The StrictModes-shaped gate reads files, and files can all look right while
the door stays shut: a sudoers drop-in that never landed, an AllowUsers or
Match block elsewhere in sshd's config. #17 names the two checks that
interrogate behavior instead, and they now run per candidate, additively,
before the drop-in installs: 'runuser -u <admin> -- sudo -n true' (NOPASSWD
sudo answers or it does not — -n never prompts; a missing runuser skips the
proof with a loud warning rather than blocking the door on a missing
prover), and 'sshd -T -C user=<admin>,host=...,addr=...' (the per-user
EFFECTIVE config — pubkeyauthentication yes, no literal DenyUsers hit,
AllowUsers if set must name them; Allow/Deny patterns match literally, fail
closed). The one thing no local check can prove remains possession of the
private key — the separate-session advisory stays load-bearing.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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