When HTTP protocol is used, the traffic is sent in plaintext. It allows the attacker to see/modify the traffic (man-in-the-middle attack). HTTPS is a secure version of HTTP – it uses SSL/TLS to protect the data of the application layer. When HTTPS is used, the following properties are achieved: authentication, data integrity.
The secure flag is an option that can be set by the application server when sending a new cookie to the user within an HTTP Response. The purpose of the secure flag is to prevent cookies from being observed by unauthorized parties due to the transmission of a the cookie in clear text.
To accomplish this goal, browsers which support the secure flag will only send cookies with the secure flag when the request is going to a HTTPS page. Said in another way, the browser will not send a cookie with the secure flag set over an unencrypted HTTP request.
By setting the secure flag, the browser will prevent the transmission of a cookie over an unencrypted channel.
Setting the Secure Flag:
Following sections describes setting the Secure Flag in respective technologies.