The filter in the tag manager required extra sanitizing to prevent a reflected XSS risk. This affects 3.9 to 3.9.1, 3.8 to 3.8.4, 3.7 to 3.7.7, 3.5 to 3.5.13 and earlier unsupported versions. Fixed in 3.9.2, 3.8.5, 3.7.8 and 3.5.14.
https://moodle.org/mod/forum/discuss.php?d=410840
http://git.moodle.org/gw?p=moodle.git&a=search&h=HEAD&st=commit&s=MDL-69340