The is_a function in PHP 5.3.7 and 5.3.8 triggers a call to the __autoload function, which makes it easier for remote attackers to execute arbitrary code by providing a crafted URL and leveraging potentially unsafe behavior in certain PEAR packages and custom autoloaders.

include/iniset.php in Roundcube Webmail 0.5.4 and earlier, when PHP 5.3.7 or 5.3.8 is used, allows remote attackers to trigger a GET request for an arbitrary URL, and cause a denial of service (resource consumption and inbox outage), via a Subject header containing only a URL, a related issue to CVE-2011-3379.