lib/classes/grades_external.php in Moodle 2.7.x before 2.7.3 does not consider the moodle/grade:viewhidden capability before displaying hidden grades, which allows remote authenticated users to obtain sensitive information by leveraging the student role to access the get_grades web service.

References

• https://nvd.nist.gov/vuln/detail/CVE-2014-7831
• https://moodle.org/mod/forum/discuss.php?d=275153
• http://git.moodle.org/gw?p=moodle.git&a=search&h=HEAD&st=commit&s=MDL-47766
• http://openwall.com/lists/oss-security/2014/11/17/11
• moodle/moodle@3b8876f
• https://web.archive.org/web/20150914064838/http://www.securitytracker.com/id/1031215