NTLM is the Microsoft Windows NT challenge and response authentication system. The local user domain authentication credentials are used to authenticate against web resources. The Windows domain controller sends the client a random challenge. The client uses this random challenge and the user’s credentials to construct a response. This response is sent to the domain controller and the domain controller decrypts the response and validates the user credentials.

NTLM authentication has special requirements that the client protocol handler must be able to fulfill. This authentication scheme will not work without a client protocol handler capable of understanding NTLM challenges and generating the correct response.