While working on the new release and testing the release candidate we unfortunately got two security issue reports from independent security researchers. We take such reports very seriously and fix them as fast as possible. We urge you to update your installations as quickly as possible. We also have patches for 2.3.x and 2.4.x if you can’t update to 2.5.7 from older installations easily.
The release contains additional inprovements for the panel and the core. You can find all information in the changelog: https://getkirby.com/changelog/kirby-2-5-7