MySQL 5.6 Reference Manual Including MySQL NDB Cluster 7.3-7.4 Reference Guide
Applications that access MySQL should not trust any data entered
by users, who 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 tries to perform SQL injection by entering
something like ; DROP DATABASE mysql; into a
form. 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 quotation marks 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 permissible 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:
Enable strict SQL mode to tell the server to be more restrictive of what data values it accepts. See Section 5.1.10, “Server SQL Modes”.
Try to enter single and double quotation 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 user name 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 SQL statements: Use SQL prepared statements and accept data values only by means of placeholders; see Section 13.5, “Prepared Statements”.
MySQL C API: Use the
mysql_real_escape_string() API
call. Alternatively, use the C API prepared statement
interface and accept data values only by means of
placeholders; see C API Prepared Statements.
MySQL++: Use the escape and
quote modifiers for query streams.
PHP: Use either the mysqli or
pdo_mysql extensions, and not the older
ext/mysql extension. The preferred API's
support the improved MySQL authentication protocol and
passwords, as well as prepared statements with placeholders.
See also Choosing an API.
If the older ext/mysql extension must be
used, then for escaping use the
mysql_real_escape_string()
function and not
mysql_escape_string() or
addslashes() because only
mysql_real_escape_string() is
character set-aware; the other functions can be
“bypassed” when using (invalid) multibyte
character sets.
Perl DBI: Use placeholders or the quote()
method.
Ruby DBI: Use placeholders or the quote()
method.
Java JDBC: Use a PreparedStatement object
and placeholders.
Other programming interfaces might have similar capabilities.
Applications should be prepared to handle errors that occur as a result of executing SQL statements, and take appropriate action. A simple approach is to intercept error messages from the database server and display them verbatim to the client. However, this runs the risk of exposing information that should remain private. For example, an application that displays a message such as this exposes both a database name and a table name to clients, which is information a client might attempt to exploit:
ERROR 1146 (42S02): Table 'mydb.mytable' doesn't exist
Instead, the application can use the error code to determine what the issue was and display to clients a message that does not expose sensitive information.