burp-exceptions

Simple trick to increase readability of exceptions raised by Burp extensions written in Python.

Rationale

Have you ever written a Burp Extender extension in Python to end up with a completely unlegible exception stack trace? Something like:

java.lang.RuntimeException: org.python.core.PyException
  at burp.fl.a(Unknown Source)
  at burp.edd.a(Unknown Source)
  at burp.e2g.a(Unknown Source)
  at burp.e2g.g(Unknown Source)
  at burp.i1c.stateChanged(Unknown Source)
  at javax.swing.JTabbedPane.fireStateChanged(JTabbedPane.java:416)
  at javax.swing.JTabbedPane$ModelListener.stateChanged(JTabbedPane.java:270)
  ...

Would you rather see something you can actually understand? Just like:

*** PYTHON EXCEPTION
Traceback (most recent call last):
  File "/Users/mb/Desktop/burp extension/exceptions_fix.py", line 8, in decorated_function
    return original_function(*args, **kwargs)
  File "/Users/mb/Desktop/burp extension/CustomEditorTab.py", line 78, in setMessage
    self._txtInput.setEsditable(self._editable)
AttributeError: 'burp.ul' object has no attribute 'setEsditable'

I'm presenting a neat solution to the problem!

Quick guide

1. Grab https://github.com/securityMB/burp-exceptions/blob/master/exceptions_fix.py and save it to your Folder for loading modules.

Hint: If you don't know what folder it is, open your Burp and go to Extender→Options→Python Environment→Folder for loading modules. If you haven't picked any folder yet, just set it now.

2. Modify your Burp extension a bit. Firstly, add the following lines in the beginning of your Python file:

from exceptions_fix import FixBurpExceptions
import sys

then find your registerExtenderCallbacks method and add one line to it:

sys.stdout = callbacks.getStdout()

and then also add one new line at the very end of the file:

FixBurpExceptions()

3. If you're not sure about the changes you're supposed to make, grab https://github.com/securityMB/burp-exceptions/blob/master/AlteredCustomEditorTab.py and look for lines with # ADDED LINE comment.

4. Just let the magic happen.

Alternatively, you might add @FixBurpExceptionForClass decorator just before the class you'd like to have exceptions fixed.

@FixBurpExceptionForClass
class SomeClass(IMessageEditorTab):
    ....

How does it work?

The code is a fine example of what one can achieve with metaprogramming in some programming languages. Let's start with the most outer function, namely FixBurpExceptions.

def FixBurpExceptions():
    for name, cls in inspect.getmembers(sys.modules['__main__'], predicate=inspect.isclass):
        FixBurpExceptionsForClass(cls)

We're just iterating over all classes defined in __main__ module (which is just the main file of your Burp extension) and calling FixBurpExceptionsForClass.

def FixBurpExceptionsForClass(cls):
    for name, method in inspect.getmembers(cls, inspect.ismethod):
        setattr(cls, name, decorate_function(method))        
    return cls

Here, for a change, we're iterating over all methods defined in a given class and overwrite them with results of decorate_function(method). So what does it do?

def decorate_function(original_function):
    @functools.wraps(original_function)
    def decorated_function(*args, **kwargs):
        try:
            return original_function(*args, **kwargs)
        except:
            sys.stdout.write('\n\n*** PYTHON EXCEPTION\n')
            traceback.print_exc(file=sys.stdout)
            raise
    return decorated_function

To better understand the code, you need to be familiar with Python decorators. At first, we use a @functools.wraps decorator so that the wrapper function will look just like the wrapped function (original_function). Then, we call the original function in try: ... except block so that we're able to catch any exception that might be raised. If some exception is raised, we're just writing the stack trace to sys.stdout and re-raise the exception.

Author

Feel free to contact me via GitHub or Twitter (@SecurityMB) if you have any questions or remarks.