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Why exception from os.path.exists()?


On Thu, Jun 7, 2018 at 11:09 PM, Steven D'Aprano
<steve+comp.lang.python at pearwood.info> wrote:
> On Thu, 07 Jun 2018 22:46:09 +1000, Chris Angelico wrote:
>
>>> I wonder how many publicly facing web servers can be induced to either
>>> crash, or serve the wrong content, this way?
>>>
>>>
>> Define "serve the wrong content". You could get the exact same content
>> by asking for "te" instead of "te%00st.html";
>
> Perhaps so, but maybe you can bypass access controls to te and get access
> to it even though it is supposed to be private.
>
> This is a real vulnerability, called null-byte injection.
>
> One component of the system sees a piece of input, truncates it at the
> NULL, and validates the truncated input; then another component acts on
> the untruncated (and unvalidated) input.
>
> https://resources.infosecinstitute.com/null-byte-injection-php/
>
> https://capec.mitre.org/data/definitions/52.html
>
> Null-byte injection attacks have lead to remote attackers executing
> arbitrary code. That's unlikely in this scenario, but given that most web
> servers are written in C, not Python, it is conceivable that they could
> do anything under a null-byte injection attack.

Fair point. So you should just truncate early and have done with it. Easy.

> Does the Python web server suffer from that vulnerability? I would be
> surprised if it were. But it can be induced to crash (an exception, not a
> seg fault) which is certainly a vulnerability.

"Certainly"? I'm dubious on that. This isn't C, where a segfault
usually comes after executing duff memory, and therefore it's
plausible to transform a segfault into a remote code execution
exploit. This is Python, where we have EXCEPTION handling. Tell me, is
this a vulnerability?

@app.route("/foo")
def foo():
    return "Kaboom", 500

What about this?

@app.route("/bar")
def bar():
    1/0
    return "Won't get here"

Put those into a Flask app and see what they do. One of them will
explicitly return a 500. The other will crash... and will return a
500. Is either of those a security problem? Now let's suppose a more
realistic version of the latter:

@app.route("/paginate/<int:size>"):
def paginate(size):
    total_pages = total_data/size
    ...

Yes, it's a bug. If someone tries a page size of zero, it'll divide by
zero and bomb. Great. But how is it a vulnerability? It is a
properly-handled exception.

It's slightly different with SimpleHTTPServer, as it fails to properly
send back the 500. That would be a bug IMO. Even then, though, all you
can do is clog the server with unfinished requests - and you can do
that much more easily by just connecting and being really slow to send
data. (And I doubt that people are using SimpleHTTPServer in
security-sensitive contexts anyway.)

ChrisA