HTTP Strict Transport Security (HSTS) is a web security policy mechanism which helps to protect secure HTTPS websites against downgrade attacks and cookie hijacking. It allows web servers to declare that web browsers (or other complying user agents) should only interact with it using secure HTTPS connections,[1] and never via the insecure HTTP protocol. HSTS is an IETF standards track protocol and is specified in RFC 6797.

The HSTS Policy[2] is communicated by the server to the user agent via an HTTP response header field named "Strict-Transport-Security". HSTS Policy specifies a period of time during which the user agent shall access the server in a secure-only fashion.

This is a header sent in the response. HSTS headers can enforce https for a given time.