SQL Injection and Prepared Statements
My teacher and I are discussing whether SQL injection into a prepared statement is possible. I understand that normally you couldn't, but the professor insists on using sql concatenation instead of using (?).
Now I am trying to break my code, but I have no luck.
public Users getUserByUsername(String username) throws SQLException {
StringBuffer sql = new StringBuffer();
sql.append("select * from users as u, user_type_lookup as l, user_types as t ");
sql.append("where u.users_id=l.user_id and l.type_id=t.user_types_id and u.username='");
sql.append(username);
sql.append("';");
System.out.println(sql.toString());
PreparedStatement ps = conn.prepareStatement(sql.toString());
ResultSet rs = ps.executeQuery(sql.toString());
if (!rs.next()) {
return null;
}
String password = rs.getString("password");
String type = rs.getString("description");
int id = rs.getInt("users_id");
int incorrect_logins = rs.getInt("incorrect_logins");
Time wait_time = rs.getTime("wait_time");
Users u = new Users(id, username, password, type, incorrect_logins,
wait_time);
return u;
}
The inserts I've tried:
string: '; DELETE FROM users WHERE 1 or users_id = '
string: ';delete from users where username<>'
//The only one that worked
string: stan' or 'a'<>'b
SQL output (results in java error):
select * from users as u, user_type_lookup as l, user_types as t where u.users_id=l.user_id and l.type_id=t.user_types_id and u.username=''; DELETE FROM users WHERE 1 or users_id = '';
SQL output (works as intended):
select * from users as u, user_type_lookup as l, user_types as t where u.users_id=l.user_id and l.type_id=t.user_types_id and u.username='stan';
Error message:
com.mysql.jdbc.exceptions.jdbc4.MySQLSyntaxErrorException: You have an error in your
SQL syntax; check the manual that corresponds to your MySQL server version for the
right syntax to use near 'DELETE FROM users WHERE 1 or users_id = ''' at line 1
Server: Tomcat 7
Database: MySQL
IDE: Eclipse
Language: Java
So please help me to break my code!
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1 answer
You cannot add a separate statement inside the SQL prepared statement, but it can be broken down like:
- using
' OR 'x' = 'x
as the username (so the query will perform a Cartesian join between all users and display types between them); it will hurt performance a lot ifusers
theyuser_type_lookup
are large tables and would be a great start to a denial of service attack. - using
' OR (SELECT stored_procedure_that_deletes_things()) = 1
(so the query will call the stored procedure, which has harmful effects).
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