Microsoft Exchange Server KB5000871: Unable to Apply “owaauth.dll”, 400 Bad Request & Server Error in ECP application

Hey, so I’ve been going through all our customers this week that’ve been identified as potentially at risk by the latest Exchange Server vulnerabilities (full details straight from Microsoft here) and as you can expect from a rapid response release, it’s not quite consistent when installing.

Please be painfully aware of the Microsoft installation notes on this one, as installing the patch via Windows Explorer with UAC enabled can completely break OWA and ECP for all users amongst other things.

The workaround for this is to open a command prompt as administrator and run the installer. Making this matter worse I’ve had some installers where no file access errors are being triggered but files have failed to be updated. At the end of this blog post are some of the error codes I’ve received as a result of this in the hopes of steering others to this resolution. This issue partially stems from lack of ability to stop and start the services when required, causing file locks and preventing files being replaced.

If you have this issue occur, you should be able to resolve by simply running a command prompt as administrator and re-running the installer as suggested in the notes. If this doesn’t your alternative options are (in my opinion, I accept no liability):

  • Roll back to a backup and run the upgrade again.
  • Pull the missing files from a backup to resolve the issue and run the installer again.
  • Reinstall Exchange and run the patch again.

Do not attempt any other suggestions on the internet regarding rebuilding your virtual directories, that’s for unrelated issues and will only delay the resolution.

I hope this helped solve your problems!

Error codes received:

Error Code 400 Bad Request OWA upon signing in.

Server Error in /ecp Application

Event 1310 Request URL: https://localhost:444/ecp/exhealth.check

Event Code: 3008

Event Message: A configuration error has occurred.

Exception message: Could not load file or assembly ‘Microsoft.Exchange.Common, Version=, Culture=neutral, PublicKeyToken=31bf3856ad364e35’ or one of its dependencies. The system cannot find the file specified.

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