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Description of problem:

Per the upstream OpenSSH documentation, unknown certificate extensions should just print a warning and continue. The version in RHEL7 erroneously treats them the same as critical extensions - any unknown one is treated as fatal. This is an OpenSSH bug, that was fixed in https://anongit.mindrot.org/openssh.git/commit/?id=ca42c1758575e592239de1d5755140e054b91a0d

Version-Release number of selected component (if applicable):

Any RHEL7 version, even in the latest (openssh-6.6.1p1-35.el7_3)

How reproducible:

Steps to Reproduce:
1. Sign an SSH certificate containing a custom extension.
2. Try to log in with it.
3. See the sshd logs.

Actual results:

Login fails, due to a fatal error due to an unknown certificate extension.

Expected results:

Login succeeds, a warning about an unknown certificate extension is logged.

Additional info:

Same is present in RHEL6 (it was a bug introduced in the original OpenSSH code, that they have since fixed, see above). I'll open a RHEL6 bug as well.

Here's the original OpenSSH bug report - https://bugzilla.mindrot.org/show_bug.cgi?id=2387