Directory traversal vulnerability in Yet another Bulletin Board (YaBB) 2.1 and earlier allows remote authenticated users to execute arbitrary Perl code via a .. (dot dot) in the userlanguage profile setting, which sets the userlanguage key of the member hash, and is propagated to the language variable in (1) HelpCentre.pl and (2) ICQPager.pl, (3) the use_lang variable in Subs.pl, and the actlang variable in (4) Post.pl and (5) InstantMessage.pl; as demonstrated by pointing userlanguage to the English folder, modifying English/HelpCentre.lng file to contain Perl statements, and then invoking the help action in YaBB.pl.