29-03-2011, 04:04 PM
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What is a SQL Injection Attack?
• Many web applications take user input from a form
• Often this user input is used literally in the construction of a SQL query submitted to a database. For example:
– SELECT productdata FROM table WHERE productname = ‘user input product name’;
• A SQL injection attack involves placing SQL statements in the user input
• An Example SQL Injection Attack
Product Search:
• This input is put directly into the SQL statement within the Web application:
– $query = “SELECT prodinfo FROM prodtable WHERE prodname = ‘” . $_POST[‘prod_search’] . “’”;
• Creates the following SQL:
– SELECT prodinfo FROM prodtable WHERE prodname = ‘blah‘ OR ‘x’ = ‘x’
– Attacker has now successfully caused the entire database to be returned.
A More Malicious Example
• What if the attacker had instead entered:
– blah‘; DROP TABLE prodinfo; --
• Results in the following SQL:
– SELECT prodinfo FROM prodtable WHERE prodname = ‘blah’; DROP TABLE prodinfo; --’
– Note how comment (--) consumes the final quote
• Causes the entire database to be deleted
– Depends on knowledge of table name
– This is sometimes exposed to the user in debug code called during a database error
– Use non-obvious table names, and never expose them to user
• Usually data destruction is not your worst fear, as there is low economic motivation
Other injection possibilities
• Using SQL injections, attackers can:
– Add new data to the database
• Could be embarrassing to find yourself selling politically incorrect items on an eCommerce site
• Perform an INSERT in the injected SQL
– Modify data currently in the database
• Could be very costly to have an expensive item suddenly be deeply ‘discounted’
• Perform an UPDATE in the injected SQL
– Often can gain access to other user’s system capabilities by obtaining their password
Defenses
• Use provided functions for escaping strings
– Many attacks can be thwarted by simply using the SQL string escaping mechanism
• ‘ à \’ and “ à \”
– mysql_real_escape_string() is the preferred function for this
• Not a silver bullet!
– Consider:
• SELECT fields FROM table WHERE id = 23 OR 1=1
• No quotes here!
More Defenses
• Check syntax of input for validity
– Many classes of input have fixed languages
• Email addresses, dates, part numbers, etc.
• Verify that the input is a valid string in the language
• Sometime languages allow problematic characters (e.g., ‘*’ in email addresses); may decide to not allow these
• If you can exclude quotes and semicolons that’s good
– Not always possible: consider the name Bill O’Reilly
• Want to allow the use of single quotes in names
• Have length limits on input
– Many SQL injection attacks depend on entering long strings
Even More Defenses
• Scan query string for undesirable word combinations that indicate SQL statements
– INSERT, DROP, etc.
– If you see these, can check against SQL syntax to see if they represent a statement or valid user input
• Limit database permissions and segregate users
– If you’re only reading the database, connect to database as a user that only has read permissions
– Never connect as a database administrator in your web application
More Defenses
• Configure database error reporting
– Default error reporting often gives away information that is valuable for attackers (table name, field name, etc.)
– Configure so that this information is never exposed to a user
• If possible, use bound variables
– Some libraries allow you to bind inputs to variables inside a SQL statement
– PERL example (from http://unixwiztechtips/sql-injection.html)
$sth = $dbh->prepare("SELECT email, userid FROM members WHERE email = ?;");
$sth->execute($email);