phpBB 2.0.17 and earlier, when register_globals is enabled and the session_start function has not been called to handle a session, allows remote attackers to bypass security checks by setting the $_SESSION and $HTTP_SESSION_VARS variables to strings instead of arrays, which causes an array_merge function call to fail.
phpBB 2.0.17 and earlier, when the register_long_arrays directive is disabled, allows remote attackers to modify global variables and bypass security mechanisms because PHP does not define the associated HTTP_* variables.
Multiple cross-site scripting (XSS) vulnerabilities in phpBB 2.0.17 and earlier allow remote attackers to inject arbitrary web script or HTML via the (1) error_msg parameter to usercp_register.php, (2) forward_page parameter to login.php, and (3) list_cat parameter to search.php, which are not initialized as variables.
SQL injection vulnerability in usercp_register.php in phpBB 2.0.17 allows remote attackers to execute arbitrary SQL commands via the signature_bbcode_uid parameter, which is not properly initialized.
usercp_register.php in phpBB 2.0.17 allows remote attackers to modify regular expressions and execute PHP code via the signature_bbcode_uid parameter, as demonstrated by injecting an "e" modifier into a preg_replace statement.