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.
The drupal CMS is vulnerable to this issue. They don't consider this as a drupal issue and have not published a fix yet.
2008-08-13: Vendor contacted
2008-09-20 Published advisory
This vulnerability was discovered by Hanno Boeck of schokokeks.org webhosting. It's licensed under the creative commons attribution license.
Hanno Boeck, http://www.hboeck.de