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Following up on my previous patch-related post, I decided to apply some well-deserved patches to the server that runs ISA Server 2004 for our office. In addition to the released patches for Windows Server 2003, the server was in need of Service Pack 2 for ISA Server 2004.

The patch is a pretty straightforward install, and all went well with the installation. Following the reboot of the server, everything worked just as it did prior to the installation.

When I arrived at the office this morning and performed my usual routine, I discovered there was a problem with certain websites when clients behind the ISA Server would attempt to connect to them. When a client would attempt to connect to one of these problem sites, the browser would return the following error:

Error Code: 500 Internal Server Error. The HTTP request includes a non-supported header. Contact your ISA Server administrator. (12156)

I did not discover this problem in my review of the documentation concerning this update, so the error message was unexpected. I immediately decided to do some digging via Google, which immediately put me to a forum thread over at ISAServer.org. After reviewing the thread, I discovered that the problem was exactly the same problem I was experiencing. At the end of the thread, one of the posters announced that Microsoft had released a patch for the problem.

I decided to check out the supporting Microsoft documentation. One of the MS KB articles–KB 915045–described a condition that was similar to my problem. The other article–KB 915422–also seemed to fit the problem I was observing. Both articles, of course, pointed me in the direction of the following patch: Update for HTTP Issues for Microsoft Internet Security and Acceleration (ISA) Server 2004 Standard Edition with Service Pack 2 (KB 916106). I applied the referenced patch and very briefly interrupted connectivity for my users, but the problems I observed went away.

So, for the sake of posterity, here are the steps one needs to take when applying SP2 to ISA Server 2004.

1. Apply SP2 for ISA Server 2004 Standard Edition. Restart your server.
2. Once the server has restarted, download the Update for HTTP Issues for ISA Server 2004 with SP2. Apply this patch to the server. Reboot your server again.
3. Once the server is restarted, all should work as expected for your clients that sit behind the ISA Server.