[openssl-project] Inappropriate fallback triggered when "holes" in client protocol list indirectly exclude TLSv1.3

[Matt Caswell]

On 15/08/18 16:46, Viktor Dukhovni wrote:
> When I configure a client with a legacy TLS 1.2 protocol exclusion,
> e.g. by setting SSL_OP_NO_TLSv1_2 (rather than the new min/max
> version interface), as a result of the new TLS 1.3 protocol
> suport configurations that previously negotiated "up to" TLS 1.1,
> now fail when communicating with a TLS 1.3 server:
> 
>   $ posttls-finger -c -p '!TLSv1.2' "[127.0.0.1]"
>   posttls-finger: SSL_connect error to 127.0.0.1[127.0.0.1]:25: -1
>   posttls-finger: warning: TLS library problem: error:1425F175:SSL routines:ssl_choose_client_version:inappropriate fallback:../openssl/ssl/statem/statem_lib.c:1939:
> 
> If I then also explicitly disable "TLSv1.3" the connection succeeds:
> 
>   $ posttls-finger -c -lmay -Lsummary -p '!TLSv1.2:!TLSv1.3' "[127.0.0.1]"
>   posttls-finger: Anonymous TLS connection established to 127.0.0.1[127.0.0.1]:25: TLSv1.1 with cipher AECDH-AES256-SHA (256/256 bits)
> 
> I think this counts as a regression, the client should notice that
> it implicitly disabled TLS 1.3, and therefore not react to the
> server's version sentinel by aborting the connection.  Thoughts?
> 

Hmm. Yes we should probably handle this scenario. Can you open a github
issue?

Matt