1. More commonly known as malware, malicious code includes: (a) Viruses: malicious code attached to other computer files, activated by users when they open infected files, (b) Worms: malware that replicates and spreads among networked computers without user interaction, (c) Trojans: harmful software that tricks users into installing it because it looks legitimate, (d) Spyware: malicious code that searches infected devices for personal information and forwards it to criminals.
Source: Guide to Cybersecurity as Risk Management: The Role of Elected Officials, CGI, https://www.cgi.com/sites/default/files/2019-03/guide_to_cybersecurity_as_risk_management_the_role_of_elected_officials_0.pdf
2. Program code intended to perform an unauthorized function or process that will have adverse impact on the confidentiality, integrity, or availability of an information system. Includes software, firmware, and scripts.
Source: Explore Terms: A Glossary of Common Cybersecurity Terminology, National Initiative for Cybersecurity Careers and Studies (NICCS), https://niccs.us-cert.gov/about-niccs/glossary
3. Software or firmware intended to perform an unauthorized process that will have adverse impact on the confidentiality, integrity, or availability of an IS [information system]. (CNSS Instruction No. 4009).
Source: Terms & Definitions of Interest for DoD Counterintelligence Professionals, Office of the National Counterintelligence, https://www.dni.gov/files/NCSC/documents/ci/CI_Glossary.pdf
4. Software or firmware intended to perform an unauthorized process that will have adverse impact on the confidentiality, integrity, or availability of an information system.
Source: U.S. Cyberspace Solarium Commission, March 2020, https://subscriber.politicopro.com/f/?id=00000170-c638-d8f7-a7f1-f63b33510000