This [1] appears to be the source of the problem, specifically "Add new RSA certificate types that that can be used in the above options and on the wire to require the use of RSA/SHA2 signatures." - unfortunately, those new certificate types don't exist/work in openssh <7.8, breaking backwards compatibility with 7.8 clients.
Christian - Correct, it doesn't matter that no Ubuntu version is shipping with openssh 7.8 today. Bleeding edge distributions are, and non-Linux users are getting updates to 7.8, which breaks connectivity to any openssh server <7.8 under these circumstances when the client is 7.8. Etienne - Thank you for providing that - it is the current workaround aside from downgrading clients to 7.7. This is not a complete solution though, as it doesn't help for environments that sign RSA user certificates through an automated service (unless that service supports EC certs, which I'm going to guess may not work with really old versions of openssh). [1] http://bugzilla.mindrot.org/show_bug.cgi?id=2799 ** Bug watch added: OpenSSH Portable Bugzilla #2799 https://bugzilla.mindrot.org/show_bug.cgi?id=2799 -- You received this bug notification because you are a member of Ubuntu Touch seeded packages, which is subscribed to openssh in Ubuntu. https://bugs.launchpad.net/bugs/1790963 Title: Unable to connect with openssh 7.8 client and certificates Status in openssh package in Ubuntu: Confirmed Bug description: Users are unable to connect to Ubuntu when using openssh client 7.8 and certificates. We have seen this with both xenial and bionic, but this affects connecting to ANY host running openssh server <7.8. It appears to be specific to using certificate authentication. The only known recourse at this time is either downgrade clients to 7.7 or a previous version of openssh, or create new keys/certificates with a different alg that is acceptable for both the older server and newer client. The error message via ssh -vvv is: debug1: Next authentication method: publickey debug1: Offering public key: RSA SHA256:REDACTED debug1: send_pubkey_test: no mutual signature algorithm When comparing the list returned from a 7.6 server and a 7.8 server via "ssh -Q key", we find that 7.8 returns rsa- sha2-512-cert-...@openssh.com and rsa-sha2-256-cert-...@openssh.com which are not present (or valid) for the earlier version server. It appears that the change noted here in the release notes[1] for 7.8 is related: * sshd(8): the semantics of PubkeyAcceptedKeyTypes and the similar HostbasedAcceptedKeyTypes options have changed. These now specify signature algorithms that are accepted for their respective authentication mechanism, where previously they specified accepted key types. This distinction matters when using the RSA/SHA2 signature algorithms "rsa-sha2-256", "rsa-sha2-512" and their certificate counterparts. Configurations that override these options but omit these algorithm names may cause unexpected authentication failures (no action is required for configurations that accept the default for these options). This is also affecting other Linux distributions as well: https://bugzilla.redhat.com/show_bug.cgi?id=1623929 https://bugs.archlinux.org/task/59838 [1] https://www.openssh.com/txt/release-7.8 To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/openssh/+bug/1790963/+subscriptions -- Mailing list: https://launchpad.net/~touch-packages Post to : touch-packages@lists.launchpad.net Unsubscribe : https://launchpad.net/~touch-packages More help : https://help.launchpad.net/ListHelp
