mantis -- session hijacking vulnerability

The mantis Team reports:

When configuring a web application to use only ssl (e. g. by
forwarding all http-requests to https), a user would expect that
sniffing and hijacking the session is impossible.
Though, for this to be secure, one needs to set the session cookie to
have the secure flag. Else the cookie will be transferred through http
if the victim’s browser does a single http-request on the same domain.