Chromium user-open error - supplied URL scheme is not allowed

I’ve been trying to use Microsoft Online Single Sign On with the chromium snap (80.0.3987.87 - rev1016). Once the SSO request has been approved (on a Microsoft website, via 2FA), a clickable button appears.

  • With the non-snap version: xdg-open will be called (which I customized to extract the urn link and save it to a temporary file).
  • With the snap version: I see this error message on stdout:

user-open error: Supplied URL scheme “urn” is not allowed

Any ideas on how to proceed on this or what to look at?

This is a known shortcoming of snapd’s strict confinement, which currently only allows a restricted list of URL schemes.