LM hash

LAN Manager was a Network Operating System (NOS) available from multiple vendors and developed by Microsoft in cooperation with 3Com Corporation. It was designed to succeed 3Com's 3+Share network server software which ran atop a heavily modified version of MS-DOS.

LAN Manager was based on the OS/2 operating system co-developed by IBM and Microsoft. It originally used the Server Message Block protocol atop either the (NBF) or a specialized version of the Xerox Network Systems (XNS) protocol. These legacy protocols had been inherited from previous products such as MS-Net for MS-DOS, Xenix-NET for MS-Xenix, and the afore-mentioned 3+Share. A version of LAN Manager for Unix-based systems called LAN Manager/X was also available.

In 1990, Microsoft announced LAN Manager 2.0 with a host of improvements, including support for TCP/IP as a transport protocol. The last version LAN Manager, 2.2, which included an MS-OS/2 1.31 base operating system, remained Microsoft's strategic server system until the release of Windows NT Advanced Server in 1993.

Many vendors shipped licensed versions, including:

LAN Manager authentication uses a particularly weak method of hashing a user's password known as the LM hash algorithm, stemming from the mid 1980s when floppy viruses were the major concern as opposed to potentially high-frequency attacks with feedback over a (high-bandwidth) network. This makes such hashes crackable in a matter of seconds using rainbow tables, or in few hours using brute force. Its use in Windows NT was replaced by NTLM, of which older versions are still vulnerable to rainbow tables, but less vulnerable to brute force attacks. NTLM is used for logon with local accounts except on domain controllers since Windows Vista and later versions no longer maintain the LM hash by default. is used in Active Directory Environments.

...
Wikipedia