phpbb -- remote PHP code execution vulnerability

FrSIRT Advisory reports:

A vulnerability was identified in phpBB, which
may be exploited by attackers to compromise a vulnerable
web server. This flaw is due to an input validation error
in the “viewtopic.php” script that does not properly filter
the “highlight” parameter before calling the “preg_replace()”
function, which may be exploited by remote attackers to execute
arbitrary PHP commands with the privileges of the web server.