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Why exception from os.path.exists()?


On Fri, Jun 8, 2018 at 12:16 PM, Steven D'Aprano
<steve+comp.lang.python at pearwood.info> wrote:
> On Thu, 07 Jun 2018 23:25:54 +1000, Chris Angelico wrote:
>> 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.
>
> Causing a denial of service is a vulnerability.

Yes, but remember, anyone can build a botnet and send large numbers of
entirely legitimate requests to your server. Since no server has
infinite capacity, a DOS is inherently unavoidable. So to call
something a "DOS vulnerability", you have to show that it makes you
*more vulnerable* than simply getting overloaded with requests. For
example:

1) If the kernel allocates resources for half-open socket connections,
a malicious client can SYN-flood the server, causing massive resource
usage from relatively few packets.

2) If the language can be induced to build a hashtable using values
that all have the same hash, the CPU load required for the O(n?)
operations can easily exceed the cost of making the requests.

3) If the app inefficiently performs many database transactions for a
simple request, a plausible number of such requests could slow the
database to a crawl.

4) If a small request results in an inordinately large response, the
server's outgoing bandwidth can be saturated by a small number of
requests.

Where in this is a simple HTTP 500 from the os.stat() call worse than
a legitimate request for an actual page?

The response is small (far smaller than many legit files - consider a
web app with a large JavaScript bundle, easily multiple megabytes). It
required zero disk operations, so it's as fast as returning a file
from cache. The only way it's more expensive is the actual exception
handling code itself, and if you reckon someone can DOS a server via
the cost of throwing and catching exceptions, I'm going to have to ask
for some serious measurements.

Apart from the one odd bug with SimpleHTTPServer not properly sending
back 500s, I very much doubt that the original concern - namely that
os.path.exists() and os.stat() raise ValueError if therels a %00 in
the URL - can be abused effectively.

ChrisA