LAN Manager - Security Vulnerability

Security Vulnerability

LAN Manager authentication uses a particularly weak method of hashing a user's password known as the LM hash algorithm. This makes the supposed one-way function 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, which is still vulnerable to rainbow tables, but less vulnerable to brute force attacks. Both protocols have subsequently been deprecated in favor of Kerberos, but remain in use for backward compatibility and inter-operability.

The major weaknesses of LAN Manager authentication protocol are:

1. Passwords are not case sensitive. All passwords are converted into uppercase before generating the hash value. Hence it takes password, PassWord, PaSsWoRd, PASSword and other similar combinations same as PASSWORD converting all characters to uppercase. Password characters are also limited to a subset of the ASCII character set.
2. Password length is limited to maximum of 14 characters
3. A 14-character password is broken into 7+7 characters and the hash is calculated for the two halves separately. This way of calculating the hash makes it exponentially easier to crack, as the attacker need to brute force 7 characters twice instead of 14 characters. This makes the effective strength of a 14-characters password equal to twice that of a 7-character password, which is significantly less complex than the strength of a 14 character password.
4. If the password is 7 characters or less, then the second half of hash will always produce the same constant value. Therefore if the length of password is less than or equal to 7 characters, then a password length of 7 characters or less can be identified visibly without using tools.
5. The hash value is sent to the server on network without salting, making it susceptible to man in the middle attacks such as replay the hash.