The SQL Injection attack is possible when the programmers who write the code behind the page neglect to properly escape strings that are used in SQL queries.
Programs common build SQL queries such to check values. For example
the SQL statement:
will return one row if the given user/password combination exists in the
SELECT name FROM users WHERE name='scott' AND password='tiger'
Of course the values
tiger are taken
from values entered by the user. To build the SQL query the Perl
programmer might use a line such as:
$sql = "SELECT name FROM users WHERE name='$Q::name' AND password='$Q::password'"
The VB programmer might use something like:
sql = "SELECT name FROM users WHERE name='" & name & "' AND '" & password & "'"
In both cases the
sql string generated will be invalid SQL
if the variable
name contains a single quote.
Worse, a sneaky user might enter a string that results in valid SQL, but SQL that generates unexpected reults.
The cure is simply to escape single quotes properly. In most cases that mean substitute a single quote with two single quotes. In Perl you could use:
$sql = sprintf 'SELECT name FROM users WHERE name=%s AND password=%s, $dbh->quote($Q::name),$dbh->quote($Q::password);
In VB you could use:
sql = "SELECT name FROM users WHERE name='" & replace(name,"'","''") & _ "' AND password='" & replace(password,"'","''")