In ASP.NET, the machine key is a cryptographic secret used to sign and encrypt application data such as authentication tokens, session material, and view state. The server uses it to verify that data has not been tampered with and, in some cases, to keep it confidential. If an attacker learns or can abuse the key, they may forge trusted data or bypass integrity checks.
Machine keys matter in cyber security because they protect the trust boundary between a web application and its users. In attacks against SharePoint and other ASP.NET applications, defenders may rotate the machine key after patching or suspected compromise to invalidate old tokens and block replay or persistence. Rotation is most effective when paired with restarting the web application or IIS so all nodes in a farm use the new secret consistently.