5.1 Security Considerations for Implementers
NTLM does not support any recent cryptographic methods, such as AES or SHA-256. It uses cyclic redundancy check (CRC) or message digest algorithms ([RFC1321]) for integrity, and it uses RC4 for encryption. Deriving a key from a password is as specified in [RFC1320] and [FIPS46-2]. Therefore, applications are generally advised not to use NTLM.<81>
The NTLM server does not require the NTLM client to send the MIC, but sending the MIC when the timestamp is present greatly increases security. Although implementations of NLMP will work without support for MIC, they will be vulnerable to message tampering.
The use of ANONYMOUS user NullSession results in a SessionBaseKey with all zeroes, which does not provide security. Therefore, applications are generally advised not to use NullSession. The use of Guest user GuestSession results in a SessionBaseKey with all zeroes, which does not provide security.
The Guest user account is disabled by default for security reasons. If the Guest user account is enabled, it is strongly recommended to set a password so that logon failures do not result in Guest logins (section 18.104.22.168.2 ). If a password is set on the Guest account, then there is a guest fallback where logons will be tried with unknown usernames against the Guest password.
When the Guest user account has been assigned a password and is used explicitly to log in by using username Guest and by providing the assigned password, login processing happens just like a regular user. This is not recommended, as the Guest user would be handled like a regular user, which it is not an intended or desired result.