Oracle MySQL 5.5.38 and earlier, 5.6.19 and earlier, and MariaDB 5.5.28a, 5.3.11, 5.2.13, 5.1.66, and possibly other versions, generates different error messages with different time delays depending on whether a user name exists, which allows remote attackers to enumerate valid usernames.
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lists.opensuse.org/opensuse-security-announce/2015-04/msg00016.html
seclists.org/fulldisclosure/2012/Dec/9
secunia.com/advisories/53372
security.gentoo.org/glsa/glsa-201308-06.xml
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mariadb.atlassian.net/browse/MDEV-3909