| Summary: | sshd treats certificate extensions as critical | ||||||
|---|---|---|---|---|---|---|---|
| Product: | Portable OpenSSH | Reporter: | Bob Van Zant <bob> | ||||
| Component: | sshd | Assignee: | Damien Miller <djm> | ||||
| Status: | CLOSED FIXED | ||||||
| Severity: | normal | CC: | djm, dtucker | ||||
| Priority: | P5 | ||||||
| Version: | 6.8p1 | ||||||
| Hardware: | amd64 | ||||||
| OS: | Linux | ||||||
| Bug Depends on: | |||||||
| Bug Blocks: | 2360 | ||||||
| Attachments: |
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Leaving aside the discussion of this particular bug, if you are adding your own extensions to the certificates you should probably be using your own namespace (ie $foo@$yourdomain, as per RFC4251 section 6), otherwise you risk future breakage if OpenSSH adds a like-named one. Created attachment 2598 [details]
accept unrecognised extensions
Yeah, that's wrong. (Do follow Darren's advice though)
committed - this will be in openssh-6.9 You guys are fast, thank you. And I'll change the format of my extension names, thank you for pointing that out. Set all RESOLVED bugs to CLOSED with release of OpenSSH 7.1 |
sshd is treating certificate extensions as critical and is disallowing logins using certificates with unknown extensions. This is happening with v01 certificates and actually the bug is quite obvious when looking for it in the code. While I am abusing this feature somewhat to encode additional data in a certificate such that it is covered by the certificate's signature this bug has the more serious side effect that it will break backwards compatibility in the future. If OpenSSH adds new cert extensions in a new version older versions of sshd (all versions prior to 6.8) will reject those certificates even though the extensions are supposed to be optional. Here's a sample certificate's ssh-keygen output: Type: ssh-rsa-cert-v01@openssh.com user certificate Public key: RSA-CERT 1c:fd:36:27:db:48:3f:ad:e2:fe:55:45:67:b1:47:99 Signing CA: RSA 62:af:90:1b:ef:b1:5a:c9:e0:2a:be:8b:3e:a9:25:18 Key ID: "bvanzant+stage@brkt.com" Serial: 1 Valid: from 2015-04-21T07:09:30 to 2015-04-21T09:11:30 Principals: ec2-user ubuntu Critical Options: (none) Extensions: ca-environment UNKNOWN OPTION (len 5) ca-reason UNKNOWN OPTION (len 17) permit-agent-forwarding permit-port-forwarding permit-pty Notice the two non-standard extensions. When I attempt to use this certificate sshd logs: sshd[30925]: error: Certificate critical option "ca-environment" is not supported The relevant code is at lines 597 and 603 of auth-options.c http://cvsweb.openbsd.org/cgi-bin/cvsweb/src/usr.bin/ssh/auth-options.c?annotate=1.65 Notice the fourth parameter is set to 1 in both the critical and extensions cases. This tells parse_option_list to treat the things being parsed as critical. I believe that the call on line 603 should have crit set to 0.