The blind-marking implementation in Moodle through 2.3.11, 2.4.x before 2.4.10, 2.5.x before 2.5.6, and 2.6.x before 2.6.3 allows remote authenticated users to de-anonymize student identities by (1) using a screen reader or (2) reading the HTML source.
https://moodle.org/mod/forum/discuss.php?d=260363
http://openwall.com/lists/oss-security/2014/05/19/1
http://git.moodle.org/gw?p=moodle.git&a=search&h=HEAD&st=commit&s=MDL-44750