iBlog - Ilia Alshanetsky

A few days ago a I noticed an interesting behavior in the session_regenerate_id() function. When it renames the session id it does not remove the old session, leaving it active and potentially usable by a would be hacker. This does not pose a problem if the function is only used during new session create as the means of preventing session fixation, which is the intended use btw. However, it makes it completely useless if used on each session based request to prevent session leakage via HTTP_REFERER and similar, since the previous session id is still usable. It also means that changing the id on “actions” as some scripts to do prevent session theft also is pointless; in fact it doubles the amount of session ids for the same user making it only simpler to assume their identity. Furthermore it means that on every call to the function there is duplication in the number of sessions entries that will hang around until they are considered expired and removed by the garbage collection process.

For this reasons, I have added an optional parameter to session_regenerate_id() in PHP 5.1, if set to true, it will force the session extension to remove the old session on an id change.