Here comes a maintenance release with version 2.5.4. Its main purpose is to fix a few security bugs.
One of them, smartly reported by Paul-Emmanuel Brun, is serious and really deserves that you update to 2.5.4 if you haven't updated to 2.6 yet. Full details on the Piwigo 2.5.4 Release Notes.
Update Piwigo from your administration home page or follow the automatic upgrade procedure.