What is URLScan? URLScan is a free tool provided by Microsoft that restricts certain kinds of HTTP requests that IIS will process. Though there are many different uses of URLScan, today I will only be covering SQL Injection blocking.
SQL Injections occur when attackers enter malformed SQL statements into data input fields. The attacker can modify or retrieve data from your database and, in some cases, they can even access data stored in your filesystem outside of SQL Server. If you find that you’re a victim of SQL Injection attacks and you’re not equipped to make all of the necessary changes to your application (or if the implementation of these changes could take a long time to implement) then URLScan may be a good fit for you until you can fix the root cause of the attack.
As of this writing, URLScan 3.1 is the latest edition and can be downloaded from Microsoft. A URLScan ISAPI filter is configured for all websites on your server after performing a default installation of URLScan. This ISAPI filter intercepts request for IIS and processes security rules defined in the URLScan.ini file against the requests.
The blocking of SQL injection attacks is handled by the DenyQueryStringSequences security rule. The rule matches IIS requests against a list of character sequences that you provide. If the request matches certain character sequences specified in the security rule, then the request is dropped, logged in the URLScan Logs, and a 404 status is returned. You can configure URLScan to only log requests that match your defined character sequences instead of blocking them. I would recommend starting off with the logging only mode so you can determine whether or not valid traffic may be blocked. If valid traffic is being blocked, you’ll need to modify the character sequences you are choosing to block.
Below is a configured DenyQueryStrinSequences security rule. I have added some of the most common SQL commands used in SQL Injection attacks:
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