Severe threats typically are remotely exploitable vulnerabilities, which can lead to system compromise. Successful exploitation does not normally require any interaction and exploits are in the wild. There exists a high possibility of potential system damage or security flaw. Attacker has complete control over your computer or install new software on your machine.
Backdoor - A backdoor is a means of access to a computer program that bypasses any computer security mechanisms. Computer attackers often use backdoors as part of an exploit or Trojan to gain access to a user's computer. In some cases, a worm is designed to take advantage of a backdoor created by an earlier attack.