So.. It's impossible to support Android 7 and older on IIS? (In a non-hacky way?)

Correct. There are workarounds but Windows builds it's own certificate chain for your certificate which it then uses (when acting as a server or a client), regardless of the chain you may have constructed in the installed PFX (stored in the machine certificate store).

Options/workarounds include:

  • Proxying your service via nginx, caddy, apache etc (these construct the chain from pem files)
  • Proxying your service via Cloudflare
  • Moving ISRG Root X1 to Untrusted (not recommended, can affect outgoing https calls from your server to other services, like Let's Encrypt).
  • Using an alternative CA

Note that there will be another round of root/intermediate expiries in 2024/25 etc and at that point those Android devices will be entirely out of luck as well.

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