@stormi
Hi, I just made a quick and dirty workaround by editing

/usr/sbin/secureboot-certs

I replaced this line:

req.add_header("User-Agent", "Mozilla/5.0")

by this line:

req.add_header("User-Agent", "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/101.0.4951.67 Safari/537.36")

Afterwards install went through without 403 error:

[18:10 xs ~]# secureboot-certs install default default default latest
Downloading https://www.microsoft.com/pkiops/certs/MicCorKEKCA2011_2011-06-24.crt...
Downloading https://www.microsoft.com/pkiops/certs/MicCorUEFCA2011_2011-06-27.crt...
Downloading https://www.microsoft.com/pkiops/certs/MicWinProPCA2011_2011-10-19.crt...
Downloading https://uefi.org/sites/default/files/resources/dbxupdate_x64.bin...
Successfully installed certificates to the XAPI DB for pool.
[18:26 xs ~]#