Directory traversal vulnerability in cgi-bin/koha/mainpage.pl in Koha 3.4 before 3.4.7 and 3.6 before 3.6.1, and LibLime Koha 4.2 and earlier allows remote attackers to read arbitrary files via a .. (dot dot) in the KohaOpacLanguage cookie to cgi-bin/opac/opac-main.pl, related to Output.pm.