Exposure of Sensitive Information to an Unauthorized Actor
message/index.php in Moodle through 2.5.9, 2.6.x before 2.6.9, 2.7.x before 2.7.6, and 2.8.x before 2.8.4 does not consider the moodle/site:readallmessages capability before accessing arbitrary conversations, which allows remote authenticated users to obtain sensitive personal-contact and unread-message-count information via a modified URL.
CWE-200 - Information Exposure
An information exposure vulnerability is categorized as an information flow (IF) weakness, which can potentially allow unauthorized access to otherwise classified information in the application, such as confidential personal information (demographics, financials, health records, etc.), business secrets, and the application's internal environment.