SQL injection is a strategy often taken by hackers for attacking databases.
An ASP page asks the user for a name and a password, and then sends the following string to the database:
SELECT FROM users WHERE username = 'whatever' AND password = 'mypassword'
It seems safe, but it isn't. A user might enter something like this as her user name:
' OR 1>0 --
When this is plugged into the SQL statement, the result looks like this:
SELECT FROM users WHERE username = '' OR 1>0 -- AND password = ''
This injection comments out the password portion of the statement. It results in a list of all the names in the users table, so any user could get into your system.
The easiest way to prevent this sort of injection is to parse the SQL string and remove any occurrences of "--" before passing the statement.
You also have to beware of injections that contain semicolons because semicolons delimit SQL statements. Think about the implications of a user name like this:
' OR 1>0 ; DELETE Customers ; --
There are numerous ways a malicious user might penetrate your system using SQL injection and various defenses, but the simplest approach is to avoid dynamic SQL. Instead, use stored procedures everywhere.
Thanks to the way SQL passes parameters, injections such as those above will produce errors, and the stored procedure will not execute.