Does my code prevent directory traversal?

Question

Is the following code snippet from a Python WSGI app safe from directory traversal? It reads a file name passed as parameter and returns the named file.

file_nam         


        

Answer 1

Your code does not prevent directory traversal. You can guard against this with the os.path module.

>>> import os.path
>>> os.curdir
'.'
>>> startdir = os.path.abspath(os.curdir)
>>> startdir
'/home/jterrace'

startdir is now an absolute path where you don't want to allow the path to go outside of. Now let's say we get a filename from the user and they give us the malicious /etc/passwd.

>>> filename = "/etc/passwd"
>>> requested_path = os.path.relpath(filename, startdir)
>>> requested_path
'../../etc/passwd'
>>> requested_path = os.path.abspath(requested_path)
>>> requested_path
'/etc/passwd'

We have now converted their path into an absolute path relative to our starting path. Since this wasn't in the starting path, it doesn't have the prefix of our starting path.

>>> os.path.commonprefix([requested_path, startdir])
'/'

You can check for this in your code. If the commonprefix function returns a path that doesn't start with startdir, then the path is invalid and you should not return the contents.

---

The above can be wrapped to a static method like so:

import os 

def is_directory_traversal(file_name):
    current_directory = os.path.abspath(os.curdir)
    requested_path = os.path.relpath(file_name, start=current_directory)
    requested_path = os.path.abspath(requested_path)
    common_prefix = os.path.commonprefix([requested_path, current_directory])
    return common_prefix != current_directory