Certificate subject match validation

Viktor Dukhovni openssl-users at dukhovni.org
Fri Mar 27 17:44:43 UTC 2020

On Fri, Mar 27, 2020 at 07:38:35PM +0200, George-Theodor Serbana wrote:

> I am writing a SSL/TLS client (using Boost.Beast but underlying it's using
> OpenSSL) and although I have set on the SSL context the 'verify_peer' flag,
> there is no verification to prove the server presents an X509 which
> contains in the Subject Alternative Names the hostname of that server.
> As this is probably the dumbest type of attack someone could do (using a
> valid certificate with another domain name), I am thinking I'm doing
> something wrong. But from the documentation, I saw that using "verify_peer"
> should perform all the verifications...

It verifies the trust chain.  To also verify the peer name, you need to
specify the peer name via:


> Now if not even this simple check is being done, how about expiration of
> the certificate, revocation status and other checks? Should they be
> performed manually as well?

No, that's what VERIFY_PEER is for.

> For now I am using X509_VERIFY_PARAM_set1_host with SSL_CTX_set1_param to
> do this specific check.

That's the slightly less convenient legacy API from OpenSSL 1.0.2.
In 1.1.0 and later, you can use SSL_set1_host() (and in some
cases also SSL_add1_host()).

See the SSL_set1_host(3) manpage for details.


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