RADIUS

The User-Password Attribute and Password Attacks

An attacker can get around any rate limits of authentication placed by the administrator of the RADIUS server because of the use of the stream cipher to protect the User-Password attribute. Here’s how it works: the hacker first tries to authenticate against a RADIUS server using a known good username and a known, but probably incorrect, password. She takes the resulting Access-Request packet and figures out the MD5 result of the request authenticator + shared secret combination, as described earlier. She can then use a brute-force password attack by switching out the passwords in the packet and using the same request authenticator and shared secret. This will only work, however, if the password is less than or equal to 16 characters, since the User-Password cipher becomes self-synchronizing at the 17^th character by including previous ciphertext in the encryption.