When curl is asked to use HSTS, the expiry time for a subdomain might overwrite a parent domain's cache entry, making it end sooner or later than otherwise intended.
This affects curl using applications that enable HSTS and use URLs with the
insecure HTTP:// scheme and perform transfers with hosts like
x.example.com as well as example.com where the first host is a subdomain
of the second host.
(The HSTS cache either needs to have been populated manually or there needs to have been previous HTTPS accesses done as the cache needs to have entries for the domains involved to trigger this problem.)
When x.example.com responds with Strict-Transport-Security: headers, this
bug can make the subdomain's expiry timeout bleed over and get set for the
parent domain example.com in curl's HSTS cache.
The result of a triggered bug is that HTTP accesses to example.com get
converted to HTTPS for a different period of time than what was asked for by
the origin server. If example.com for example stops supporting HTTPS at its
expiry time, curl might then fail to access http://example.com until the
(wrongly set) timeout expires. This bug can also expire the parent's entry
earlier, thus making curl inadvertently switch back to insecure HTTP earlier
than otherwise intended.
