Malware
Malware authors target bugs, or loopholes, to exploit. A common method is exploitation of a buffer overrun vulnerability, where software designed to store data in a specified region of memory does not prevent more data than the buffer can accommodate being supplied. Malware may provide data that overflows the buffer, with malicious executable code or data after the end; when this payload is accessed it does what the attacker, not the legitimate software, determines.
Sality
Sality’s downloader mechanism downloads and executes additional malware as listed in the URLs received using the peer-to-peer component. The distributed malware may share the same “code signature” as the Sality payload, which may provide attribution to one group and/or that they share a large portion of the code. The additional malware typically communicates with and reports to central command and control (C&C) servers located throughout the world. According to Symantec, the "combination of file infection mechanism and the fully decentralized peer-to-peer network [...] make Sality one of the most effective and resilient malware in today's threat landscape."