The Web Application Hacker’s Handbook Discovering and Exploiting Security Flaws

Chapter 6 

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Attacking Authentication

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N OT E

The subtleties of devising a secure authentication mechanism run 

deep here. If care is not taken in the asking of a randomly varying question,

then this can lead to new opportunities for username enumeration. For

example, in order to prevent an attacker from choosing his own question, an

application may store within each user’s profile the last question that user was

asked, and continue presenting that question until the user answers it correctly.

An attacker who initiates several logins using any given user’s username will 

be met with the same question. However, if the attacker carries out the same