5.7. General Security Issues

MySQL 5.0

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5.7. 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 5.8, “The MySQL Access Privilege System”.

5.7.1. 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 accounts) access to the table in the database! This is critical.

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

    Checklist:

    • Try . 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 user with full privileges! Review the MySQL installation instructions, paying particular attention to the information about setting a password. See Section 2.10.3, “Securing the Initial MySQL Accounts”.

    • Use the statement to check which accounts have access to what. Then use the 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 , , 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 . 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 is the hostname or IP number of the host on which your MySQL server runs: 

      shell>  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 “”. 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 when a user enters the value , the user can enter the value to cause the application to generate the query . 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: . 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 non-numeric 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 (‘’), (‘’), and (‘’) 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 API call.

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

    • PHP: Use the function (available as of PHP 4.3.0, prior to that PHP version use , and prior to PHP 4.0.3, use ). Note that only is character set-aware; the other functions can be “bypassed” when using (invalid) multi-byte character sets. In PHP 5, you can use the extension, which supports the improved MySQL authentication protocol and passwords, as well as prepared statements with placeholders.

    • Perl DBI: Use placeholders or the method.

    • Ruby DBI: Use placeholders or the method.

    • Java JDBC: Use a 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>

    (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.

5.7.2. 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 5.8.9, “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 5.9.7, “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 if has no password. If all account have a password, connecting using another user's account becomes much more difficult.

    For a discussion of methods for setting passwords, see Section 5.9.5, “Assigning Account Passwords”.

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

    mysqld can (and should) be run as an ordinary, unprivileged user instead. You can create a separate Unix account named to make everything even more secure. Use this account only for administering MySQL. To start mysqld as a different Unix user, add a option that specifies the username in the group of the 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 5.7.5, “How to Run MySQL as a Normal User”.

    Running mysqld as a Unix user other than does not mean that you need to change the username in the 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 option.) This is especially important if you run mysqld as , because anyone that has write access to the server's data directory then could delete any file in the system! See Section 7.6.1.2, “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 or privilege to non-administrative users. The output of mysqladmin processlist and 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 .

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

    The 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 privilege to non-administrative users. Any user that 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 do not overwrite existing files and are writable by everyone.

    The 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 to load into a table, which then can be displayed with .

  • 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 variable in mysqld. The statement also supports resource control options for limiting the extent of server use allowed to an account. See Section 13.5.1.3, “ Syntax”.

  • Options that begin with specify whether to allow clients to connect via SSL and indicate where to find SSL keys and certificates. See Section 5.9.7.3, “SSL Command Options”.

5.7.3. Security-Related mysqld Options

The following mysqld options affect security:

  • This option controls whether user-defined functions that have only an 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. See Section 24.2.4.6, “User-Defined Function Security Precautions”.

  • If you start the server with , clients cannot use in statements. See Section 5.7.4, “Security Issues with ”.

  • 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 5.8.9, “Password Hashing as of MySQL 4.1”.

  • (OBSOLETE)

    In previous versions of MySQL, this option caused the 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 as this is now the default behavior, and there is a privilege that can be used to control access to database names on a per-account basis. See Section 13.5.1.3, “ Syntax”.

  • If this option is enabled, a user cannot create new MySQL users by using the statement unless the user has the privilege for the 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 ''@'';

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

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

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

  • 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 statement. This option also suppresses loading of user-defined functions (UDFs).

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

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

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

5.7.4. Security Issues with LOAD DATA LOCAL

The 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 keyword is specified.

There are two potential security issues with supporting the version of 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 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 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 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 option, to be compatible with MySQL 3.23.48 and before.

  • If you build MySQL from source but do not invoke configure with the option, cannot be used by any client unless it is written explicitly to invoke . See Section 22.2.3.48, “”.

  • You can disable all commands from the server side by starting mysqld with the option.

  • For the mysql command-line client, can be enabled by specifying the option, or disabled with the option. Similarly, for mysqlimport, the or 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 in Perl scripts or other programs that read the group from option files, you can add the option to that group. However, to keep this from causing problems for programs that do not understand , specify it using the prefix: 

    [client]
loose-local-infile=1

  • If 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

5.7.5. 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 user for security reasons. To change mysqld to run as a normal unprivileged Unix user , you must do the following:

  1. Stop the server if it's running (use mysqladmin shutdown).

  2. Change the database directories and files so that has privileges to read and write files in them (you might need to do this as the Unix user): 

    shell>

    If you do not do this, the server will not be able to access databases or tables when it runs as .

    If directories or files within the MySQL data directory are symbolic links, you'll also need to follow those links and change the directories and files they point to. might not follow symbolic links for you.

  3. Start the server as user . If you are using MySQL 3.22 or later, another alternative is to start mysqld as the Unix user and use the option. mysqld starts up, then switches to run as the Unix user before accepting any connections.

  4. To start the server as the given user automatically at system startup time, specify the username by adding a option to the group of the option file or the option file in the server's data directory. For example: 

    [mysqld]
user=

If your Unix machine itself isn't secured, you should assign passwords to the MySQL accounts in the grant tables. Otherwise, any user with a login account on that machine can run the mysql client with a 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 2.10, “Post-Installation Setup and Testing”.

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