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.