By: Kurt Seifried, [email protected], Copyright Kurt Seifried, 2001
This section has been added since many people use databases to store the login and password information that is used to authenticate. Doing this securely is much more difficult then most people realize.
14.1 Basic security considerations
The authentication backend must be secured heavily of course, there are several "best practices" you should follow:
The authentication server should only be an authentication server, it should not have other services such as DNS, WWW, POP, IMAP and so forth. Any additional services only increase the chances of a compromise.
The authentication server should have a minimal number of users that are allowed to log in, the best is to only allow administrators as needed.
The authentication server should be firewalled and possibly placed on a separate segment.
If this is a publicly accessible web server using the authentication server then you may want to place the authentication server outside of your internal network. This prevents attackers from potentially compromising an internal machine (using an SQL insertion attack for example).
Where possible create a special user that only has read access to the database that is used for authentication queries. In this manner it is much harder for an attacker to modify data in the database.
14.2 Exporting authentication information
As it is preferable to have authentication servers isolated as much as possible it is not often that people allow externally placed authentication servers to pull data from internal authentication servers. The most popular solution is to export data from the internal servers, this also allows you to only export the accounts that are needed instead of everything. Exporting information depends heavily on the exist authentication backend used.
Typically the simplest was is to simply export a subset of the data to a file and then upload it to authentication server where it is imported to the appropriate table(s). Another method would be to use an automated SQL client to connect to the external authentication server and update accounts with standard SQL commands (i.e. INSERT records for new accounts). This can be combined with database triggers (not available in all databases) so that whenever a password is modified for example the internal server connects to the external one and updates the information. The last method is to use database replication (again not available in all databases) and simply replicate the database out to the external server. Depending on the product in use you may be able to filter what goes out.
14.2.2 Windows 2000
Using the LDAP server in Windows 2000 you can replicate data to other LDAP servers, allowing for relatively easy updates to an external server.
14.3 SQL insertion attacks
As always you need to filter data that is provided by users. Just blindly using the contents of the "name" field can be very dangerous. Inserting special characters can allow an attacker to modify the SQL command used, such as inserting wildcard characters (i.e. set username and password to "*" so that it simply matches the first record). Several papers are available on this topic:
http://www.roses-labs.com - Abusing poor programming techniques in webserver scripts V 1.0
http://www.digitaloffense.net/csw/slides/CanSec_files/frame.htm - Making NT Server Bleed
To combat SQL insertion attacks you should sanitize data before it is sent to the database server. This must not be done client side through the use of java script for example since it is trivial for attackers to circumvent preventive measures that run on their machines. Probably the best piece of advice is to create a list of allowed characters (i.e. a-z, A-Z, 0-9) for usernames and the appropriate characters for passwords, not allowing "special" characters like "/", "\", "*", "&" and so forth will greatly reduce the number of potential problems. You should then escape any remaining "special" characters so they are interpreted as data and not parts of the SQL command. For Perl there is a DBI quote function, for recent version of PHP this is done automatically for example.
14.4 Preventing brute forcing
As with any authentication system attackers can (and will) try to brute force their way in by using different usernames and passwords. Preventing brute forcing is difficult, for example using account lockouts results in a new problem, attackers simply try to guess the password X number of times and then the account is locked out and users cannot access the system. Using rate limits and back off procedures is typically more robust, increasing the time between authentication methods by 5 seconds will force an attacker to connect repeatedly, increasing the time needed to try passwords. With all of these solutions there is one major common flaw. The assumption is that attackers will choose a user and then try various passwords. Attackers can just as easily choose a password and try different usernames. This works especially well with systems that use default passwords when accounts are created or reset for example (for example my telephone company and my bank use default passwords, the phone company changes it every year, and the bank four times a year). If you use some sort of lockout or backoff you should take this into consideration.
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