phpbb -- NULL byte injection vulnerability

Secunia reports:

ShAnKaR has discovered a vulnerability in phpBB, which can be exploited by malicious users to compromise a vulnerable system.

Input passed to the "avatar_path" parameter in admin/admin_board.php is not properly sanitised before being used as a configuration variable to store avatar images. This can be exploited to upload and execute arbitrary PHP code by changing "avatar_path" to a file with a trailing NULL byte.

Successful exploitation requires privileges to the administration section.

[source]