The message_options function in includes/ucp/ucp_pm_options.php in phpBB before 3.0.13 does not properly validate the form key, which allows remote attackers to conduct CSRF attacks and change the full folder setting via unspecified vectors.
https://wiki.phpbb.com/Release_Highlights/3.0.13
https://tracker.phpbb.com/browse/PHPBB3-13526
https://security.gentoo.org/glsa/201701-25
https://github.com/phpbb/phpbb/pull/3311
https://github.com/phpbb/phpbb/commit/23069a13e203985ab124d1139e8de74b12778449
https://exchange.xforce.ibmcloud.com/vulnerabilities/100671