Web application security remains a fun and thriving subject. We enjoyed writing this book as much as we continue to enjoy hacking into web applications on a daily basis. We hope that you will also take pleasure from learning about the different techniques we describe and how you can defend against them.
Before going any further, we should mention an important caveat. In most countries, attacking computer systems without the owner's permission is against the law. The majority of the techniques we describe are illegal if carried out without consent.
The authors are professional penetration testers who routinely attack web applications on behalf of clients to help them improve their security. In recent years, numerous security professionals and others have acquired criminal records — and ended their careers — by experimenting on or actively attacking computer systems without permission. We urge you to use the information contained in this book only for lawful purposes.