Re: [TLS] Last Call: draft-ietf-tls-rfc4366-bis (Transport Layer Security (TLS) Extensions: Extension Definitions) to Proposed Standard

Dean Anderson <> Wed, 23 September 2009 19:03 UTC

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Date: Wed, 23 Sep 2009 15:04:00 -0400
From: Dean Anderson <>
To: Simon Josefsson <>
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Subject: Re: [TLS] Last Call: draft-ietf-tls-rfc4366-bis (Transport Layer Security (TLS) Extensions: Extension Definitions) to Proposed Standard
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Is that insecure?

If the client is authorized by certificate, then it seems that it has 
that identity in addition to any application level identities.

The only insecurity is if the certifiate private key has been
compromised, which isn't something that TLS can protect against.

One problem with using TLS for virtual web hosts is that the server
names cannot match the single name allowed in the certificate.  I don't
want to see that get worse; I'd like to see it get better.


On Wed, 23 Sep 2009, Simon Josefsson wrote:

> I am aware that the IETF-wide last call has ended, but Daniel Black
> provided a suggestion (posted on the gnutls-devel list) for the Security
> Considerations that I agree with and believe can be important.  Quoting
> him, slightly reworded:
>   also maybe 11.1. could say, in response to the last paragraph of
>   section 3, + "Server applications SHOULD validate server_name against
>   any application layer equivalent field."
> The last paragraph of section 3 reads:
>    If an application negotiates a server name using an application
>    protocol and then upgrades to TLS, and if a server_name extension is
>    sent, then the extension SHOULD contain the same name that was
>    negotiated in the application protocol. If the server_name is
>    established in the TLS session handshake, the client SHOULD NOT
>    attempt to request a different server name at the application layer.
> It appears security relevant for the server to actual verify that the
> client do not use another server name at the application layer to
> circumvent authorization decisions.  I cannot find any MUST/SHOULD
> requirement in the document for servers to test this right now.
> One attack could works like this:
> 1) Client establish an client-authenticated HTTPS session with a TLS SNI
> for and sends a HTTP GET with a Host: header for
> 2) The server TLS code authenticate and authorize the client (using the
> certificate) for use with the domain.  The server HTTP
> code serves the web page to the client.
> This system would be insecure but still compliant with RFC 4366bis as
> far as I can tell, which seems suboptimal.  Adding a requirement for
> servers to check for this attack would solve the problem.
> /Simon
> _______________________________________________
> TLS mailing list

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