The User::matchEditToken function in includes/User.php in MediaWiki before 1.23.12, 1.24.x before 1.24.5, 1.25.x before 1.25.4, and 1.26.x before 1.26.1 does not perform token comparison in constant time before determining if a debugging message should be logged, which allows remote attackers to guess the edit token and bypass CSRF protection via a timing attack, a different vulnerability than CVE-2015-8623.