Local Security Authority Subsystem Service (LSASS) is a process in Microsoft Windows operating systems that is responsible for enforcing the security policy on the system. It verifies users logging on to a Windows computer or server, handles password changes, and creates access tokens. It also writes to the Windows Security Log.
Forcible termination of lsass.exe will result in the system losing access to any account, including NT AUTHORITY, prompting a restart of the machine.
Because lsass.exe is a crucial system file, its name is often faked by malware. The lsass.exe file used by Windows is located in the directory %WINDIR%\System32 and the description of the file is Local Security Authority Process. If it is running from any other location, that lsass.exe is most likely a virus, spyware, trojan or worm. Due to the way some systems display fonts, malicious developers may name the file something like Isass.exe (capital "i" instead of a lowercase "L") in efforts to trick users into installing or executing a malicious file instead of the trusted system file. The Sasser worm spreads by exploiting a buffer overflow in the LSASS on Windows XP and Windows 2000 operating systems.