mod/lesson/pagetypes/matching.php in Moodle through 2.2.11, 2.3.x before 2.3.8, 2.4.x before 2.4.5, and 2.5.x before 2.5.1 allows remote authenticated users to obtain sensitive answer information by reading the HTML source code of a document.
https://moodle.org/mod/forum/discuss.php?d=232500
https://euvd.enisa.europa.eu/vulnerability/EUVD-2013-2194
http://git.moodle.org/gw?p=moodle.git&a=search&h=HEAD&st=commit&s=MDL-39546