Anonymous Users

If you can see users with blank names:

mysql> select user, host, password
    -> from mysql.user
    -> where user = '';
+------+-----------+----------+
| user | host      | password |
+------+-----------+----------+
|      | localhost |          |
+------+-----------+----------+
1 row in set (0.01 sec)

mysql>

These are anonymous users, which allow you to login like this:

C:\Program Files\MySQL\MySQL Server 5.5\bin>mysql
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 4
Server version: 5.5.19 MySQL Community Server (GPL)

Copyright (c) 2000, 2011, Oracle and/or its affiliates. All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql>

... or this:

C:\Program Files\MySQL\MySQL Server 5.5\bin>mysql -u ''
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 5
Server version: 5.5.19 MySQL Community Server (GPL)

Copyright (c) 2000, 2011, Oracle and/or its affiliates. All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql>

... or this:

C:\Program Files\MySQL\MySQL Server 5.5\bin>mysql -u anonymous
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 7
Server version: 5.5.19 MySQL Community Server (GPL)

Copyright (c) 2000, 2011, Oracle and/or its affiliates. All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql>

... or this:

C:\Program Files\MySQL\MySQL Server 5.5\bin>mysql -u anonymous@localhost
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 8
Server version: 5.5.19 MySQL Community Server (GPL)

Copyright (c) 2000, 2011, Oracle and/or its affiliates. All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql>

If you don’t need them, you can delete them as follows:

C:\Program Files\MySQL\MySQL Server 5.5\bin>mysql -u root -p
Enter password: *********
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 9
Server version: 5.5.19 MySQL Community Server (GPL)

Copyright (c) 2000, 2011, Oracle and/or its affiliates. All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql> select count(*) from mysql.user
    -> where user = '';
+----------+
| count(*) |
+----------+
|        1 |
+----------+
1 row in set (0.01 sec)

mysql> delete from mysql.user
    -> where user = '';
Query OK, 1 row affected (0.02 sec)

mysql> select count(*) from mysql.user
    -> where user = '';
+----------+
| count(*) |
+----------+
|        0 |
+----------+
1 row in set (0.00 sec)

mysql>

At this stage, the anonymous user can still login:

C:\Program Files\MySQL\MySQL Server 5.5\bin>mysql
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 13
Server version: 5.5.19 MySQL Community Server (GPL)

Copyright (c) 2000, 2011, Oracle and/or its affiliates. All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql> exit
Bye

C:\Program Files\MySQL\MySQL Server 5.5\bin>mysql -u ''
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 14
Server version: 5.5.19 MySQL Community Server (GPL)

Copyright (c) 2000, 2011, Oracle and/or its affiliates. All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql> exit
Bye

C:\Program Files\MySQL\MySQL Server 5.5\bin>mysql -u anonymous
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 15
Server version: 5.5.19 MySQL Community Server (GPL)

Copyright (c) 2000, 2011, Oracle and/or its affiliates. All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql>

To finish the user deletion, you need to run the flush privileges command as root:

C:\Program Files\MySQL\MySQL Server 5.5\bin>mysql -u root -p
Enter password: *********
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 16
Server version: 5.5.19 MySQL Community Server (GPL)

Copyright (c) 2000, 2011, Oracle and/or its affiliates. All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql> flush privileges;
Query OK, 0 rows affected (0.05 sec)

mysql>

Then the user can no longer login:

C:\Program Files\MySQL\MySQL Server 5.5\bin>mysql
ERROR 1045 (28000): Access denied for user 'ODBC'@'localhost' (using password: N
O)

C:\Program Files\MySQL\MySQL Server 5.5\bin>mysql -u ''
ERROR 1045 (28000): Access denied for user ''@'localhost' (using password: NO)

C:\Program Files\MySQL\MySQL Server 5.5\bin>mysql -u anonymous
ERROR 1045 (28000): Access denied for user 'anonymous'@'localhost' (using passwo
rd: NO)

C:\Program Files\MySQL\MySQL Server 5.5\bin>