Re: [secdir] SECDIR review of draft-ietf-httpbis-cdn-loop-01

Mark Nottingham <mnot@mnot.net> Thu, 20 December 2018 05:11 UTC

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From: Mark Nottingham <mnot@mnot.net>
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Date: Thu, 20 Dec 2018 16:11:24 +1100
Cc: "iesg@ietf.org" <iesg@ietf.org>, draft-ietf-httpbis-cdn-loop.all@ietf.org, secdir@ietf.org
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To: Donald Eastlake <d3e3e3@gmail.com>
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Subject: Re: [secdir] SECDIR review of draft-ietf-httpbis-cdn-loop-01
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> On 20 Dec 2018, at 10:18 am, Donald Eastlake <d3e3e3@gmail.com> wrote:
> 
>>> I believe this document should RECOMMEND that CDN-Loop headers include some sort of MAC (Message Authentication Code) covering the header so a CDN node can reliably recognize CDN-Loop headers that it has added. Since it need only recognize its own headers, the MAC need not be further specified or interoperable. (CDN-Loop information in an HTTP message can grow by the appending of entries or by additional of another CDN-Loop header. Since I have little confidence in the stability of header order, I would suggest MACs added as a parameter to a CDN-Loop header by the last parameter for that entry and sign that entry and all previous entries in that CDN-Loop header.) This could be done by modifying the 3rd paragraph of the Security Considerations section.
>> 
>> The authors have discussed this, and we don't see much utility in this; a MAC would protect against the modification of the key/value pairs, but they're not crucial to the functionality of the header. If an attacker were able to modify header values, something much more than a MAC would be needed. Also, as you note, interoperability isn't necessary here. So, while I think we're fine with mentioning in Security Considerations that parameters can be used to secure the field, I for one would be wary of going any further than that in this specification.
> 
> Well, OK I guess. It just seems so easy to add a MAC in the header
> covering the preceding part of that header that I would do it. And it
> would help rule out certain causes when debugging problems. But if the
> WG has decided not to recommend this, so be it.

Just to make sure you saw it, Security Considerations already includes:

"""
It is possible to sign the contents of the header (either by putting the signature directly into
the field's content, or using another header field), but such use is not defined (or required) by
this specification.
"""

Cheers,

--
Mark Nottingham   https://www.mnot.net/