Manual:Edit token

An edit token is a random string that is passed between a client and the MediaWiki server when performing actions that change pages. It is used to check that the user really intended to make the change, rather than being tricked into requesting the change on the wiki while visiting an external site (i.e. cross-site request forgery).

Note: the information on this page needs to be verified by a developer.

Why is it necessary
Edit tokens are used as an additional security measure when performing changes. If the user identity were checked using cookies only, an external site could use a link like the following one to have visitors perform changes to the wiki.

http://en.wikipedia.org/w/index.php?title=Image:Abcd.jpg&action=delete&oldimage=324242234

Following such a link would lead an administrator to unknowingly request deletion of an image. If the administrator is still logged in, the server would check the cookies and grant the request.

For this reason, actions performing changes require an additional piece of data that is passed as an HTTP parameter, the edit token. An edit token is embedded into web pages from which the user can request a change; this includes the edit form (where one can change a page by pressing "Save change") but also the image description pages (where an administrator can request deletion of an old version of an image), contributor histories (where administrators can rollback), etc. When the user actually request the change to be done (by pressing a button or following a link), the edit token is sent back to the server. This provides information to the server that the user has requested the change directly from the site and not from an external site, as external sites do not have access to the edit token of the user.

How does it work
An edit token is a random string stored in the PHP session, which is an associative array that is stored in the server and maintained across sessions because of a cookie (e.g.,  on the English Wikipedia). The edit token is in particular contained in the  element of the PHP session.

Edit tokens are embedded into web pages from where the user can request a change. When such a page is to be generated, the edit token is retrieved from the  element of the PHP session, if such an element exists; otherwise, a random string is generated and stored in that element.

What is actually embedded into the web page is not the  element itself. Rather, this element is concatenated to the salt, which is a string that depends on the particular action and page; the resulting string is then MD5-hashed; this is what is embedded in the web page. When the user actually requests the action, this string is sent back to the server via an HTTP parameter. The server can then check the correctness of this parameter: it repeats the procedure used to generate it from the PHP session and checks if the result is is equal to the parameter.

Source code
Edit tokens are mainly dealt with in the User.php source file, and in particular by the following methods.


 * editToken(salt) : returns the MD5 hash of the concatenation of the  element of the PHP session with the salt; if such an element does not exist in the PHP session, a random one is generated;


 * generateToken(salt) : generate a random string (depends on the salt parameter)


 * matchEditToken(token, salt) : checks whether its first argument is a valid edit token with respect to the salt; this is done by repeating the procedure of generation and then comparing the result with the first argument; in particular, this function calls  and then compares the result with the first argument;

Salt
The default salt is the empty string; most actions use this default value. As a result, an edit token string received from a server to perform one such an action on a page can also be used to perform other actions on other pages. However, since an edit token is stored in the PHP session, it can be used only as long as the session is kept in the server and the client has the corresponding session token cookie (e.g., the enwiki_session cookie).

An edit token hash generated using a salt can be used for performing other actions only if the salt used by them is the same. As a result, if the salt for example depend on the page where the action is to be performed, the edit token hash can only be used on that page. Actions not using the default empty salt are:


 * rollback : the salt is the title of the article (including the namespace prefix) concatenated with the name of the user whose edits are to be reverted;
 * delete the old version of an image : the salt is the  parameter (when deleting all version this parameter is the empty string, which is also the default salt);
 * SpecialUserrights : the salt is the username of the user whose properties are to be changed;
 * SpecialWatchlist/clear : the salt is the string 'clearwatchlist'

The edit token suffix
Since revision 18112, a trailing backslash has been added to edit tokens, and edit tokens made of a single backslash introduced for anonymous users. This change has been done to prevent broken proxies from editing: proxies that cannot correctly handle the backslash typically also mess up the wiki markup code. This suffix has been changed to  in r23287 to also catch broken proxies which mangle the '+' character.