Question about TLS 1.3 and openssl -cipher aNULL option

[Benjamin Kaduk]

On Thu, Sep 03, 2020 at 11:45:28PM +0000, Yury Mazin via openssl-users wrote:
> Hello,
> 
> We have a server was originaly using OpenSSL 1.0.2h.
> Server is configured to use SSL ciphers as following
> ALL:!aNULL:!ADH:!EDH:!eNULL:!EXPORT
> When openssl client tries to connect to this server with command
> openssl s_client -connect localhost:8101-cipher aNULL
> it fails, because any aNULL ciphers are not available per server configuration.
> We have now upgraded server to use OpenSSL 1.1.1f.
> The current behavior is this:  client can connect using the same command
> openssl s_client -connect localhost:8101 -cipher aNULL
> or
> openssl s_client -tls1_3 -connect localhost:8101 -cipher aNULL
> 
> while the same connect attempt using TLS1.2 protocol would still fail
> 
> openssl s_client -tls1_2 -connect localhost:8001-cipher aNULL
> 
> Would the fact that I can connect to the server using TLS 1.3 using the following command (specifically, using -cipher aNULL, while server is configured to exclude all aNULL cipher suites) considered a security violation?
> 
> openssl s_client -tls1_3 -connect localhost:8001 -cipher aNULL
> 
> Also, if this a security violation, how this can be addressed in the server configuration?
> Lastly, if this is not a security violation, please explain.

It is not a security violation, because you are using TLS 1.3 ciphers,
and there are not any NULL-encryption TLS 1.3 ciphers.

Configuration of TLS 1.3 ciphers and ciphers for previous versions
of TLS are separate (since, at a protocol level, they serve different roles).
See the documentation for s_client/s_server -ciphersuites for more
information about TLS 1.3 ciphers.

-Ben