A dropper is a malware installer that surreptitiously carries viruses, back doors and other malicious software so they can be executed on the compromised machine. Droppers don’t cause harm directly but can deliver a malware payload onto a target machine without detection.
Droppers are often disguised and hidden in a computer’s directories (folders), so that although they are visible, they look like valid programs or file types. The dropper itself may contain one or a number of types of malware, perhaps with other features to prevent detection by antivirus software and facilitate stealthy installation.
Droppers aren't associated with any file extensions, which makes them harder to detect. The software is often in the form of a Trojan horse that gains entry to a system via an email attachment or along with a download. As such, droppers are often part of a spear phishing attempt. Droppers may require user execution but they can also be executed through exploitation of a security vulnerability.
Although droppers are traditionally standalone programs, their capabilities are increasingly included as part of a malware package. In late 2014, for example, the FBI reported that malware used in attacks on Sony associated with their movie The Interview came wrapped in an executable dropper that installed itself as a Windows service.