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Re: draft-ietf-tls-http-upgrade reissued

From: Julien Pierre <jpierre@netscape.com>
Date: Wed, 03 May 2000 22:42:42 -0700
Message-ID: <39110DD2.A35FEEFD@netscape.com>
To: Scott Lawrence <lawrence@agranat.com>
CC: IETF HTTP List <http-wg@hplb.hpl.hp.com>
Hi,

I know this is an old messsage, but I must ask : has anybody tried to
implement this TLS upgrade draft ? Or is anybody contemplating it ?
I would certainly like to get the benefits of secure software virtual
servers, but I have concerns about this draft.

At first, as a server implementer, the proposal in this draft looks good,
and quite straightforward to implement . All I have to do is parse the
clear-text TLS upgrade request from the client, find the right keys and
certs for the virtual host requested, and upgrade the existing connection to
TLS. Also, I can force a connection upon the client if an ACL is set. This
is nothing major and looks clean.

However, upon further examination, there are logistical and security
problems with doing a TLS upgrade. Here are a few.

1) It is a fact that many server products out there use confidential data,
like application session keys, or other confidential state information, as
part of the URI.

Section 8.1 states :
"servers may apply resource access rules such as 'the FORM on this page must
be served and submitted using TLS'

Even if a server enforces such a restriction, the URI will already be
compromised as part of the TLS upgrade process, because it is transmitted in
clear by the client before the server instructs it to upgrade to TLS.

2) Nowadays, web users know if they have an http link that the connection is
not secure, and that if it's https it is secure (SSL). It's possible by
looking at the source for an HTML form to determine ahead of time if the
data you will submit will go out in clear or not. The new upgrade draft
makes it unpredictable, by its use of regular http URLs.

With the new draft, there may be http:// link on a web form, for example to
submit (POST) confidential data, made with the assumption that the server
has an ACL requiring a TLS upgrade and therefore that the connection will be
secured.

A current-generation HTTP client will create a request with the form data
from the user, and submit it in clear text to the server, compromising the
data even before the server gets a chance to deny it by requesting a TLS
upgrade with an HTTP 426 return code. This compromises security in order to
retain compatibility with existing HTTP clients. I do not think it is an
acceptable compromise.

3) Section 8.1 states :

"8.1 Implications for the https: URI Scheme

   While nothing in this memo affects the definition of the 'https' URI
   scheme, widespread adoption of this mechanism for HyperText content
   could use 'http' to identify both secure and non-secure resources.

   The choice of what security characteristics are required on the
   connection is left to the client and server.  This allows either
   party to use any information available in making this determination.
   For example, user agents may rely on user preference settings or
   information about the security of the network such as 'TLS required
   on all POST operations not on my local net', or servers may apply
   resource access rules such as 'the FORM on this page must be served
   and submitted using TLS'. "

When it comes to security, I don't think ambiguity is a good thing, and it
is not good to leave it entirely
"up to the client and server" to determine what security characteristics are
needed for the connection.
I think the draft should define at least one standard way to tell clients
that an upgrade is in order before connection establishment.

Take the HTML form example described in my second remark.

What is a new HTTP client aware of the TLS upgrade draft supposed to do when
submitting such an HTML form ? Should it always try first thing to upgrade
the connection to any HTTP server to TLS, before issuing any meaningful
request (containing URI and maybe also POST data) ?

That would certainly work technically and ensure best possible security, but
it would be a major waste of bandwidth in 99% of the cases since current
servers will not understand the upgrade request and reject it. Many existing
servers don't support keep-alive and will even close the connection,
introducing extra latency due to the need to reconnect.

It may also be a waste of CPU for both the server and client, since perhaps
the connection didn't really need to be secure, but ended up as TLS because
of the way the client negotiated the upgrade for every server, and the
server just accepted it.

I think a key problem even for a next-generation HTTP client that is aware
of the upgrade mechanism in this draft, and would be able to perform the
upgrade to TLS - is that the original HTML form fails to specify that the
client should not POST the data in clear, but must request upgrade to TLS.

There needs to be a way to inform the client to request security prior to
the establishment of the connection. I think something still needs to be
specified in the URL to let the client know ahead of time that security is
required for the connection. It cannot be a regular https:// URL since that
will not allow the server to differentiate the virtual host for which the
data is intended. I believe a clean way to do it is to instead have a
distinct protocol name in the URL.

Perhaps there should be other ways to specify it than a protocol name, and
perhaps it doesn't belong in the HTML and not HTTP specification, but I
think that this is a major hole in the draft. Certainly, a new protocol name
means that some clients will not be able to follow some links on new servers
that require a TLS step up, but I think it is acceptable in the name of
uncompromised security.


I believe there is another approach to solving this problem of tying secure
virtual servers to IP addresses. It is completely different than the
proposal in this draft, and does not address the issues of using dual ports
for non-secure and TLS, but I believe is is still relevant.

Instead of trying to define a mechanism to upgrade the connection from
non-secure to secure, the virtual host negotiation could be made a standard
part of the SSL/TLS protocols. I think that makes sense since these
protocols exchange certificates which are tied to the virtual host names.
This is how I would envision it would work :

Let's take two virtual hosts, say secure.sun.com and secure.netscape.com .
They both resolve to the same IP address.

A client wants to connect to https://secure.sun.com . The server happens to
present its public key with the certificate for secure.netscape.com . At
that point, the client normally brings a pop-up dialog saying that the
certificate doesn't match the host requested - which was secure.sun.com ,
then asks the user if he wants to continue, and the user usually stops the
connection.

Instead, the client would by default agree to communicate with the server on
the current SSL/TLS socket, specifically to issue a "virtual host change"
request for secure.sun.com .

The server would then realize that it also knows the key and cert for
secure.sun.com, and would be given the opportunity to renegotiate the SSL
connection with the client, this time presenting secure.sun.com's public key
and certificate. The client could then verify that certificate and accept
it, and the whole process would be transparent for the user.

As you will note, this process requires two full SSL/TLS handshakes, so it
is more expensive in CPU. But that can be optimized. The secure virtual
server could have a unique public key, with several certificates signing
that same key - one certificate for each virtual host. It would then only
have to transmit the certificate for secure.sun.com, without having to
transmit a different public key. This second certificate would satisfy the
client, and would be much faster since it would save a second full
handshake. As part of TLS, this process would be generically applicable to
any other type of TCP client/server application with TLS, not specifically
tied to HTTP.

If you have made it this far, I'm now waiting for all your objections, so go
ahead and shoot !

Scott Lawrence wrote:

> The secretariat didn't forward this announcement to this list, so I
> will.
>
> The only change from version -04 is the addition of the standard
> boilerplate and references  regarding the keywords (MUST, SHOULD,
> MAY, etc).
>
> A New Internet-Draft is available from the on-line Internet-Drafts
> directories.
> This draft is a work item of the Transport Layer Security Working
> Group of the IETF.
>
>         Title           : Upgrading to TLS Within HTTP/1.1
>         Author(s)       : R. Khare, S. Lawrence
>         Filename        : draft-ietf-tls-http-upgrade-05.txt
>         Pages           : 13
>         Date            : 05-Jan-00
>
> This memo explains how to use the Upgrade mechanism in HTTP/1.1 to
> initiate Transport Layer Security (TLS) over an existing TCP
> connection. This allows unsecured and secured HTTP traffic to share
> the same well known port (in this case, http: at 80 rather than
> https: at 443). It also enables 'virtual hosting,' so a single HTTP
> + TLS server can disambiguate traffic intended for several hostnames
> at a single IP address.
> Since HTTP/1.1[1] defines Upgrade as a hop-by-hop mechanism, this
> memo also documents the HTTP CONNECT method for establishing
> end-to-end tunnels across HTTP proxies. Finally, this memo
> establishes new IANA registries for public HTTP status codes, as
> well as public or private Upgrade product tokens.
>
> A URL for this Internet-Draft is:
> http://www.ietf.org/internet-drafts/draft-ietf-tls-http-upgrade-05.t
> xt
>
> Internet-Drafts are also available by anonymous FTP. Login with the
> username
> "anonymous" and a password of your e-mail address. After logging in,
> type "cd internet-drafts" and then
>         "get draft-ietf-tls-http-upgrade-05.txt".
>
> A list of Internet-Drafts directories can be found in
> http://www.ietf.org/shadow.html
> or ftp://ftp.ietf.org/ietf/1shadow-sites.txt
>
> Internet-Drafts can also be obtained by e-mail.
>
> Send a message to:
>         mailserv@ietf.org.
> In the body type:
>         "FILE /internet-drafts/draft-ietf-tls-http-upgrade-05.txt".
>
> NOTE:   The mail server at ietf.org can return the document in
>         MIME-encoded form by using the "mpack" utility.  To use this
>         feature, insert the command "ENCODING mime" before the "FILE"
>         command.  To decode the response(s), you will need "munpack" or
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> readers
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>
> --
> Scott Lawrence      Director of R & D        <lawrence@agranat.com>
> Agranat Systems   Embedded Web Technology   http://www.agranat.com/

--
for a good time, try kill -9 -1




Received on Thursday, 4 May 2000 06:43:37 EDT

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