Re: [Captive-portals] Secdir last call review of draft-ietf-capport-rfc7710bis-04

Tommy Pauly <tpauly@apple.com> Tue, 02 June 2020 05:33 UTC

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From: Tommy Pauly <tpauly@apple.com>
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Date: Mon, 01 Jun 2020 22:33:14 -0700
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Cc: Erik Kline <ek.ietf@gmail.com>, Martin Thomson <mt@lowentropy.net>, Benjamin Kaduk <kaduk@mit.edu>, Barry Leiba <barryleiba@computer.org>, IETF SecDir <secdir@ietf.org>, d.thakore@cablelabs.com
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To: Rifaat Shekh-Yusef <rifaat.s.ietf@gmail.com>, captive-portals <captive-portals@ietf.org>
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Archived-At: <https://mailarchive.ietf.org/arch/msg/captive-portals/a5oVMHg1nDPp58WmTGAgnjhqdSA>
Subject: Re: [Captive-portals] Secdir last call review of draft-ietf-capport-rfc7710bis-04
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Hi Rifaat,

Your comments make it clear that the recommendation to make the API server name visible isn’t necessarily clear. I think it’s not a harmful thing to show, as a way to give troubleshooting information and transparency to the user, but it is not a security-critical point.

It seems appropriate to either explain the rationale more in depth, or remove that sentence entirely to avoid the misinterpretation.

Do others in the CAPPORT group have thoughts on this sentence, and any background on why we decided to go that way? Would there be any objections to removing the sentence?

Thanks,
Tommy

> On Jun 1, 2020, at 6:05 PM, Rifaat Shekh-Yusef <rifaat.s.ietf@gmail.com> wrote:
> 
> 
> 
> 
>> On Sun, May 31, 2020 at 2:07 AM Erik Kline <ek.ietf@gmail.com> wrote:
>> On Wed, May 20, 2020 at 4:37 AM Rifaat Shekh-Yusef
>> <rifaat.s.ietf@gmail.com> wrote:
>> >
>> > Adding SecDir back to this thread.
>> >
>> >
>> > >Martin Thomson <mt@lowentropy.net> Tue, 19 May 2020 01:02 UTCShow header
>> > >
>> > >On Tue, May 19, 2020, at 07:08, Rifaat Shekh-Yusef wrote:
>> > >>    it provides the client of the API
>> > >>    an opportunity to authenticate the server that is hosting the API.
>> > >>    This authentication is aimed at *allowing a user to be reasonably
>> > >>    confident that the entity providing the Captive Portal API has a
>> > >>    valid certificate for the hostname in the URI*
>> > >[...]
>> > >> An end user should be able to validate that the name is example.com and
>> > >> not any other form of the URI.
>> > >> It would be much more difficult for the end user to make sense and
>> > >> validate an IP address.
>> > >
>> > >I think that you missed the point of my comments.  This validation, performed by
>> > >a user, has no meaningful security value.  The text you cite says that the server
>> > >has a certificate for the name it chooses, which is not the same as "has a certificate
>> > >for a name the client expects".  The difference is important.
>> > >
>> >
>> > This is not the way I read these statements from section 4.1 titled Server Authentication.
>> >
>> > Here is the use case I have in mind when I read this section:
>> > If I walk into an airport and I see an ad for a paid Internet service from example.com, then as an
>> > end user it is reasonable to expect that I would have some ability to make sense of the name
>> > presented to me and make sure it is from example.com if I choose to get such a service.
>> >
>> > If this is not the case, then yo might want to make it clear that this is not about Server Authentication
>> > as the title of the section and the text inside that section is suggesting.
>> 
>> If we changed the API document's section 4.1 title from "Server
>> Authentication" to "API Server Authentication", would that be more
>> clear?
>> 
>> I reality, users should never see the API URL. 
> 
> The following is a quote from section 4.1 of the API document, which implies otherwise:
> "The hostname of the API SHOULD be displayed to the user in order to indicate the entity which is providing the API service."
>  
> Regards,
>  Rifaat
> 
> 
>  
>> They'll just be
>> connecting to "Cool Cafe" or "Awesome Bookshop" ESSIDs.  If anything
>> will only be one or both of the "user-portal-url" or "venue-info-url"
>> URLs that might be visible in the browser that's opened for the user's
>> interaction.
>> 
>> -ek
>> 
>> > Regards,
>> >  Rifaat
>> >
>> >
>> >
>> > >In a typical web scenario, a person types a string in and (ignore the case where there
>> > >is an extra hop via a search engine) that string determines what is acceptable as a
>> > >server identity.  The exposure to confusables is limited (under a set of other assumptions,
>> > >HSTS, etc...).  Here, the network has free reign to do as they choose with homoglyphs and
>> > >other such nonsense.  Any expectation you might have about this really being a trustworthy
>> > >entity is meaningless in this context.
>> > >
>> > >There are protections against this, but they all lie firmly in the anti-phishing domain.
>> > >Most of those rely on having a certificate though, so the requirement for HTTPS is in
>> > >service of that, not in terms of ensuring that an untrained human can make a security
>> > >critical decision based on poisoned information.
>> >
>> > On Mon, May 18, 2020 at 5:08 PM Rifaat Shekh-Yusef <rifaat.s.ietf@gmail.com> wrote:
>> >>
>> >> Adding Ben.
>> >>
>> >>
>> >> On Sun, May 17, 2020 at 9:26 PM Martin Thomson <mt@lowentropy.net> wrote:
>> >>>
>> >>> Adding more lists..
>> >>>
>> >>> On Sun, May 17, 2020, at 02:50, Rifaat Shekh-Yusef wrote:
>> >>> > > Here is a quote form the API document:
>> >>> > > "The hostname of the API SHOULD be displayed to the user in order to indicate the entity which is providing the API service."
>> >>> > >
>> >>> > > This seems to suggest that the user is expected to inspect the displayed name and make sure it is make sense in the context of whoever is providing that service.
>> >>>
>> >>> I don't think that is the case.  If this were a security mechanism, then it would use "MUST".  This is likely for the purpose of enabling some sort of accountability.  In other words, this is to offer maximal information about what is going on.
>> >>>
>> >> Here is the sentence just before the above quote from the API document:
>> >>
>> >>    it provides the client of the API
>> >>    an opportunity to authenticate the server that is hosting the API.
>> >>    This authentication is aimed at allowing a user to be reasonably
>> >>    confident that the entity providing the Captive Portal API has a
>> >>    valid certificate for the hostname in the URI
>> >>
>> >>
>> >>
>> >>
>> >>>
>> >>> > > Since this would be an easier attack compared to the interception attack, and IP address is still permitted, then an attacker might force the use of IP address to make it harder for the user to make sense of the displayed name.
>> >>>
>> >>> I don't think that is materially different than getting a name with confusable characters (or using the prefix hack, example.com.<some-guid>.example, in an attempt to confuse).
>> >>
>> >>
>> >> An end user should be able to validate that the name is example.com and not any other form of the URI.
>> >> It would be much more difficult for the end user to make sense and validate an IP address.
>> >>
>> >> Regards,
>> >>  Rifaat
>> >>
>> > _______________________________________________
>> > Captive-portals mailing list
>> > Captive-portals@ietf.org
>> > https://www.ietf.org/mailman/listinfo/captive-portals