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Authentication weaknesses

This tip focuses on hardening network access against brute-force attacks.

Ah, the good 'ole login screen. Is any secure system complete without one? Whether it's a Web site login screen or a Unix login prompt, most systems' security relies solely on a valid userID and password to prove one's identity. Since this is usually the only access requirement, it's worth putting your authentication system under the magnifying glass to see just how well it holds up to a curious hacker.

It's extremely common for hackers to try to brute force their way into a system by guessing commonly used userIDs and passwords. You should avoid using "admin," "test," "user" and any default userIDs. Common passwords to avoid are the userID, "password," "pass" and any default passwords. Some systems make it easy for a user to discover a valid userID, displaying a message when a logon failure occurs. Such messages may say, "Invalid userID," telling the hacker that they should keep guessing userIDs. When a valid userID is found, they may then be shown another revealing message, such as, "Invalid password." Ideally, a system's logon failure message should be generic, such as, "Invalid userID or password," regardless of the reason for failure. Otherwise, the hacker could enumerate a valid userID and start guessing passwords, looking for a weak one, which brings us to the next item.

Weak passwords are often the weakest link in most authentication systems. If at all possible, enforce password rules for every system on your network, especially for systems at the network border. Password and account rules should at least require a mix of letters and numbers, and should specify a minimum password length, password history, account lockout and password expiration. If possible, set password rules that do not allow a password to be the same as the userID or the user's first or last name, as these are easy to guess. The goal is to force users to choose strong passwords.

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To really beef up your authentication mechanism, you should enforce a two- or three-factor authentication system. Multifactor authentication means there are at least two different types of credentials that must be submitted to be authenticated. There are three categories of authentication factors: something you have, something you know and something you are. Each factor in the authentication mechanism should be from a different category than the others. In other words, a userID and password is still one-factor authentication since both pieces are something you know. Some valid combinations would be a key-fob token and a PIN, a thumbprint and a password or a retina scanner and your voice.

By improving your authentication mechanisms you are making it very tough for hackers to brute force their way into your systems. With the exception of multi-factor authentication systems, the above recommendations should not cost much, if anything, to implement.

About the author
Vernon Haberstetzer, president of security seminar and consulting company, has seven years of in-the-trenches security experience in healthcare and retail environments.

This tip originally appeared on our sister site,

This was last published in March 2005

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