Another HSTS bypass via IDN

curl’s HSTS check could be bypassed to trick it to keep using HTTP.

Using its HSTS support, curl can be instructed to use HTTPS instead of using
an insecure clear-text HTTP step even when HTTP is provided in the URL.

The HSTS mechanism could be bypassed if the hostname in the given URL first
uses IDN characters that get replaced to ASCII counterparts as part of the IDN
conversion. Like using the character UTF-8 U+3002 (IDEOGRAPHIC FULL STOP)
instead of the common ASCII full stop (U+002E). Then in a subsequent request,
it does not detect the HSTS state and makes a clear text transfer. Because it
would store the info IDN encoded but look for it IDN decoded.

Reproducible like this:

curl --hsts hsts.txt https://curl%E3%80%82se
curl --hsts hsts.txt http://curl%E3%80%82se