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140 Chapter 5 n Bypassing Client-Side Controls Subject to these obfuscation defenses, decompiling bytecode normally is the preferable route to understanding and attacking browser extension components. This allows you to review business logic, assess the full functionality of the client-side application, and modify its behavior in targeted ways. Downloading the Bytecode The first step is to download the executable bytecode for you to start working on. In general, the bytecode is loaded in a single file from a URL specified within the HTML source code for application pages that run the browser extension. Java applets generally are loaded using the tag, and other components generally are loaded using the tag. For example: In some cases, the URL that loads the bytecode may be less immediately obvious, since the component may be loaded using various wrapper scripts provided by the different browser extension frameworks. Another way to identify the URL for the bytecode is to look in your proxy history after your browser has loaded the browser extension. If you take this approach, you need to be aware of two potential obstacles: n Some proxy tools apply filters to the proxy history to hide from view items such as images and style sheet files that you generally are less interested in. If you cannot find a request for the browser extension bytecode, you should modify the proxy history display filter so that all items are visible. n Browsers usually cache the downloaded bytecode for extension components more aggressively than they do for other static resources such as images. If your browser has already loaded the bytecode for a component, even doing a full refresh for a page that uses the component may not cause the browser to request the component again. In this eventuality, you may need to fully clear your browser’s cache, shut down every instance of the browser, and then start a fresh browser session to force your browser to request the bytecode again. When you have identified the URL for the browser extension’s bytecode, usually you can just paste this URL into your browser’s address bar. Your browser then prompts you to save the bytecode file on your local filesystem. TIP If you have identified the request for the bytecode in your Burp Proxy history, and the server’s response contains the full bytecode (and not a reference to an earlier cached copy), you can save the bytecode directly to file
Chapter 5 n Bypassing Client-Side Controls 141 from within Burp. The most reliable way to do this is to select the Headers tab within the response viewer, right-click the lower pane containing the response body, and select Copy to File from the context menu. Decompiling the Bytecode Bytecode usually is distributed in a single-file package, which may need to be unpacked to obtain the individual bytecode files for decompilation into source code. Java applets normally are packaged as .jar (Java archive) files, and Silverlight objects are packaged as .xap files. Both of these file types use the zip archive format, so you can easily unpack them by renaming the files with the .zip extension and then using any zip reader to unpack them into the individual files they contain. The Java bytecode is contained in .class files, and the Silverlight bytecode is contained in .dll files. After unpacking the relevant file package, you need to decompile these files to obtain source code. Flash objects are packaged as .swf files and don’t require any unpacking before you use a decompiler. To perform the actual bytecode decompilation, you need to use some specific tools, depending on the type of browser extension technology that is being used, as described in the following sections. Java Tools Java bytecode can be decompiled to into Java source code using a tool called Jad (the Java decompiler), which is available from: www.varaneckas.com/jad Flash Tools Flash bytecode can be decompiled into ActionScript source code. An alternative approach, which is often more effective, is to disassemble the bytecode into a human-readable form, without actually fully decompiling it into source code. To decompile and disassemble Flash, you can use the following tools: n Flasm — www.nowrap.de/flasm n Flare — www.nowrap.de/flare n SWFScan — www.hp.com/go/swfscan (this works for Actionscript 2 and 3) Silverlight Tools Silverlight bytecode can be decompiled into source code using a tool called .NET Reflector, which is available from: www.red-gate.com/products/dotnet-development/reflector/
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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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xxvi Introduction Chapters 6, 7, an
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xxviii Introduction internal workin
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xxx Introduction If you want to foc
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xxxii Introduction web users that d
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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 3 Web Application Technolog
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CHAPTER 7 Attacking Session Managem
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Chapter 7 n Attacking Session Manag
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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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432 Chapter 12 n Attacking Users: C
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CHAPTER 13 Attacking Users: Other T
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Chapter 13 n Attacking Users: Other
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616 Chapter 15 n Exploiting Informa
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CHAPTER 16 Attacking Native Compile
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Chapter 16 n Attacking Native Compi
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CHAPTER 19 Finding Vulnerabilities
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Chapter 19 n Finding Vulnerabilitie
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748 Chapter 20 n A Web Application
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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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