The KexAlgorithms line in ssh-audit-hardening.conf began with a space.
In sshd_config a leading whitespace marks a line continuation, so the
entire directive was absorbed as arguments to the (non-existent)
previous directive. The effective kexalgorithms collapsed to only the
two trailing GSSAPI entries (gss-curve25519-sha256-, gss-group16-sha512-),
which no normal OpenSSH client can negotiate.
Result: after secharden-ssh deployed this file, every SSH connection to
the host died in [preauth] with no usable key exchange algorithm. sshd -t
still returned 0, so the breakage was completely silent.
Drop the leading space so the directive is parsed as intended. This
restores normal client compatibility while keeping the hardened
algorithm set.
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mrcharles