Re: [TLS] draft-ietf-tls-dnssec-chain-extensions security considerations

Paul Wouters <paul@nohats.ca> Tue, 05 June 2018 13:51 UTC

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Date: Tue, 5 Jun 2018 09:51:03 -0400 (EDT)
From: Paul Wouters <paul@nohats.ca>
To: Benjamin Kaduk <bkaduk=40akamai.com@dmarc.ietf.org>
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Subject: Re: [TLS] draft-ietf-tls-dnssec-chain-extensions security considerations
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On Mon, 4 Jun 2018, Benjamin Kaduk wrote:

Hi Ben,

> I've taken a stab at putting together some security considerations text for
> draft-ietf-tls-dnssec-chain-extension that reflects my understanding of the
> current state of affairs.  It's in a pull request at
> https://github.com/tlswg/dnssec-chain-extension/pull/19 , along with Viktor's
> commit to update the text about the actual DNS records involved (which as
> far as I can tell seems to improve the technical accuracy of the text), and
> also inserting a variable-length array that's reserved for future attempts
> to mitigate the (now-)documented security considerations.

Thanks for writing the proposed text. The new opaque Reserved field
along with the denial of existence changes you propose addresses all
my concerns.

One corner case that might be worth mentioning with the proposed DoE text
is the odd corner case of hitting a server with a Host header for which
it is not configured. You will hit the "default vhost" but you have no
appropriate extension data to populate for that hostname. So either the
server will return the (cached) TLSA/DoA data of the 'wrong' hostname,
or it could just omit the extension. I'd mostly like to say something to
prevent an implementation of forgetting this corner case and causing
some vulnerability in their code later on by pointing to bogus memory or
crashing on dereferencing a NULL.

Some nits:

 	If the server supports this extension it performs the appropriate DNS queries,

In my PR I had changed this into "obtains the appropriate DNS RRsets",
because I envision that TLS servers might not do DNS lookups themselves,
but just read this via a file or other method. Eg a cron job that
regenerates these hourly for all the hosted domains. It also reduces
some fear that all TLS servers suddenly could need to link against
a DNSSEC library. They don't if they can just reload a blob from disk
regularly to serve up.

Similarly:

 	it will need to rebuild

is better written as "it will need rebuilt data"


 	"authenticates the chain"

That should probably be "validate the chain", but the document has more
mixups of validate vs authenticate. In my PR request I had not corrected
them in an effort to keep the diff as small as possible.

I had modified the examples slightly, so the NSEC example would show a
simpler NSEC chain (from www to zzz) instead of the one wrapping around
from www to the zone apex. The example quoted is the wrong NSEC record
(the one from apex to www.example.com does not show that the
_443._tcp.www.example.com does not exist, you need the one from
www.example.com to the apex at the end of the zone)

I'm not sure what you mean with:

 	validated negative TTL

I guess you mean to say the shortest TTL of any data in the chain? But
sometimes the negative cache time is larger then the TTL (eg if TTL = 0)
Since this is just generic DNS handling, I would probably just write
something like "up to the appropriate lifetime" and leave it out of
this document.

 	denial of existence RRset

Technically that should be "RRsets" since you usually will need more then
one RRset to prove this (to disprove existence of the wildcard record)

 	-      DNSSEC authentication chain extension from a server, SHOULD use this
 	+      DNSSEC authentication chain extension from a server, uses this

I'm not sure why the SHOULD was removed here? Since it is describing TLS
client behaviour the "uses this" confuses me as I dont know if that
means MAY, SHOULD or MUST.

 	+ Specifically, the relevant DANE records are included
 	+ in the TLS handshake transcript hash, so both sides
 	+ possess identical records;

I'm not sure why you suggest to add this text. A TLS client might still
have cached a valid TLSA records from a previous connection 1 second
ago and might decide to not use this information at all. Also, it seems
to suggest that the transscript hash protects this data, but it is
"protection" using the webpki or proof of lack of MITM, but the data
does not need this protecting as it is protected by DNSSEC signatures.

 	+ the client remains responsible
 	+ for actually performing the domain name validation of
 	+ the DANE records.

Well, if it just fired up a previous TLS connection, perhaps it will be
content with the same pubkey used and skip re-validating this data? I
guess we sort of say the same thing due to the lack of a MUST ?

 	+ The Reserved field is reserved for future updates to this
 	+ document; in particular it is hoped that the issues discussed
 	+ in <xref target="increasing-trust" /> can be addressed.
 	+ Implementations of this specification MUST send a zero-length vector

Perhaps that last line can be clarified a bit:

 	Implementation not implementing any updates to this document
 	MUST send a zero-length vector

This sentence does not parse:

 	"whether or not to the exclusion"

This text is a little confusing to me:

 	+ certified by the DNS key hierarchy, that the
 	+ server does not support DANE authentication

The TLS server itself might support "DANE authentication", but there is
simply no data.  Perhaps say that the server's configured hostname does
not support DANE authentication?

Thanks again for the write up!

Paul