What is NOT a valid method of obtaining Nessus updates?

Downloading from a third-party website is not a valid method of obtaining Nessus updates because it raises significant security concerns. Third-party sources may not provide reliable or authorized updates and could potentially deliver malicious software that compromises the integrity of both the Nessus software and the security of the system it is intended to protect. Official channels, such as DISA's plugin server or the DoD Patch Repository, ensure that updates are validated and received from trusted sources, which is crucial for maintaining the security posture of systems within the Department of Defense and other federal entities. This emphasis on using official and secure methods helps to prevent unauthorized tampering and ensures compliance with security standards.