A reconnaissance attack is not a true attack because nothing is exploited as a result. It is used for gathering information about target systems and services so that vulnerabilities can be discovered and exploited using other types of attacks.

Some types of security threats can come in the form of malicious software or malware. All of the options are example of malware

An attacker develops worm software and deposits it on a system. From that point on, the worm replicates itself and spreads to other systems through their vulnerabilities, then replicates and spreads again and again.

Viruses are malware that can propagate between systems more readily. To spread, virus software must inject itself into another application, then rely on users to transport the infected application software to other victims.