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Preventing a Brute Force or Dictionary Attack

January 10th, 2007

To understand and then combat a brute force attack, also known as a dictionary attack, we must start by understanding why it might be an appealing tool for a hacker. To a hacker, anything that must be kept under lock and key is probably worth stealing. If your Web site (or a portion of it) requires a user to login and be authenticated, then the odds are good that a hacker has tried to break into it.

In terms of processing power, it is expensive for a Web site to require authentication, so it is usually only required when the site stores valuable private information. Corporate intranet sites can contain confidential data such as project plans and customer lists. E-commerce sites often store users’ email addresses and credit card numbers. Bypassing or evading authentication in order to steal this data is clearly high on a hacker’s priority list, and today’s hackers have a large library of authentication evasion techniques at their disposal.

Session hijacking attacks such as Cross-site Scripting can steal a user’s authentication token and transmit it to a malicious third party, who can then use it to impersonate the legitimate user. SQL injection attacks can also be very effective at bypassing authentication. By sending a specially-formatted username and password combination containing SQL code to the login form, an attacker can often trick the server into granting him unauthorized access. These types of attacks get a lot of attention since they are creative, elegant, and effective. However, there is another type of attack that can be just as effective, if not as elegant or creative. A brute force attack (or dictionary attack) can still be a dangerous threat to your Web site unless proper precautions are taken.

The brute force attack is about as uncomplicated and low-tech as Web application hacking gets. The attacker simply guesses username and password combinations until he finds one that works. It may seem like a brute force or dictionary attack is unlikely to ever succeed. After all, what are the odds of someone randomly guessing a valid username and password combination? Surprisingly, the odds for a brute force attack can be quite good if the site is not properly configured. There are several factors that work to the hacker’s advantage, the most important of which is human laziness.

Don’t Be Lazy – Choose a Password Carefully!

Generally, people do not remember complicated passwords very well. If users are allowed to create their own passwords, they will often create very simple ones like “password”, “1234”, their spouse’s name, or their favorite sports team. Passwords like these are easy for the user to remember, but unfortunately they are also easy for someone else to guess. Furthermore, any serious hacker who attempts a brute force attack will not be sitting at a Web browser, guessing at authentication credentials and typing them in. He will be using an automated tool for the brute force attack that can make thousands of requests per minute with credentials generated from a large list of possible values. Often this list is an actual dictionary, hence the term “dictionary attack.” If a user chooses a common password, such as a dictionary word, the automated tool will eventually guess it, and the user’s account will be compromised.

Also, once the brute force attack has revealed a valid username and password combination for one Web site, the hacker knows that the same combination is likely to work for other Web sites. In a study conducted by the University of Wichita, more than half of the test subjects reported using the exact same password for multiple sites. This laziness works to the hacker’s advantage. If, for example, a hacker is able to use a dictionary attack to obtain a valid user credential for Amazon.com, then it is probable that the same credential would be valid for other popular Web sites, such as eBay.

Sidestepping a Dictionary Attack with Username Selection

Of course, a password is only half of the required login credential. A username is also required. While it is less likely that a dictionary word would be used as a username, there are still some common usernames that hackers are certain to try with a brute force attack. First among these are “admin” and “administrator”. These names are especially dangerous since they are not only easily guessed, but the accounts they represent are usually highly privileged administrative accounts. If the hacker’s dictionary attack could gain access to an administrative account, he could probably do much more damage to the system than he could if he gained access to a regular user’s account.

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