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February 04 2013


Effective AMF Remoting Message fuzzing with Blazer v0.3

After several weeks of extensive testing and debugging, Blazer v0.3 is finally out!
It's been a long ride since the first lines of code, back in 2011. In this post, I am going to present all new features and describe Tips&Tricks to make your AMF security testing even more effective.

If you are not familiar with Blazer, have a look at the project page: http://code.google.com/p/blazer/.
New to Burp Suite? Have a look at the video tutorials and consider to buy Instant Burp Suite Starter.

What's new?

Blazer v0.3 includes a few interesting new features presented during my DeepSec talk, but even more important is the result of extensive testing on Windows, Mac OS X and Linux using multiple Java Runtime Environments and recent Burp Suite releases.

  • Java classes and source code import feature
    In addition to JARs, it is now possible to import directories containing .class and .java files. The ability to import source code, in addition to application libraries, allows to partially use Blazer even during black-box security testing.
  • AMF request/response export functionality (AMF2XML)
    Sharing details of security vulnerabilities triggered by AMF messages was annoying, as it was not possible to export AMF requests and responses in an intelligible format. Using the AMF2XML feature, it is now possible to export those messages in a file or console.

  • Sandbox feature using a custom security manager 
    The rationale behind the introduction of this feature is to prevent any malicious action caused by application libraries. Blazer uses Java reflection and fairly complex heuristics to automatically instantiate and populate objects by using the application libraries. Application objects are created on the tester's computer and methods are locally invoked to populate attributes before sending the AMF message to the remote service. As a result, untrusted application libraries may end up writing files, opening network sockets or other involuntary IO operations.

  • Numerous bugs and performance issues fixed
    I've fixed more than 20 bugs and multiple performance issues, including an annoying GUI refresh bug on OS X and Windows. This version has been extensively tested on multiple platforms; I've specifically delayed the release to make sure that all issues I've encountered during my testing have been fixed.

BlackBox vs GrayBox testing with Blazer

Blazer is a security tool for gray-box testing. It has been designed and built with the assumption that the application libraries are available to the tester. All Java classes exchanged between client and server should be imported in the tool. This is a realistic assumption if you are doing vulnerability research, not if you are performing a standard pentest.

However, starting from this release, it is actually possible to partially use Blazer during black-box testing. If your application is using primitive types and libraries which can be downloaded from the Internet, you can benefit from Blazer's automatic objects generation by manually crafting a fake .java file including all method signatures:

1. Decompile the client-side Flex components (e.g. SWF files) or monitor the network traffic in order to enumerate all remote methods. Deblaze tool can be used for it. 

2. Create a .java file containing method signatures as observed in the traffic. Something like the following:
package flex.samples.product;
public class ProductService{
public Product getProduct(int prodId){}
3. In Blazer, import the crafted Java source file and all application libraries referenced in the application. At this stage, Blazer can be used to automatically generate objects and perform fuzzing.

Tips & Tricks 

Fuzzing complex applications containing multiple custom classes isn't trivial. To improve coverage and effectiveness, the following recommendations can save you precious time:

  • Always increment the amount of memory that your computer makes available to Burp Suite. If you are generating a large number of AMF messages, consider to chain two instances of Burp Suite. The first instance can be used to intercept the application requests and launch Blazer. In Blazer, set the proxy within tab 3 to point to the second Burp Suite instance. The latter will collect all requests generated by Blazer. In Burp Suite Pro, you can also set automatic backups to prevent any data loss.

  • As of Burp Suite v1.5.01, Burp Extender has a new API. Blazer has been improved to support both old and new Burp Extender APIs. Standard output and error can be displayed within Burp Extender, to a file or in the console screen. During testing, I suggest to redirect those streams to two separate files in order to record all operations and exceptions.

  • Balancing the number of permutations, attack vectors and probability is the magic sauce of Blazer. Read the original whitepaper/presentation, make sure to understand those settings and tune the tool. Even better, check the implementation of the ObjectGenerator class.

  • Divide et impera by breaking up numerous application method signatures into small groups. Start testing a few methods and make sure that you have imported all required application libraries. Finally, review the server responses and monitor the server's status to detect security vulnerabilities. For example - if you are looking for SQL injections - use Burp's filter by search term to identify AMF messages that triggered visible errors and grep for similar strings in the server logs. Blazer appends a custom HTTP header to all AMF requests that can be used to correlate message and method signature. Also, the newest export functionality can be used to review the AMF payload. 

  • Feel free to email me if you have any question.  Also, let me know if you find bugs using Blazer!

    January 14 2013


    Anti-debugging techniques and Burp Suite


    No matter how good a Java obfuscator is, the bytecode can still be analyzed and partially decompiled. Also, using a debugger, it is possible to dynamically observe the application behavior at runtime making reverse engineering much easier. For this reason, developers often use routines to programmatically detect the execution under a debugger in order to prevent easy access to application's internals. Unfortunately, these techniques can be also extremely annoying for people with good intents.


    Burp Suite

    Over the course of the years, starting from the very first release, I have been an enthusiastic supporter of Burp Suite. Not only @PortSwigger was able to create an amazing tool, but he also built a strong community that welcome each release as a big event. He has also been friendly and open to receive feedback from us, ready to implement suggested features. Hopefully, he won't change his attitude now.

    Since a few releases, both Burp Suite Free and Pro cannot be executed under a debugger. Unfortunately, this is a severe limitation - especially considering the latest Extensibility API.  The new extensibility framework is a game-changer: it is now possible to fully integrate custom extensions in our favorite tool. But, how to properly debug extensions in an IDE? Troubleshooting fairly complex extensions (e.g. Blazer) requires lot of debugging. Setting breakpoints, stepping in and out of methods, ... are must-have operations.

    Inspired by necessity, I spent a few hours to review the anti-debugging mechanism used in Burp Suite Free. According to Burp's EULA (Free Edition), reversing does not seem to be illegal as long as it is "essential for the purpose of achieving inter-operability". Not to facilitate any illegal activity, this post will discuss details related to the Free edition only.  
    Disclaimer: Don't be a fool, be cool. If you use Burp Pro, you must have a valid license.


    Automatic detection of a debugger

    In Java, it is possible to enable remote debugging with the following options:

    -Xdebug -agentlib:jdwp=transport=dt_socket,server=y,address=8000,suspend=n 

    and attach a debugger with:

     jdb --attach [host]:8000

    A common technique to programmatically understand if a program is running under a debugger involves checking the input arguments passed to the Java Virtual Machine. The following is the pseudo-code of a very common technique:
     for(ManagementFactory.getRuntimeMXBean().getInputArguments() ...){
                    if(Argument.contains("-Xdebug") || Argument.contains("-agentlib") ...){
                       // Do something annoying for the user
    In practice, ManagementFactory returns the managed bean for the runtime system of the current Java Virtual Machine that can be used to retrieve the execution arguments (see RuntimeMXBean API for further details). In case of Burp Free, the application gets shutdown via a System.exit(0);


    Bypass techniques, an incomplete list

    First of all, it is always possible to attach the debugger once the Java process is already up and running. Any check performed during the application startup won't block the execution:   

    jdb -connect sun.jvm.hotspot.jdi.SAPIDAttachingConnector:pid=[Process ID]

    Unfortunately, this is a read-only mechanism and cannot be used within traditional IDEs. A few better solutions require tweaking the application in order to modify the program execution. This can be achieved via static changes in the .class files or using static/dynamic bytecode instrumentation. The code above is pretty simple and can be bypassed in several ways:
    • Using ClassEditor, reJ or any other tool that allow .class manipulation, it is just necessary to identify all strings in the constant pool used during the string comparison within the if-statement. For instance, you could replace all strings with a bunch of "a" so that the program won't even enter in the if-statement body
    Manually changing the Constant Pool of a .class file

    •  An even more portable solution, especially when strings obfuscation is used, consist of editing the bytecode using JavaAssist or similar libraries. This allows to write a piece of code that search a class and patch it:
      • For instance, we could force the getInputArguments() to return an empty List;
      • Or, we could insert an arbitrary unconditional jump jsr to skip the program shutdown;
      • Or again, it is possible to override the System.exit() method with a local method using an empty body. First, we need to create a fake static exit(int) method. Then, we replace System.exit() with the custom method within our class.
    Using JavaAssist to replace an existent method within a Class

    Patching Burp Free for debugging your custom extensions

    With the honest intent to simplify the life of coders writing custom Burp's extensions, I have developed a small utility (BurpPatchMe) to patch your own copy of Burp Free - which will allow you to debug your code in NetBeans, Eclipse, etc.
      A few important details:
      • BurpPatchMe works for Burp Suite Free only. I have included a specific check for it as well as I have used a technique compatible with that release only. Again, you won't be able to remove debugging in Burp Suite Pro using this tool. Go and buy your own copy of this amazing tool!
      • BurpPatchMe is compiled without debugging info and it has been obfuscated too. A quick skiddie prevention mechanism to avoid abuses
      • BurpPatchMe does not contain any Burp's code, library or resource. It is your own responsability to accept the EULA agreement and its conditions, before downloading Burp Free. Also, this tool is provided as it is - please do not send emails/comments asking for "features"
      • Java JDK is required in order to use this tool. All other dependencies are included within the jar
    You can download BurpPatchMe here and launch it with:
    $ java -jar BurpPatchMe.jar -file burpsuite_free_v1.5.jar   
     Long life Burp Suite and happy extensions!
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