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.
Regin (malware)
Symantec says that both it and Kaspersky identified the malware as "Backdoor. Regin". Most antivirus programs, including Kaspersky, (as of October 2015) do NOT identify the sample of Regin released by The Intercept as malware. On 9 March 2011 Microsoft added related entries to its Malware Encyclopedia; later two more variants, "Regin. B" and "Regin. C" were added. Microsoft appears to call the 64-bit variants of Regin "Prax. A" and "Prax. B". The Microsoft entries do not have any technical information. Both Kaspersky and Symantec have published white papers with information they learned about the malware.