NTLM

In a Windows network, NT (New Technology) LAN Manager (NTLM) is a suite of Microsoft security protocols intended to provide authentication, integrity, and confidentiality to users. NTLM is the successor to the authentication protocol in Microsoft LAN Manager (LANMAN), an older Microsoft product. The NTLM protocol suite is implemented in a Security Support Provider, which combines the LAN Manager authentication protocol, NTLMv1, NTLMv2 and NTLM2 Session protocols in a single package. Whether these protocols are used or can be used on a system, which is governed by Group Policy settings, for which different versions of Windows have different default settings.

NTLM passwords are considered weak because they can be brute-forced very easily with modern hardware.

Protocol

NTLM is a challenge–response authentication protocol which uses three messages to authenticate a client in a connection-oriented environment (connectionless is similar), and a fourth additional message if integrity is desired.

First, the client establishes a network path to the server and sends a NEGOTIATE_MESSAGE advertising its capabilities.

Next, the server responds with CHALLENGE_MESSAGE which is used to establish the identity of the client.

Finally, the client responds to the challenge with an AUTHENTICATE_MESSAGE.

The NTLM protocol uses one or both of two hashed password values, both of which are also stored on the server (or domain controller), and which through a lack of salting are password equivalent, meaning that if you grab the hash value from the server, you can authenticate without knowing the actual password. The two are the LM hash (a DES-based function applied to the first 14 characters of the password converted to the traditional 8-bit PC charset for the language), and the NT hash (MD4 of the little endian UTF-16 Unicode password).