Re: anyone using certificates with an empty principals section?

Sorry, I should have been more precise:

If I set the principals to the empty array [], then a pre-10.3 client will trigger the special case in sshkey.c when nprincipals==0 and treat it as a wildcard:

	// Pre 10.3 client
	if (k->cert->nprincipals == 0) {
		if (require_principal) {
			*reason = "Certificate lacks principal list";
			return SSH_ERR_KEY_CERT_INVALID;
		}
	}

So I can't set any principals or that wildcard special case will not trigger.

> The better alternative would be to make your hostname stable somehow

For some situations yes, but hosts that I am concerned about are embedded devices with no DNS/name resolution capabilities who can only be accessed by IP address (and their IP address is whatever they get from DHCP)

________________________________________
From: Jörn Heissler <[email protected]>
Sent: Saturday, 8 August 2026 02:55
To: Ashutosh Ghosh <[email protected]>
Cc: [email protected] <[email protected]>
Subject: Re: anyone using certificates with an empty principals section?
 
On Sat, Aug 08, 2026 at 07:52:01 +0000, Ashutosh Ghosh via openssh-unix-dev wrote:
> I'm very late to this party, sorry! I have been using host certificates with empty principals and today I got the first user with an OpenSSH 10.3 client report an issue (I think macOS 26.6 released a few days ago is bundling OpenSSH 10.3p1).
>
> I think the way this patch is implemented makes it impossible to have a backwards compatible host certificate. If I set the principals to "*", then a pre-10.3 client will not accept it; if I don't set it to "*" for a wildcard certificate then a post-10.3 client will not accept it.
>
> For my use case, the hostname and IP address are not stable so I don't have a way to express the set of valid hostnames in advance.
>
> Am I missing something? Is there a way to issue a certificate that is effectively a wildcard for both old and new clients?

Hi,

did you try creating a cert with two principals, "" and "*"?

The better alternative would be to make your hostname stable somehow.
Perhaps the HostKeyAlias client option can help you with that.

Jörn