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I have developed an Excel add-in, and trying to run it I get the following error:

Cert error

"The content is blocked because it isn't signed by a valid security certificate."

Add-in's binding

  • The add-in is a taskpane developed in Visual Studio
  • I've deployed the add-in with Microsoft 365 admin center
  • The add-in is hosted on a server running IIS, on port 443
  • I'm connected to the server via VPN.

I've tried

  • creating a self-signed certificate in IIS manager, and installing it in the client machine's Trusted Root Certification Authorities
  • creating a certificate in Visual Studio, importing it to IIS, installing it on the client
  • adding the server's IP to the client's trusted sites
  • ports 4433 and 443
  • setting the add-in's binding to "All unassigned"
  • something I've already forgotten

Somehow the add-in works in the web version of Excel.

Patrick Mevzek
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emaj
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  • This error is due to the certificate, to fix this issue you need to install this certificate on your local machine inside certificate store to accept that as valid certificate. – samwu Mar 07 '23 at 09:53
  • @samwu That is indeed the first thing I did, but no luck. – emaj Mar 07 '23 at 11:01
  • Does this answer your question? [The content is blocked because it isn't signed by a valid security certificate error in Outlook addin](https://stackoverflow.com/questions/64818707/the-content-is-blocked-because-it-isnt-signed-by-a-valid-security-certificate-e) – Eugene Astafiev Mar 07 '23 at 19:19
  • @EugeneAstafiev No, as far as I know local dev certs aren't relevant at this point, as the add-in is hosted on a remote server. – emaj Mar 08 '23 at 10:53
  • Your certificate must be trusted on your machine. Does it work correctly if you run your solution locally? – Eugene Astafiev Mar 08 '23 at 11:00
  • @EugeneAstafiev The add-in runs successfully while debugging locally in Visual Studio. – emaj Mar 08 '23 at 11:14
  • Do you get this error in a web browser or desktop application? – Eugene Astafiev Mar 08 '23 at 11:15
  • @EugeneAstafiev I get the error on desktop, the add-in works without error in browser-based Excel. – emaj Mar 08 '23 at 11:32
  • Are you sure your test certificate includes the URL where your add-in is hosted, not only for localhost? – Eugene Astafiev Mar 08 '23 at 11:41
  • @EugeneAstafiev the certificate is issued to hostname.domain.local. – emaj Mar 20 '23 at 07:02
  • Does it correspond to the one specified in the manifest file? – Eugene Astafiev Mar 20 '23 at 07:47
  • Yes, AppDomain, SourceLocation etc. point to the same url. – emaj Mar 20 '23 at 12:27
  • It is difficult to reproduce your problem based on your description, I suggest you open a case via: https://support.microsoft.com. – samwu Mar 21 '23 at 10:22

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