Machine keys are cryptographic secrets used by IIS and ASP.NET to sign and encrypt web application data, including authentication cookies, viewstate, and other tokens. They are typically shared across a server or farm so the application can verify that its own state has not been tampered with.
They matter in cyber security because anyone who steals these keys may be able to forge trusted cookies, decrypt protected data, or create valid-looking tokens that survive logouts and password changes. In practice, attackers look for machine keys after compromising a web server, application backup, or configuration file. With the keys, they can maintain access, impersonate users, or bypass some controls without needing to keep exploiting the original flaw. Defenders protect them by restricting file and backup access, rotating keys after suspected compromise, and reviewing IIS and ASP.NET configuration for exposure.