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Client Programming Security Guidelines

573

6.1.7. Client Programming Security Guidelines

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 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 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 5.1.7, “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:

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