4.7. General Security Issues

This section describes some general security issues to be aware of and what you can do to make your MySQL installation more secure against attack or misuse. For information specifically about the access control system that MySQL uses for setting up user accounts and checking database access, see Section 4.8, "The MySQL Access Privilege System."

4.7.1. General Security Guidelines

Anyone using MySQL on a computer connected to the Internet should read this section to avoid the most common security mistakes.

In discussing security, we emphasize the necessity of fully protecting the entire server host (not just the MySQL server) against all types of applicable attacks: eavesdropping, altering, playback, and denial of service. We do not cover all aspects of availability and fault tolerance here.

MySQL uses security based on Access Control Lists (ACLs) for all connections, queries, and other operations that users can attempt to perform. There is also support for SSL-encrypted connections between MySQL clients and servers. Many of the concepts discussed here are not specific to MySQL at all; the same general ideas apply to almost all applications.

When running MySQL, follow these guidelines whenever possible:

• Do not ever give anyone (except MySQL root accounts) access to the user table in the mysql database! This is critical. The encrypted password is the real password in MySQL. Anyone who knows the password that is listed in the user table and has access to the host listed for the account can easily log in as that user.

• Learn the MySQL access privilege system. The GRANT and REVOKE statements are used for controlling access to MySQL. Do not grant more privileges than necessary. Never grant privileges to all hosts.

  Checklist:

  • Try mysql -u root. If you are able to connect successfully to the server without being asked for a password, anyone can connect to your MySQL server as the MySQL root user with full privileges! Review the MySQL installation instructions, paying particular attention to the information about setting a root password. See Section 2.9.3, "Securing the Initial MySQL Accounts."

  • Use the SHOW GRANTS statement to check which accounts have access to what. Then use the REVOKE statement to remove those privileges that are not necessary.

• Do not store any plain-text passwords in your database. If your computer becomes compromised, the intruder can take the full list of passwords and use them. Instead, use MD5(), SHA1(), or some other one-way hashing function and store the hash value.

• Do not choose passwords from dictionaries. Special programs exist to break passwords. Even passwords like "xfish98" are very bad. Much better is "duag98" which contains the same word "fish" but typed one key to the left on a standard QWERTY keyboard. Another method is to use a password that is taken from the first characters of each word in a sentence (for example, "Mary had a little lamb" results in a password of "Mhall"). The password is easy to remember and type, but difficult to guess for someone who does not know the sentence.

• Invest in a firewall. This protects you from at least 50% of all types of exploits in any software. Put MySQL behind the firewall or in a demilitarized zone (DMZ).

  Checklist:

  • Try to scan your ports from the Internet using a tool such as nmap. MySQL uses port 3306 by default. This port should not be accessible from untrusted hosts. Another simple way to check whether or not your MySQL port is open is to try the following command from some remote machine, where server_host is the hostname or IP number of the host on which your MySQL server runs:

    shell> telnet server_host 3306
    

    If you get a connection and some garbage characters, the port is open, and should be closed on your firewall or router, unless you really have a good reason to keep it open. If telnet hangs or the connection is refused, the port is blocked, which is how you want it to be.

• Do not trust any data entered by users of your applications. They can try to trick your code by entering special or escaped character sequences in Web forms, URLs, or whatever application you have built. Be sure that your application remains secure if a user enters something like "; DROP DATABASE mysql;". This is an extreme example, but large security leaks and data loss might occur as a result of hackers using similar techniques, if you do not prepare for them.

  A common mistake is to protect only string data values. Remember to check numeric data as well. If an application generates a query such as SELECT * FROM table WHERE ID=234 when a user enters the value 234, the user can enter the value 234 OR 1=1 to cause the application to generate the query SELECT * FROM table WHERE ID=234 OR 1=1. As a result, the server retrieves every row in the table. This exposes every row and causes excessive server load. The simplest way to protect from this type of attack is to use single quotes around the numeric constants: 'SELECT * FROM table WHERE ID=''234'. If the user enters extra information, it all becomes part of the string. In a numeric context, MySQL automatically converts this string to a number and strips any trailing nonnumeric characters from it.

  Sometimes people think that if a database contains only publicly available data, it need not be protected. This is incorrect. Even if it is allowable to display any row in the database, you should still protect against denial of service attacks (for example, those that are based on the technique in the preceding paragraph that causes the server to waste resources). Otherwise, your server becomes unresponsive to legitimate users.

  Checklist:

  • Try to enter single and double quote marks ( ''' and '"') in all of your Web forms. If you get any kind of MySQL error, investigate the problem right away.

  • Try to modify dynamic URLs by adding %22 ( '''), %23 ('#'), and %27 ( ''') to them.

  • Try to modify data types in dynamic URLs from numeric to character types using the characters shown in the previous examples. Your application should be safe against these and similar attacks.

  • Try to enter characters, spaces, and special symbols rather than numbers in numeric fields. Your application should remove them before passing them to MySQL or else generate an error. Passing unchecked values to MySQL is very dangerous!

  • Check the size of data before passing it to MySQL.

  • Have your application connect to the database using a username different from the one you use for administrative purposes. Do not give your applications any access privileges they do not need.

• Many application programming interfaces provide a means of escaping special characters in data values. Properly used, this prevents application users from entering values that cause the application to generate statements that have a different effect than you intend:

  • MySQL C API: Use the mysql_real_escape_string() API call.

  • MySQL++: Use the escape and quote modifiers for query streams.

  • PHP: Use the mysql_escape_string() function, which is based on the function of the same name in the MySQL C API. (Prior to PHP 4.0.3, use addslashes() instead.) In PHP 5, you can use the mysqli extension, which supports the improved MySQL authentication protocol and passwords, as well as prepared statements with placeholders.

  • Perl DBI: Use the quote() method or use placeholders.

  • Ruby DBI: Use placeholders.

  • Java JDBC: Use a PreparedStatement object and placeholders.

  Other programming interfaces might have similar capabilities.

• Do not transmit plain (unencrypted) data over the Internet. This information is accessible to everyone who has the time and ability to intercept it and use it for their own purposes. Instead, use an encrypted protocol such as SSL or SSH. MySQL supports internal SSL connections as of version 4.0. Another technique is to use SSH port-forwarding to create an encrypted (and compressed) tunnel for the communication.

• Learn to use the tcpdump and strings utilities. In most cases, you can check whether MySQL data streams are unencrypted by issuing a command like the following:

  shell> tcpdump -l -i eth0 -w - src or dst port 3306 | strings
  

  (This works under Linux and should work with small modifications under other systems.) Warning: If you do not see plaintext data, this doesn't always mean that the information actually is encrypted. If you need high security, you should consult with a security expert.

4.7.2. Making MySQL Secure Against Attackers

When you connect to a MySQL server, you should use a password. The password is not transmitted in clear text over the connection. Password handling during the client connection sequence was upgraded in MySQL 4.1.1 to be very secure. If you are still using pre-4.1.1-style passwords, the encryption algorithm is not as strong as the newer algorithm. With some effort, a clever attacker who can sniff the traffic between the client and the server can crack the password. (See Section 4.8.9, "Password Hashing as of MySQL 4.1," for a discussion of the different password handling methods.)

All other information is transferred as text, and can be read by anyone who is able to watch the connection. If the connection between the client and the server goes through an untrusted network, and you are concerned about this, you can use the compressed protocol to make traffic much more difficult to decipher. You can also use MySQL's internal SSL support to make the connection even more secure. See Section 4.9.7, "Using Secure Connections." Alternatively, use SSH to get an encrypted TCP/IP connection between a MySQL server and a MySQL client. You can find an Open Source SSH client at http://www.openssh.org/, and a commercial SSH client at http://www.ssh.com/.

To make a MySQL system secure, you should strongly consider the following suggestions:

• Require all MySQL accounts to have a password. A client program does not necessarily know the identity of the person running it. It is common for client/server applications that the user can specify any username to the client program. For example, anyone can use the mysql program to connect as any other person simply by invoking it as mysql -uother_user db_name if other_user has no password. If all accounts have a password, connecting using another user's account becomes much more difficult.

  For a discussion of methods for setting passwords, see Section 4.9.5, "Assigning Account Passwords."

• Never run the MySQL server as the Unix root user. This is extremely dangerous, because any user with the FILE privilege is able to cause the server to create files as root (for example, ~root/.bashrc). To prevent this, mysqld refuses to run as root unless that is specified explicitly using the --user=root option.

  mysqld can (and should) be run as an ordinary, unprivileged user instead. You can create a separate Unix account named mysql to make everything even more secure. Use this account only for administering MySQL. To start mysqld as a different Unix user, add a user option that specifies the username in the [mysqld] group of the my.cnf option file where you specify server options. For example:

  [mysqld]
  user=mysql
  

  This causes the server to start as the designated user whether you start it manually or by using mysqld_safe or mysql.server. For more details, see Section 4.7.5, "How to Run MySQL As a Normal User."

  Running mysqld as a Unix user other than root does not mean that you need to change the root username in the user table. Usernames for MySQL accounts have nothing to do with usernames for Unix accounts.

• Do not allow the use of symlinks to tables. (This capability can be disabled with the --skip-symbolic-links option.) This is especially important if you run mysqld as root, because anyone that has write access to the server's data directory then could delete any file in the system! See Section 6.6.1.2, "Using Symbolic Links for Tables on Unix."

• Make sure that the only Unix user with read or write privileges in the database directories is the user that mysqld runs as.

• Do not grant the PROCESS or SUPER privilege to non-administrative users. The output of mysqladmin processlist and SHOW PROCESSLIST shows the text of any statements currently being executed, so any user who is allowed to see the server process list might be able to see statements issued by other users such as UPDATE user SET password= PASSWORD( not_secure').

  mysqld reserves an extra connection for users who have the SUPER privilege, so that a MySQL root user can log in and check server activity even if all normal connections are in use.

  The SUPER privilege can be used to terminate client connections, change server operation by changing the value of system variables, and control replication servers.

• Do not grant the FILE privilege to non-administrative users. Any user who has this privilege can write a file anywhere in the filesystem with the privileges of the mysqld daemon. To make this a bit safer, files generated with SELECT ... INTO OUTFILE do not overwrite existing files and are writable by everyone.

  The FILE privilege may also be used to read any file that is world-readable or accessible to the Unix user that the server runs as. With this privilege, you can read any file into a database table. This could be abused, for example, by using LOAD DATA to load /etc/passwd into a table, which then can be displayed with SELECT.

• If you do not trust your DNS, you should use IP numbers rather than hostnames in the grant tables. In any case, you should be very careful about creating grant table entries using hostname values that contain wildcards.

• If you want to restrict the number of connections allowed to a single account, you can do so by setting the max_user_connections variable in mysqld. The GRANT statement also supports resource control options for limiting the extent of server use allowed to an account.

4.7.3. Security-Related mysqld Options

The following mysqld options affect security:

• --allow-suspicious-udfs

  This option controls whether user-defined functions that have only an xxx symbol for the main function can be loaded. By default, the option is off and only UDFs that have at least one auxiliary symbol can be loaded; this prevents attempts at loading functions from shared object files other than those containing legitimate UDFs. For MySQL 5.0, this option was added in MySQL 5.0.3.

• --local-infile[={0|1}]

  If you start the server with --local-infile=0, clients cannot use LOCAL in LOAD DATA statements. See Section 4.7.4, "Security Issues with LOAD DATA LOCAL."

• --old-passwords

  Force the server to generate short (pre-4.1) password hashes for new passwords. This is useful for compatibility when the server must support older client programs. See Section 4.8.9, "Password Hashing as of MySQL 4.1."

• --safe-show-database (OBSOLETE)

  In previous versions of MySQL, this option caused the SHOW DATABASES statement to display the names of only those databases for which the user had some kind of privilege. In MySQL 5.0, this option is no longer available because this is now the default behavior, and there is a SHOW DATABASES privilege that can be used to control access to database names on a per-account basis.

• --safe-user-create

  If this option is enabled, a user cannot create new MySQL users by using the GRANT statement unless the user has the INSERT privilege for the mysql.user table. If you want a user to have the ability to create new users that have those privileges that the user has right to grant, you should grant the user the following privilege: GRANT INSERT(user) ON mysql.user TO user_name'@'host_name';

  This ensures that the user cannot change any privilege columns directly, but has to use the GRANT statement to give privileges to other users.

• --secure-auth

  Disallow authentication for accounts that have old (pre-4.1) passwords.

  The mysql client also has a --secure-auth option, which prevents connections to a server if the server requires a password in old format for the client account.

• --skip-grant-tables

  This option causes the server not to use the privilege system at all. This gives anyone with access to the server unrestricted access to all databases. You can cause a running server to start using the grant tables again by executing mysqladmin flush-privileges or mysqladmin reload command from a system shell, or by issuing a MySQL FLUSH PRIVILEGES statement. This option also suppresses loading of user-defined functions (UDFs).

• --skip-name-resolve

  Hostnames are not resolved. All Host column values in the grant tables must be IP numbers or localhost.

• --skip-networking

  Do not allow TCP/IP connections over the network. All connections to mysqld must be made via Unix socket files.

• --skip-show-database

  With this option, the SHOW DATABASES statement is allowed only to users who have the SHOW DATABASES privilege, and the statement displays all database names. Without this option, SHOW DATABASES is allowed to all users, but displays each database name only if the user has the SHOW DATABASES privilege or some privilege for the database. Note that any global privilege is a privilege for the database.

4.7.4. Security Issues with LOAD DATA LOCAL

The LOAD DATA statement can load a file that is located on the server host, or it can load a file that is located on the client host when the LOCAL keyword is specified.

There are two potential security issues with supporting the LOCAL version of LOAD DATA statements:

• The transfer of the file from the client host to the server host is initiated by the MySQL server. In theory, a patched server could be built that would tell the client program to transfer a file of the server's choosing rather than the file named by the client in the LOAD DATA statement. Such a server could access any file on the client host to which the client user has read access.

• In a Web environment where the clients are connecting from a Web server, a user could use LOAD DATA LOCAL to read any files that the Web server process has read access to (assuming that a user could run any command against the SQL server). In this environment, the client with respect to the MySQL server actually is the Web server, not the remote program being run by the user who connects to the Web server.

To deal with these problems, we changed how LOAD DATA LOCAL is handled as of MySQL 3.23.49 and MySQL 4.0.2 (4.0.13 on Windows):

• By default, all MySQL clients and libraries in binary distributions are compiled with the --enable-local-infile option to be compatible with MySQL 3.23.48 and before.

• If you build MySQL from source but do not invoke configure with the --enable-local-infile option, LOAD DATA LOCAL cannot be used by any client unless it is written explicitly to invoke mysql_options(... MYSQL_OPT_LOCAL_INFILE, 0).

• You can disable all LOAD DATA LOCAL commands from the server side by starting mysqld with the --local-infile=0 option.

• For the mysql command-line client, LOAD DATA LOCAL can be enabled by specifying the --local-infile[=1] option, or disabled with the --local-infile=0 option. Similarly, for mysqlimport, the --local or -L option enables local data file loading. In any case, successful use of a local loading operation requires that the server is enabled to allow it.

• If you use LOAD DATA LOCAL in Perl scripts or other programs that read the [client] group from option files, you can add the local-infile=1 option to that group. However, to keep this from causing problems for programs that do not understand local-infile, specify it using the loose- prefix:

  [client]
  loose-local-infile=1
  

• If LOAD DATA LOCAL INFILE is disabled, either in the server or the client, a client that attempts to issue such a statement receives the following error message:

  ERROR 1148: The used command is not allowed with this MySQL version
  

4.7.5. How to Run MySQL As a Normal User

On Windows, you can run the server as a Windows service using a normal user account.

On Unix, the MySQL server mysqld can be started and run by any user. However, you should avoid running the server as the Unix root user for security reasons. To change mysqld to run as a normal unprivileged Unix user user_name, you must do the following:

shell> chown -R user_name /path/to/mysql/datadir

[mysqld]
user=user_name

If your Unix machine itself isn't secured, you should assign passwords to the MySQL root accounts in the grant tables. Otherwise, any user with a login account on that machine can run the mysql client with a --user=root option and perform any operation. (It is a good idea to assign passwords to MySQL accounts in any case, but especially so when other login accounts exist on the server host.) See Section 2.9, "Post-Installation Setup and Testing."