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Language: SQL
Expertise: Intermediate

May 23, 2005

A common mistake many developers make is using their MySQL database to validate a user name and password combination.

Here's a typical SQL query checking for the existence of a user trying to login:

SELECT 1 FROM <USERTABLE> WHERE USERNAME="<USERNAME>" AND 

USERPASSWORD="<USERPASSWORD>"

If a record matches, the database returns that record and the process continues.

But in MySQL, the above query allows a person with knowledge of the just the USERNAME alone to gain access to the system without much difficulty.

For instance, suppose a user keys in the value for USERNAME as <USERNAME>##" and for PASSWORD, he keys in anything. Because MySQL interprets the character "##" as a comment, the query is terminated when the "##" character is found and there definitely will be a record matching just the username. The query thus returns a record and allows this user to login.

Here's an effective query:

SELECT 1 FROM <USERTABLE> WHERE USERNAME="<USERNAME>"

Always be sure to check whether the username contatins a character like "##" and throw an appropriate error before even sending it to the database.