Re: [PATCH 3/3] imap-send: only check the CN if no SAN DNS names are present

["brian m. carlson" <sandals@crustytoothpaste.net>]

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On 2026-09-07 at 21:12:10, Beat Bolli wrote:
> Checking the certificate subject's common name may only be done if the
> subjectAltNames extension contains no DNS entries. If no SAN DNS name
> matches, there's no match.
> 
> Per RFC 6125 section 6.4.4[1]:
> 
>     As noted, a client MUST NOT seek a match for a reference identifier
>     of CN-ID if the presented identifiers include a DNS-ID, SRV-ID,
>     URI-ID, or any application-specific identifier types supported by the
>     client.
> 
> This change was inspired by a similar commit in the HAProxy project[2].

TLS is not supposed to use the CN at all these days and Go's
implementation completely ignores it.  subjectAltName is supposed to be
used in all cases.

> diff --git a/imap-send.c b/imap-send.c
> index 9a807cdde8..66d3dbfaa5 100644
> --- a/imap-send.c
> +++ b/imap-send.c
> @@ -256,11 +256,11 @@ static int verify_hostname(X509 *cert, const char *hostname)
>  #endif
>  	const X509_NAME_ENTRY *cname_entry;
>  	const ASN1_STRING *cname;
> -	int i, found;
> +	int i, found, has_san_dns;
>  	STACK_OF(GENERAL_NAME) *subj_alt_names;
>  
>  	/* try the DNS subjectAltNames */
> -	found = 0;
> +	found = has_san_dns = 0;
>  	if ((subj_alt_names = X509_get_ext_d2i(cert, NID_subject_alt_name, NULL, NULL))) {
>  		int num_subj_alt_names = sk_GENERAL_NAME_num(subj_alt_names);
>  		for (i = 0; !found && i < num_subj_alt_names; i++) {
> @@ -268,13 +268,18 @@ static int verify_hostname(X509 *cert, const char *hostname)
>  			GENERAL_NAME *subj_alt_name = sk_GENERAL_NAME_value(subj_alt_names, i);
>  			ASN1_STRING *subj_alt_str = GENERAL_NAME_get0_value(subj_alt_name, &ntype);
>  
> -			if (ntype == GEN_DNS && host_matches(hostname, subj_alt_str))
> -				found = 1;
> +			if (ntype == GEN_DNS) {
> +				has_san_dns = 1;
> +				if (host_matches(hostname, subj_alt_str))
> +					found = 1;
> +			}

This handles certificates with DNS names but not IP addresses.  So, for
instance, this match wouldn't work for the certificates for 1.1.1.1
(assuming they had public IMAP service).

>  		}
>  		sk_GENERAL_NAME_pop_free(subj_alt_names, GENERAL_NAME_free);
>  	}
>  	if (found)
>  		return 0;
> +	if (has_san_dns)
> +		return error("none of the subjectAltNames matches hostname '%s'", hostname);

I know OpenSSL has built-in hostname verification that can be used as of
OpenSSL 1.0.2[0].  Is there a reason we're still doing this by hand?

Relying on OpenSSL's verification would mean that (a) we would not have
to worry about getting verification wrong in a security-sensitive way
and (b) OpenSSL would handle the policy and standards compliance
functionality.

[0] https://wiki.openssl.org/index.php/Hostname_validation
-- 
brian m. carlson (they/them)
Toronto, Ontario, CA

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