ISAPI Filter ‘C:\Windows\Microsoft.NET\Framework\v4.0.30319\\aspnet_filter.dll’ could not be loaded due to a configuration problem

Problem

After monthly server patching, I also made some security enhancements to the web.config files in the C:\Windows\Microsoft.NET\FrameworkXX\vXXX\CONFIG\ directories. It was therefore important that I test both 32bit and 64bit application pool modes. I found that tests running 64bit mode were suddenly failing with the following error message:

ISAPI Filter ‘C:\Windows\Microsoft.NET\Framework\v4.0.30319\\aspnet_filter.dll’ could not be loaded due to a configuration problem. The current configuration only supports loading images built for a AMD64 processor architecture. The data field contains the error number. To learn more about this issue, including how to troubleshooting this kind of processor architecture mismatch error, see http://go.microsoft.com/fwlink/?LinkId=29349.

It was of no surprise what so ever that the link included in the error message was 100% useless (thanks again Microsoft). One article I found suggested I uninstall .NET 4 – that’s like turning a monitor off to fix a dead pixel.

Solution

Upon closer inspection of the error message I noticed the problem:

v4.0.30319\\aspnet_filter.dll

Windows doesn’t handle double slashes very well as opposed to Linux. What is strange is none of these settings are manually changed which suggests it was perhaps a Windows update that caused this, however I have no evidence beyond change control logs.

I fixed with the following process:

Opened IIS Manager and selected the IIS web server node:

In the middle pane, browse to the IIS section and select ISAPI Filters:

Expand the columns as required, looking for an Executable path with \\ in it, then click Edit:

From the above screenshot, change the \\ to \ so the path reads:

C:\Windows\Microsoft.NET\Framework\v4.0.30319\aspnet_filter.dll

Just keep in mind that the folders Framework and Framework64 or the .NET version may change depending on your implementation.

After clicking OK, all my testing started to work again and the 500 Internal Server errors went away.

Conclusion

A possible Microsoft update (exact cause unknown) made an error updating an existing or installing a new ISAPI handler to IIS. By mishandling the path, it inserted \\ instead of \ into the executable path causing IIS to be unable to load the filter.

This caused 500 Internal Server errors to sites/applications running application pools in 64bit mode.

Hopefully this helps someone out. If it does, leave me some feedback! Thanks all!

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109 Responses to ISAPI Filter ‘C:\Windows\Microsoft.NET\Framework\v4.0.30319\\aspnet_filter.dll’ could not be loaded due to a configuration problem

  1. Jeff Peterson says:

    Thank you sir for being spot on. You are clear, concise and correct. Good eye to catch double back slash in that tiny font!

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  2. Padraig Walsh says:

    Thank you for this, very simple fix which worked immediately, but I would never have found.

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  3. Brendan says:

    Sounds like a tough week!

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  4. Chucky says:

    I had this exact same issue and your post saved my day! Thanks so much. Just a week after having some System Centre Endpoint Protection definition updates screw up my terminal server environment until it was uninstalled completely, I’m not having much love for MS updates at the moment….. Thank you so much.
    Cheers
    Chucky

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  5. Simon says:

    I accidentally install .net version 1.1 (was looking for .net core 1.1.2) as well as dotnetfx (.net core sdk) and found that this error cropped up. It may be a coincidence but who knows. Thanks for your blog post, helped me out of a sticky spot.

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