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.
Locky
Locky is ransomware malware released in 2016. It is delivered by email (that is allegedly an invoice requiring payment) with an attached Microsoft Word document that contains malicious macros. When the user opens the document, it appears to be full of garbage, and includes the phrase "Enable macro if data encoding is incorrect," a social engineering technique. If the user does enable macros, they save and run a binary file that downloads the "actual" encryption trojan, which will encrypt all files that match particular extensions. Filenames are converted to a unique 16 letter and number combination. Initially, only the .locky file extension was used for these encrypted files. Subsequently, other file extensions have been used, including .zepto, .odin, .aesir, .thor, and .zzzzz. After encryption, a message (displayed on the user's desktop) instructs them to download the Tor browser and visit a specific criminal-operated Web site for further information. The Web site contain instructions that demand a payment of between 0.5 and 1 bitcoin (as of November 2017, one bitcoin varies in value between $9,000 and $10,000 via a bitcoin exchange). Since the criminals possess the private key and the remote servers are controlled by them, the victims are motivated to pay to decrypt their files.