Werk #766: Changed transid implemtation to work as CSRF protection (Fixes CVE-2014-2330)

This change fixes possible attacks against Check_MK Multisite users. In previous versions a possible attacker could try to make the browsers of authenticated users open URLs of the Check_MK Multisite GUI to execute actions e.g. within WATO without knowledge of the attacked user.

To make such an attack possible, there are several things needed: The user must be authenticated with multisite and have enough permission within multisite to execute the actions the attacker wants to use, the attacker needs to know the exact URL to the Multisite GUI. Then the attacker needs to make the user either click on a manipulated link or open a manipulated webpage which makes the browser of the user, where the user is authenticated with multisite, open the URL the attacker wants to make it open.

The multisite GUI makes use of transids (transaction ids) when processing form submissions or actions. The transids were mainly used to prevent double execution of actions when reloading the page which performed the action in the browser. Now we changed internal handling of the transid to make it also prevent CSRF attacks. The transid is now some kind of shared secret between the webserver and the browser of the user. This ensures a form submission is intended by a previously requested page.

This change impicates an incompatible change: In case you use a script which opens multisite pages to perform an action, e.g. set a downtime and use this with a regular user account which authenticates by username/password, the script won't work anymore after this change. The way to go is to adapt the script and change the user to authenticate with an automation secret instead of a password. For this kind of authentication, you will need to user other URL parameters (_username=... and _secret=...).

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