fl wrote:
No, they can't. As far as I am aware, MediaWiki is released under the GNU General Public License[1], which stipulates, among other things, the requirement to release a program's source code to the public and to release any derived changes under the same license[2].
If the WMF were to try and convert MediaWiki to a closed source project, they would be liable to legal actions against them.
That requirement is only valid if they distribute the changed version (and only for people which get that version). They could improve mediawiki and keep the changes to themselves.
There are many reasons that would be a really bad idea. But from a strictly legal POV, they can do it.
For a license which requires releasing the code to people browsing the site, see the Affero GPL.