Re: [Spud] [Privsec-program] Detecting and Defeating TCP/IP Hypercookie Attacks

Mirja Kühlewind <mirja.kuehlewind@tik.ee.ethz.ch> Fri, 29 July 2016 14:48 UTC

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Date: Fri, 29 Jul 2016 16:48:33 +0200
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References: <409B6F52-B637-4333-915B-A8127C80C98B@trammell.ch> <d27266cf-87f6-17b1-3038-e0f614c6c773@cs.tcd.ie>
To: Stephen Farrell <stephen.farrell@cs.tcd.ie>
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Cc: Brian Trammell <ietf@trammell.ch>, privsec-program@iab.org, spud <spud@ietf.org>
Subject: Re: [Spud] [Privsec-program] Detecting and Defeating TCP/IP Hypercookie Attacks
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Hi Stephen,

I believe that what you think PLUS is, is not what we propose. Maybe we did not make a great job so far to define PLUS narrowly enough but we believe the actual protocol specification work should be done in a wg to ensure that a broad community can participate, and all concern, including privacy concerns, can be addressed.

The point of draft-trammell-privsec-defeating-tcpip-meta is to further explain that the things you describe as risks are nothing new. And by new, we mean they are even standard-conform; because this is the main point of your concern if I understand you correctly, right? 

I assume your point is that we propose a protocol that is intended to signal information from middleboxes to the endpoint (amongst other features). I assume this based on the following statement you made:

> Should
> we standardise a method for such abuse, then I think it's quite
> possible the attacker may argue that their behaviour is not an
> attack as it's just a part of "the standard.“

and 

> And to the extent that
> PLUS could enable and standardise such things, that is, for me,
> a major reason to oppose PLUS. 

Otherwise can you please further explain what you mean by „such abuse“ and „such things“?

If the thing you are concerned about in PLUS is, that we propose to standardize ‚option‘ space that explicitly allows middleboxes to add information to a packet, I guess you know that it is possible to define an experimental TCP option (in the ISE stream) without IESG approval that allows the insertion of private information by middleboxes. Even thought TCP options are not intended to be altered by network nodes, there is also no standard that forbids this.

Let me say two things about what we ACTUALLY propose with PLUS:
1) We do NOT propose to add space to add arbitrary information. The semantic of the field must be well-defined in an IESG approved RFC and registered respectively. This will not only make it not-standard conform to use such a field for something different, it also restrict the set of valid values which then could even be checked by later middleboxes on the path, and erased if needed.
2) Further only PLUS provides an additional function that allows to detect any mangling of data/bits that was not intended. This is urgently needed because all the TCP (and higher) layer mangling we see today is the root cause for the problem we have right now.

Further, I would like to say that not all middlebox mangling is automatically bad or an attack. If we don’t provide a standardized way to communicate with middleboxes, it will be even harder to distinguish an attack from something that actually supports the network service provided or even makes the services possible at all. Without standardization there is no control at all. I don’t think we can ignore what’s already happening the Internet any further and providing standardized mechanisms to support the good in-network functions is the only way to improve security for these functions.

To make this even more clear, you wrote:
> the argument seems to ignore the downsides of standardising and thus
> legitimising "bad" behaviour including behaviours that your draft
> properly calls an attack.

We not at all want to legitimate „bad“ behavior and I really don’t know why you think that what we propose would do so. We propose to standardize a protocol that allows middleboxes to provide information they have been requested for (by the endhost). This information is well define and under IESG approval. Any other use of such a protocol will not be standard-conform and as bad as the misuse we can see today with existing protocols.

Mirja


  
> Am 29.07.2016 um 15:23 schrieb Stephen Farrell <stephen.farrell@cs.tcd.ie>;:
> 
> 
> Hi Brian,
> 
> On 29/07/16 13:33, Brian Trammell wrote:
>> Greetings, all,
>> 
>> During the PLUS BoF last week, concern was expressed that a generic
>> signaling mechanism such as proposed opened two new attack surfaces:
> 
> No necessarily "opened...new" perhaps more "risked making much
> more ubiquitous." At the meeting I didn't hear anyone claim these
> were new attacks. (I did hear you say they were not new.)
> 
>> 
>> (1) A method for endpoints to allow path elements to add
>> non-integrity protected signals presents a surface for metadata
>> injection attacks, where an entity who can place devices on a user's
>> access network and has information about the user's identity could
>> exfiltrate that information to third parties. For purposes of giving
>> it a name, let's call this a hypercookie injection attack ("hyper"
>> since it exists in a space completely inaccessible to the
>> application).
>> 
>> (2) Even if path elements are not allowed to say anything, a
>> mechanism to allow endpoints to add integrity-protected signals to
>> their traffic presents a surface for coercion attacks. An access
>> provider can force a user to tag traffic with their user ID or some
>> other token (a signed assertion that an advertisement has been viewed
>> to the end, or maybe even just straight-up bitcoins) in order to get
>> "better" connectivity, or even any connectivity at all. A more
>> classically Orwellian dystopian variant of this attack has a
>> government requiring citizens to tag all their outgoing traffic with
>> some government-issued identifier. Let's call this a hypercookie
>> coercion attack.
>> 
>> I am less concerned about the surface PLUS presents to these attacks
>> than those who have raised the concerns in the BoF and on the mailing
>> list, because the current Internet architecture is already quite
>> vulnerable to them. As I said during my presentation last Thursday,
>> Ted Hardie and I sat down to think about this at lunch a couple of
>> months ago, and found six ways one could execute hypercookie
>> injection or coercion today before our pizza showed up.
> 
> Wrt injection I can buy that totally. Having read your draft
> I don't find enough there to accept your assertion wrt
> coercion - ISTM that coercion attacks have not been analysed
> yet in your draft.
> 
> As a side-note, meta-data doesn't have to be person-specific to
> be controversial - "over 18" and anything with similar semantics,
> e.g. "member of <this> minority" can very clearly be equally or
> more damaging, yet totally non-identifying if the relevant set of
> folks is large enough. So I wonder if the "hypercookie" concept
> is even the right starting point here. And to the extent that
> PLUS could enable and standardise such things, that is, for me,
> a major reason to oppose PLUS. (That's a side-note for this
> email, but perhaps a quite fundamental thing to consider in the
> overall discussion.)
> 
>> 
>> I sat down a little longer to write these up. I found five more,
>> without even considering trivial out-of-band metadata leaks or
>> steganographic side channels.
>> https://tools.ietf.org/html/draft-trammell-privsec-defeating-tcpip-meta-00
>> is the result. The conclusion: these attacks are trivially easy to
>> execute today by exploiting the gap between valid TCP traffic and
>> what will be ignored by TCP-indifferent devices and endpoints, as
>> well as all those juicy bits IPv6 gives you. Unless we're willing to
>> rely on the widespread, altruistic deployment of stateful TCP
>> firewalls to reject traffic (I think we can use our experience with
>> BCP38 as guidance as to how well *that* will work, and in any case I
>> think it would be kind of rich for me of all people to recommend
>> throwing more TCP-meddling middleboxes into the mix) the only way I
>> can see out is to add integrity protection to all transport and
>> network-layer headers, as well as confidentiality protection to those
>> headers the path does not need to see.
> 
> I don't agree. Observatories seem to me like a mitigation that
> your draft does not consider. If the attacker here does not want
> to be seen to be attacking, then those can be effective. Should
> we standardise a method for such abuse, then I think it's quite
> possible the attacker may argue that their behaviour is not an
> attack as it's just a part of "the standard."
> 
> Such a mitigation could be attempted against the attack in 4.1.3
> of your draft for example so I disagree with the draft's assertion
> that "no user-initiated mitigation is possible" in that case at
> least and maybe others.
> 
> I think it'd be a fine thing to see further analysis of the attacks
> and potential mitigations as your draft develops.
> 
>> 
>> This is, of course, the whole point of PLUS. We can and should have a
>> discussion of what the endpoints should be able to say, and what the
>> endpoints should be able to let the path say. But if we're concerned
>> about this attack, the general approach is AFAICT the only way out.
> 
> I disagree. And I think it was clear that a whole bunch of folks
> in the room last week also clearly disagreed.
> 
> I believe your "only way out" conclusion isn't logically justified as
> the argument seems to ignore the downsides of standardising and thus
> legitimising "bad" behaviour including behaviours that your draft
> properly calls an attack.
> 
> Frankly, I was and remain puzzled by SPUD/PLUS. ISTM that we have
> different sets of sensible folks reaching diametrically opposed
> conclusions based on the same facts and arguments. Perhaps the tl;dr
> in your abstract may be a hint there - I do not think everything
> is ruined myself, so maybe one's level of opt/pess-imism affects
> one's view of the valid conclusions to reach in this space.
> 
> Cheers,
> S.
> 
>> 
>> Cheers,
>> 
>> Brian
>> 
>> 
>> 
>> _______________________________________________ Privsec-program
>> mailing list Privsec-program@iab.org 
>> https://www.iab.org/mailman/listinfo/privsec-program
>> 
> 
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