If you use SSL but do not use HSTS (nor HPKP) you allow an attacker to downgrade to HTTP.
Having SSL, without HSTS, is the same as nothing. You cannot turn on HSTS (nor HKPK), this will break access to HTTP pages.
The absence of this feature allows HTTPS pages to go to HTTP. The attacker can do this and completely ignore SSL.
If you are carrying script
or img
in HTTP and without the use of SRI and without CSP, you allow the attacker to modify the content. But, this is irrelevant, because it can already modify the whole page. Whereas the attacker does some DNS Poisoning or ARP Poisoning, for example.
If you have a secure subdomain, such as seguro.site.com
, you have to hold that this uses HSTS at least.
That still has other problems. For example, you should enable "Secure" in the cookie, so that the cookie does not go to the unsafe area, and restrict it to the subdomain. Otherwise, it becomes possible to steal sessions when the user enters the insecure area. This ignores the possibility of a Session Fixation, setting a cookie in the unsafe area to be used in the safe area.
Outside privacy. Now, the provider knows which site he accessed and what information he got, by being on HTTP. If it was on HTTPS (and with HSTS Preload) only the IP would be obtained, the does not even guarantee which site was accessed.