[VOIPSEC] Soft Phone Vulnerabilities

[Randell Jesup]

Jon Callas <jon at pgpeng.com> writes:
>One of the things Tom told me was that he liked their architecture,  
>but he found bugs in their code, and suggested tweaks they could make  
>to the core system.
>
>But he also said that these are guys who literally grew up under  
>Soviet occupation, and they see no need to bow to anyone. *That* is  
>the attitude I want to see. I've seen eBay follow up on it, as well.

That can be good and bad.  It can make them ignore outside criticism
(witness the discussion here of lack of peer review).  It may give them
a predeliction for security-by-obscurity (witness the lack of information
about Skype in general).  It could (and there's no proof of this) make
them more likely to roll over quietly if serious governmental threat/weight
is dropped on them while still professing to be secure.

>There's no difference between reverse-engineering and malware. I *am*  
>smiling when I say that, but I do mean it.

I think there's a big difference.  Most people define 'malware' as
"software that by design does or can do things that aren't properly
disclosed to or expected by the user".  Even if you don't define it that
way, it's easier to exploit bugs in code if you can reverse-engineer it,
but it's far from required to do so.  All you need to know is that certain
inputs cause it to do bad things.  But this is off the topic...

>I was at a conference that included law enforcement people a few  
>months ago, and the Skype/eBay folks were there. They had a big slide  
>that said
>
>	Skype is software
>	  not a service
>
>as part of their preso. I interpreted this as saying that they do not  
>believe CALEA applies to them. That is another bit of info that I  
>noted and thought favorable.

I'm sure they'd prefer it that way.  The current FCC rulings say otherwise.
And Skype now touches (in both directions) the PSTN, and they're now owned
by a US company.  And how do you know they haven't cut a deal to provide
CALEA support?  They don't have to notify anyone that they accept CALEA
requests.  By definition, the target is not supposed to be able to know,
and that feeds into their obscured network design well.  For that matter,
you can't know if they're funneling selected streams and signalling through
NSA-run skype 'supernodes'.

>> Comparing Skype and the GSM or 3G networks is a straw-man argument.  The
>> *only* part of the GSM network protocol that is not disclosed is certain
>> parts of the encryption scheme as well as the various mechanisms that
>> vendors use for encrypting the SIMs (I'm not an expert here, so please
>> feel free to demolish me on this point. But provide references, please
>> :)
>>
>> Every other part of the GSM and 3G standards (as far as I know) is
>> available as an open standard. These protocols have been implemented
>> countless times and have been subjected to probably millions of
>> man-hours of review.
>>
>> Skype has a looong way to go before I will consider it to be in the same
>> state of review as GSM or 3G, or even SIP or H.323.
>
>Well, I mentioned GSM solely because I have a GSM phone with which I  
>have a love-hate affair. I had my CSO hat on, not my protocol  
>designer hat on. I apologize for a lack of clarity. I think the  
>data / voice capabilities of other mobile protocols are the same.
>
> From the CSO perspective, if Skype represents a threat, but the same  
>threat is posed by cell phones or wireless cards, then banning Skype  
>is merely shifting the threat. It also shifts it to a place that I  
>have less control over. The actual protocol matters not.

Skype is a larger threat than cellphones.  If Skype is compromised by a
3rd-party, that may give access to anything running on that PC, and
anything that PC has access to.  (Much like any trojan or exploited
app/OS.)  A compromised cellphone only will give access to calls and
perhaps act as a roving microphone, not give access to all networked data.
Cellphones and in particular the cell network are harder to physically hack
(regardless of the security levels of the protocols themselves) than
computer networks (which are often easy to attack sitting in your bathrobe
1/2-way around the world). Yes, I may be glossing over a few issues, but
you get my point.  And this assumes that you can totally trust Skype and
everyone who works for them (and yes, those arguments can apply to
cellphones as well - but it's easier to assess the vulnerability in a
mostly-open design).

Random example: someone figures out a way to (due to bugs) tweak the
security setup exchange when acting as a supernode, such that the data
streams are vulnerable.  Then they insinuate themselves as a convenient
supernode (by putting it on a good connection, or by blocking other
potential supernodes the target might use) and look for interesting calls
to attack.  Not to say this is possible (in a given version of Skype) - but
it could be.  We can't know.  And again, this assumes that it isn't
possible for someone inside Skype/eBay to be bribed to cause Skype to route
your calls to an attacker with an attackable key.  If that possible?  We
don't know.  If they support CALEA or equivalent now or in the future, then
the ability to do so will probably be built in, and if it's built in, then
the odds of it getting misused are significant.  Also, if it's there it's a
prime target for hackers.  Even if one machine at Skype is compromised, an
attacker might insinuate themselves into the system, even into the
binaries, in theory even without access to a machine at Skype.  But a
single carefully-placed vulnerability if it made it into a binary release
could compromise millions of PCs.

And don't forget the Thompson attack:
http://cm.bell-labs.com/who/ken/trust.html

All of this doesn't mean Skype is an unacceptable threat.  Just that
compared to a cellphone, overall it's probably a larger threat (risk) due
to the lack of information.  Cellphones are a more well-defined threat.

-- 
Randell Jesup, Worldgate (developers of the Ojo videophone), ex-Amiga OS team
rjesup at wgate.com
"The fetters imposed on liberty at home have ever been forged out of the weapons
provided for defence against real, pretended, or imaginary dangers from abroad."
		- James Madison, 4th US president (1751-1836)