October CMS through 1.0.428 does not prevent use of .htaccess in themes, which allows remote authenticated users to execute arbitrary PHP code by downloading a theme ZIP archive from /backend/cms/themes, and then uploading and importing a modified archive with two new files: a .php file and a .htaccess file. NOTE: the vendor says "I don't think [an attacker able to login to the system under an account that has access to manage/upload themes] is a threat model that we need to be considering.
Cross-Site Request Forgery exists in OctoberCMS 1.0.426 (aka Build 426) due to improper validation of CSRF tokens for postback handling, allowing an attacker to successfully take over the victim's account. The attack bypasses a protection mechanism involving X-CSRF headers and CSRF tokens via a certain _handler postback variable.