Solved: .NET 5 “SEC_ERROR_INADEQUATE_KEY_USAGE” in Firefox

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Ran across an issue I had to fix where a .NET 5 application, which happened to be running in a Docker container (this ended up being irrelevant), couldn’t start up in Firefox due to a certificate error, even though I’d accepted the .NET certificate at the root. I saw a SEC_ERROR_INADEQUATE_KEY_USAGE error in the browser.

Solution

This is because Firefox is not utilizing the Windows certificate store, so I have to force it to do so.

  • Open the Firefox settings by going to about:config
  • Search for security.enterprise_roots.enabled
  • Ensure that the setting is set to true.

This will tell Firefox to use the Windows Cert store, which allows the certificate to be seen and the site trusted for local development.