The KDC RC4-HMAC Downgrade rule detects attempts to exploit a vulnerability identified in CVE-2022-37966, which involves a security bypass and privilege escalation related to the Kerberos authentication protocol. Specifically, this rule focuses on the improper negotiation of weaker RC4-HMAC encryption methods during authentication requests. An attacker may exploit this vulnerability to force a downgrade of the encryption method, allowing for potential unauthorized access and elevation of privileges within a Windows environment. The detection is based on monitoring event ID 42 from the Kerberos Key Distribution Center (KDC), which signals a negotiation attempt involving the problematic encryption algorithm. By flagging such events, administrators can be alerted to suspicious authentication activities that could lead to security breaches.