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Disable Weak Key Exchange Algorithm, CBC Mode in SSH

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source link: https://blog.51cto.com/wemux/5576385
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Disable Weak Key Exchange Algorithm, CBC Mode in SSH

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wemux 2022-08-15 09:09:08 博主文章分类：Linux ©著作权

文章标签 SSH CBC 文章分类 Linux 系统/运维 yyds干货盘点 阅读数178

Backup /etc/sysconfig/sshd and /etc/ssh/sshd_config

cp /etc/sysconfig/sshd /etc/sysconfig/sshd.before
cp /etc/ssh/sshd_config /etc/ssh/sshd_config.before



Edit /etc/sysconfig/sshd to uncomment the
CRYPTO_POLICY setting



from:



# CRYPTO_POLICY=



CRYPTO_POLICY=



Copy the following ciphers, MACs, and KexAlgorithms
to /etc/ssh/sshd_config.



KexAlgorithms [email protected],ecdh-sha2-nistp521,ecdh-sha2-nistp384,ecdh-sha2-nistp256,diffie-hellman-group-exchange-sha256
Ciphers [email protected],[email protected],[email protected],aes256-ctr,aes192-ctr,aes128-ctr
MACs [email protected],[email protected],[email protected],hmac-sha2-512,hmac-sha2-256,[email protected]



Verify the configuration file before restarting the
SSH server.



sshd -t



If there are no errors reported, then restart the
SSHD service.



systemctl restart sshd



Test weak CBC ciphers by executing the below
command.



ssh -vv -oCiphers=3des-cbc,aes128-cbc,aes192-cbc,aes256-cbc
<server>



If successful, it will prompt for a password. This
means weak ciphers are enabled.



If it fails, indicating cbc ciphers are disabled,
you should receive a message like this:



Unable to negotiate with 10.30.11.11 port 22: no matching cipher found.
Their offer: [email protected],[email protected],[email protected],aes256-ctr,aes192-ctr,aes128-ctr



To test if weak MAC algorithms are enabled, run the
below command:



ssh -vv -oMACs=hmac-md5,hmac-md5-96,hmac-sha1,hmac-sha1-96,[email protected],[email protected],[email protected],[email protected],[email protected] <server>



RHEL8 default ciphers include a number which have
implicit MACs, testing like above will actually negotiate successfully and
securely even though it would appear it is using hmac-md5. Look for the
concluding negotiation where you will see something similar to:



debug1: kex: server->client cipher: [email protected] MAC: <implicit> compression: none
debug1: kex: client->server cipher: [email protected] MAC: <implicit> compression: none

        

        

            

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