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Re: cve requests for python-oauth2
From: Kurt Seifried <kseifried () redhat com>
Date: Thu, 12 Sep 2013 13:55:51 -0600

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I have some clarifying questions, see below

On 09/12/2013 11:34 AM, Seth Arnold wrote:
Hello Kurt, all, I recently gave python-oauth2 a quick audit and
believe it needs three CVE entries:

- _check_signature() ignores the nonce value when validating signed
urls

def _check_signature(self, request, consumer, token): timestamp,
nonce = request._get_timestamp_nonce() 
self._check_timestamp(timestamp) signature_method =
self._get_signature_method(request)

try: signature = request.get_parameter('oauth_signature') except: 
raise MissingSignature('Missing oauth_signature.')

# Validate the signature. valid = signature_method.check(request,
consumer, token, signature)

if not valid: key, base = signature_method.signing_base(request,
consumer, token)

raise Error('Invalid signature. Expected signature base ' 'string:
%s' % base)

Ignoring the nonce value enables replay attacks.

This appears to already be known (ignoring the misleading title): 
https://github.com/simplegeo/python-oauth2/issues/129

Yeah ignoring nonces is not good. Oddly enough CWE only has CWE-323
"Reusing a Nonce, Key Pair in Encryption" there is nothing for
"completely ignored nonce". So this gets a CVE. Please use
CVE-2013-4346 for this issue.

- _check_timestamp() does not constrain how far into the future
times may be, (also does not prevent negative times, but probably
not relevant for a CVE)

def _check_timestamp(self, timestamp): """Verify that timestamp is
recentish.""" timestamp = int(timestamp) now = int(time.time()) 
lapsed = now - timestamp if lapsed > self.timestamp_threshold: 
raise Error('Expired timestamp: given %d and now %s has a ' 
'greater difference than threshold %d' % (timestamp, now, 
self.timestamp_threshold))

The timestamps are probably most useful to limit the number of
nonces that must be stored and compared but it seems generally
useful to prevent timestamps from the distant future from being
allowed.

I see how this can be a problem, but with proper nonces it shouldn't
be an issue on it on, correct? As such I'm leaning towards classifying
this one as security hardening.

- make_nonce(), generate_nonce(), and generate_verifier() use a
poor prng:

@classmethod def make_nonce(cls): """Generate pseudorandom
number.""" return str(random.randint(0, 100000000))


def generate_nonce(length=8): """Generate pseudorandom number.""" 
return ''.join([str(random.randint(0, 9)) for i in range(length)])


def generate_verifier(length=8): """Generate pseudorandom
number.""" return ''.join([str(random.randint(0, 9)) for i in
range(length)])

Nonces may not need full-blown /dev/urandom but the Python
'random' documentation clearly states the results are repeatable.
The lack of seeding in this module makes me think this is too weak
for this use.

The safety of oauth depends upon the verifier being unguessable,
and this is both too short, with too few character choices, and
probably does need full-blown /dev/urandom style randomness.

The poor PRNG for the nonce has been known since 2010-04-24 (silly
github, hover your _mouse pointer_ over the "3 years ago" text in
the bug report): 
https://github.com/simplegeo/python-oauth2/issues/9

Yeah to quote Python random():

However, being completely deterministic, it is not suitable for all
purposes, and is completely unsuitable for cryptographic purposes.

so even with a 'random' seed can attacker could conceivably take a
sample and then brute force the original seed allowing them to predict
future values. So this would go under insufficient randomness and get
a CVE. Please use CVE-2013-4347 for this issue.



Thanks



- -- 
Kurt Seifried Red Hat Security Response Team (SRT)
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