ssh-hardening

設定例

/etc/ssh/sshd_config.d/ssh-audit_hardening.conf

# Restrict key exchange, cipher, and MAC algorithms, as per sshaudit.com
# hardening guide.
KexAlgorithms [e-mail],curve25519-sha256,[e-mail],diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,diffie-hellman-group-exchange-sha256

Ciphers [e-mail],[e-mail]

MACs    [e-mail],[e-mail]

HostKeyAlgorithms [e-mail],[e-mail],[e-mail],[e-mail],[e-mail],ssh-ed25519,rsa-sha2-512,rsa-sha2-256

PubkeyAcceptedKeyTypes [e-mail],[e-mail],[e-mail],[e-mail],[e-mail],[e-mail],[e-mail],ssh-ed25519,ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,rsa-sha2-256,rsa-sha2-512,ssh-rsa
HostKeyAlgorithms      [e-mail],[e-mail],[e-mail],[e-mail],[e-mail],[e-mail],[e-mail],ssh-ed25519,ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,rsa-sha2-256,rsa-sha2-512,ssh-rsa

ssh-audit

ssh-audit localhost

general

(gen) banner: SSH-2.0-OpenSSH_9.6
(gen) software: OpenSSH 9.6
(gen) compatibility: OpenSSH 8.5+, Dropbear SSH 2018.76+
(gen) compression: enabled ([e-mail])

key exchange algorithms

(kex) [e-mail] -- [info] available since OpenSSH 8.5
(kex) curve25519-sha256 -- [info] available since OpenSSH 7.4, Dropbear SSH 2018.76
- [info] default key exchange since OpenSSH 6.4 (kex) [e-mail] -- [info] available since OpenSSH 6.4, Dropbear SSH 2013.62 - [info] default key exchange since OpenSSH 6.4
(kex) diffie-hellman-group16-sha512 -- [info] available since OpenSSH 7.3, Dropbear SSH 2016.73
(kex) diffie-hellman-group18-sha512 -- [info] available since OpenSSH 7.3
(kex) diffie-hellman-group-exchange-sha256 (3072-bit) -- [info] available since OpenSSH 4.4
`- [info] OpenSSH's GEX fallback mechanism was triggered during testing. Very old SSH clients will still be able to create connections using a 2048-bit modulus, though modern clients will use 3072. This can only be disabled by recompiling the code (see https://github.com/openssh/openssh-portable/blob/V_9_4/dh.c#L477).
(kex) ext-info-s -- [info] pseudo-algorithm that denotes the peer supports RFC8308 extensions
(kex) [e-mail] -- [info] pseudo-algorithm that denotes the peer supports a stricter key exchange method as a counter-measure to the Terrapin attack (CVE-2023-48795)

host-key algorithms

(key) rsa-sha2-512 (3072-bit) -- [info] available since OpenSSH 7.2
(key) rsa-sha2-256 (3072-bit) -- [info] available since OpenSSH 7.2
(key) ssh-ed25519 -- [info] available since OpenSSH 6.5

encryption algorithms (ciphers)

(enc) [e-mail] -- [info] available since OpenSSH 6.2
(enc) [e-mail] -- [info] available since OpenSSH 6.2
(enc) aes256-ctr -- [info] available since OpenSSH 3.7, Dropbear SSH 0.52
(enc) aes192-ctr -- [info] available since OpenSSH 3.7
(enc) aes128-ctr -- [info] available since OpenSSH 3.7, Dropbear SSH 0.52

message authentication code algorithms

(mac) [e-mail] -- [info] available since OpenSSH 6.2
(mac) [e-mail] -- [info] available since OpenSSH 6.2

fingerprints

(fin) ssh-ed25519:
(fin) ssh-rsa:

# algorithm recommendations (for OpenSSH 9.5)
(rec) [e-mail]        -- enc algorithm to append 

for Terappin Attack

[e-mail]を無効にする。
Terrapin attack について #SSH - Qiita

設定ファイルのデバッグ

sudo /usr/bin/sshd -dddt

参考サイト
SSH Hardening Guides
ssh-audit
JVNTA#95077890: SSH接続の安全性を低下させる攻撃手法Terrapin Attackについて
OpenSSHの暗号化周りの設定について #SSH - Qiita