The Web Application Hacker's Handbook, 2nd Edition

Chapter 6Attacking Authentication

On the face of it, authentication is conceptually among the simplest of all the security mechanisms employed within web applications. In the typical case, a user supplies her username and password, and the application must verify that these items are correct. If so, it lets the user in. If not, it does not.

Authentication also lies at the heart of an application's protection against malicious attack. It is the front line of defense against unauthorized access. If an attacker can defeat those defenses, he will often gain full control of the application's functionality and unrestricted access to the data held within it. Without robust authentication to rely on, none of the other core security mechanisms (such as session management and access control) can be effective.

In fact, despite its apparent simplicity, devising a secure authentication function is a subtle business. In real-world web applications authentication often is the weakest link, which enables an attacker to gain unauthorized access. The authors have lost count of the number of applications we have fundamentally compromised as a result of various defects in authentication logic.

This chapter looks in detail at the wide variety of design and implementation flaws that commonly afflict web applications. These typically arise because application designers and developers fail to ask a simple question: What could an attacker achieve if he targeted our authentication mechanism? In the ...