This is the most common interpretation. The is a core Windows process responsible for enforcing security policies, verifying logins, and handling password changes.
Exploiting a vulnerability in the LSASS interface to spread across networks. Lsass Exe Error
This is often caused by corrupted system files, hardware failing (like RAM), or malware trying to mess with your security settings. 2. The Malware/Virus (Sasser or imitation) This is the most common interpretation