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698 Chapter 18 n Attacking the Application Server At a high level, we do not need to distinguish between a global input validation filter, host-based agent, or network-based web application firewall. The following steps apply to all in equal measure. HACK STEPS The presence of a web application firewall can be deduced using the following steps: 1. Submit an arbitrary parameter name to the application with a clear attack payload in the value, ideally somewhere the application includes the name and/or value in the response. If the application blocks the attack, this is probably due to an external defense. 2. If a variable can be submitted that is returned in a server response, submit a range of fuzz strings and encoded variants to identify the behavior of the application defenses to user input. 3. Confirm this behavior by performing the same attacks on variables within the application. You can try the following strings to attempt to bypass a web application firewall: 1. For all fuzzing strings and requests, use benign strings for payloads that are unlikely to exist in a standard signature database. Giving examples of these is, by definition, not possible. But you should avoid using /etc/passwd or /windows/system32/config/sam as payloads for file retrieval. Also avoid using terms such as in an XSS attack and using alert() or xss as XSS payloads. 2. If a particular request is blocked, try submitting the same parameter in a different location or context. For instance, submit the same parameter in the URL in a GET request, within the body of a POST request, and within the URL in a POST request. 3. On ASP.NET, also try submitting the parameter as a cookie. The API Request.Params[“foo”] retrieves the value of a cookie named foo if the parameter foo is not found in the query string or message body. 4. Review all the other methods of introducing user input provided in Chapter 4, choosing any that are unprotected. 5. Determine locations where user input is (or can be) submitted in a nonstandard format such as serialization or encoding. If none are available, build the attack string by concatenation and/or by spanning it across multiple variables. (Note that if the target is ASP.NET, you may be able to use HPP to concatenate the attack using multiple specifications of the same variable.) Many organizations that deploy web application firewalls or IDSs do not have them specifically tested according to a methodology like the one described in this section. As a result, it is often worth persevering in an attack against such devices.
Chapter 18 n Attacking the Application Server 699 Summary As with the other components on which a web application runs, the web server represents a significant area of attack surface via which an application may be compromised. Defects in an application server can often directly undermine an application’s security by giving access to directory listings, source code for executable pages, sensitive configuration and runtime data, and the ability to bypass input filters. Because of the wide variety of application server products and versions, locating web server vulnerabilities usually involves some reconnaissance and research. However, this is one area in which automated scanning tools can be highly effective at quickly locating known vulnerabilities within the configuration and software of the server you are attacking. Questions Answers can be found at http://mdsec.net/wahh. 1. Under what circumstances does a web server display a directory listing? 2. What are WebDAV methods used for, and why might they be dangerous? 3. How can you exploit a web server that is configured to act as a web proxy? 4. What is the Oracle PL/SQL Exclusion List, and how can it be bypassed? 5. If a web server allows access to its functionality over both HTTP and HTTPS, are there any advantages to using one protocol over the other when you are probing for vulnerabilities?
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The Web Application Hacker’s Hand
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About the Authors Dafydd Stuttard i
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MDSec: The Authors’ Company Dafyd
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Acknowledgments We are indebted to
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Contents Introduction xxiii Chapter
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Contents xi Chapter 5 Bypassing Cli
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Contents xiii Testing Restrictions
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Contents xv Payloads for XSS Attack
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Contents xvii “Off-by-One” Vuln
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Contents xix Technical Challenges F
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Contents xxi 12 Miscellaneous Check
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xxiv Introduction attacker. If you
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xxviii Introduction internal workin
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CHAPTER 1 Web Application (In)secur
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Chapter 1 n Web Application (In)sec
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CHAPTER 5 Bypassing Client-Side Con
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CHAPTER 7 Attacking Session Managem
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CHAPTER 8 Attacking Access Controls
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Chapter 8 n Attacking Access Contro
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288 Chapter 9 n Attacking Data Stor
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358 Chapter 10 n Attacking Back-End
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406 Chapter 11 n Attacking Applicat
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CHAPTER 16 Attacking Native Compile
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854 Index n A-A hacker’s methodol
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856 Index n B-B meaningful, 212 URL
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858 Index n C-C XSS, 450-451 “cha
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860 Index n D-D CSS. See Cascading
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862 Index n F-F detecting hits, 574
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864 Index n H-H script injection, 8
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866 Index n J-J error messages, 388
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868 Index n M-O secret questions, 1
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870 Index n Q-R user input, 379-380
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872 Index n S-S second-order XSS. S
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874 Index n T-T signatures of commo
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876 Index n V-W provisos, 305-306 S
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878 Index n X-Z WHERE clause DELETE
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