Book Image

ModSecurity 2.5

Book Image

ModSecurity 2.5

Overview of this book

With more than 67% of web servers running Apache and web-based attacks becoming more and more prevalent, web security has become a critical area for web site managers. Most existing tools work on the TCP/IP level, failing to use the specifics of the HTTP protocol in their operation. Mod_security is a module running on Apache, which will help you overcome the security threats prevalent in the online world. A complete guide to using ModSecurity, this book will show you how to secure your web application and server, and does so by using real-world examples of attacks currently in use. It will help you learn about SQL injection, cross-site scripting attacks, cross-site request forgeries, null byte attacks, and many more so that you know how attackers operate. Using clear, step-by-step instructions this book starts by teaching you how to install and set up ModSecurity, before diving into the rule language with examples. It assumes no prior knowledge of ModSecurity, so as long as you are familiar with basic Linux administration, you can start to learn right away. Real-life case studies are used to illustrate the dangers on the Web today ñ you will for example learn how the recent worm that hit Twitter works, and how you could have used ModSecurity to stop it in its tracks. The mechanisms behind these and other attacks are described in detail, and you will learn everything you need to know to make sure your server and web application remain unscathed on the increasingly dangerous web. Have you ever wondered how attackers figure out the exact web server version running on a system? They use a technique called HTTP fingerprinting, and you will learn about this in depth and how to defend against it by flying your web server under a "false flag". The last part of the book shows you how to really lock down a web application by implementing a positive security model that only allows through requests that conform to a specific, pre-approved model, and denying anything that is even the slightest bit out of line.
Table of Contents (17 chapters)
ModSecurity 2.5
Credits
About the Author
About the Reviewers
Preface
Directives and Variables
Index

Blocking proxied requests


Requests routed via proxy servers can be problematic for some sites. If you run any type of discussion forum, users can hide behind the perceived anonymity of a proxy server and launch anything from profanity-laden tirades in forum posts to outright denial of service attacks. You may therefore want to block proxied requests if you find that they cause problems on your site.

One way to do this is to check for the presence of the X-Forwarded-For header in the HTTP request. If this header exists, it means that the request was made by a proxy server on behalf of the real user.

This rule detects and blocks requests by proxy servers that use the X-Forwarded-For header:

SecRule &REQUEST_HEADERS:X-Forwarded-For "@gt 0" deny

The rule uses the & operator to return the number of request headers present with the name X-Forwarded-For. If this number is greater than zero, that means such a header is present, and the request is blocked.

Another similar header used by...