tor proxy, OnionDuke packs the original file and adds a malicious stub to it. When the downloaded file is run, the stub first downloads malware and installs it on a computer, and then unpacks the legitimate file and removes itself in order to be unnoticed.
Most of the time, the user gets infected by using some unauthenticated online resources. Infections are often consequences of activities like:
They may also be installed without user interaction, carried by various exploit kits.
Downloaders often appear in non-persistent form. They install the malicious module and remove themselves automatically. In such a case, after a single deployment they are no longer a threat. If for some reason they haven’t removed themselves, they can be deleted manually.
More dangerous variants are persistent. They copy themselves to some random, hidden file and create registry keys to run after the system is restarted, attempting to download the malicious modules again. In such cases, to get rid of the downloader it is necessary to find and remove the created keys and the hidden file.
What remains to do is to take appropriate steps in order to neutralize the real weapon carried by the dropper. The difficulty level of cleaning the system varies as the payload may be of different types. The most universal way is to use good quality, automated anti-malware tools and run a full system scan.
Keeping good security habits, such as being careful about visiting certain websites and not opening unknown attachments minimizes the risk of being affected by malicious downloaders. However, in some cases it is not enough. Exploit kits can still install the malicious software on the vulnerable machine, even without any interaction. That’s why it is important to have good quality anti-malware software.