PCI failure - weak ssh hashing and weak key exchange algorithms supported

Symptoms

PCI scanners will report a failure similar to the below:

"SSH data integrity is protected by including with each packet a MAC that is computed from a shared secret, packet sequence number, and the contents of the packet. The algorithms supported by this SSH service use cryptographically weak hashing (MAC) algorithms for data integrity."

Description 

When reviewing a PCI scan, one of the common issues is that the SSHD supports weak hashing algorithms. More often than not, this issue can occur when a server is using the default SSHD settings. Modifications to the SSHD configuration will resolve this issue.

Solution

For the Macs, Ciphers, and Algorithms, the below SSHD edits will be sufficient. The PCI scan concern is to disable the below insecure hashing algorithms:

Mac hmac-sha1, hmac-sha1-etm@openssh.com, umac-64-etm@openssh.com

KexAlgorithms diffie-hellman-group1-sha1,diffie-hellman-group14-sha1,diffie-hellman-group-exchange-sha1

The steps to accomplish the task:

Note: Login to SSH, CLI, or terminal as root is required to edit the SSHD configuration file.

1. Edit the SSHD configuration file "/etc/ssh/sshd_config" to add below lines:
   

   KexAlgorithms curve25519-sha256@libssh.org,ecdh-sha2-nistp521,ecdh-sha2-nistp384,ecdh-sha2-nistp256,diffie-hellman-group-exchange-sha256
   
   Ciphers aes256-gcm@openssh.com,aes128-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-ctr
   
   MACs hmac-sha2-512-etm@openssh.com,hmac-sha2-256-etm@openssh.com,umac-128-etm@openssh.com,hmac-sha2-512,hmac-sha2-256,umac-128@openssh.com

2. Restart SSHD:
   

   /scripts/restartsrv_sshd

3. Now retest to confirm the algorithms in question have been disabled successfully with SSHD commands.
   Command to test available "KexAlgorithms" in SSHD:
   

   sshd -T |grep ^kexalgorithms

   Command to test available "MACs" in SSHD:
   

   sshd -T |grep ^macs

   Command to test available "Ciphers" in SSHD:
   

   sshd -T |grep ^ciphers

Note: If the server is AlmaLinux 8, you must also edit "/etc/crypto-policies/back-ends/opensshserver.config" as this file overrides the settings applied to sshd_config.

Replacing line CRYPTO_POLICY with the below text will disable the weak hashing methods.

CRYPTO_POLICY='-oCiphers=aes256-gcm@openssh.com,aes128-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-ctr -oMACs=hmac-sha2-512-etm@openssh.com,hmac-sha2-256-etm@openssh.com,umac-128-etm@openssh.com,hmac-sha2-512,hmac-sha2-256,umac-128@openssh.com -oGSSAPIKexAlgorithms=gss-curve25519-sha256-,gss-nistp256-sha256-,gss-group14-sha256-,gss-group16-sha512-,gss-gex-sha1-,gss-group14-sha1- -oKexAlgorithms=curve25519-sha256@libssh.org,ecdh-sha2-nistp521,ecdh-sha2-nistp384,ecdh-sha2-nistp256,diffie-hellman-group-exchange-sha256 -oHostKeyAlgorithms=ecdsa-sha2-nistp256,ecdsa-sha2-nistp256-cert-v01@openssh.com,ecdsa-sha2-nistp384,ecdsa-sha2-nistp384-cert-v01@openssh.com,ecdsa-sha2-nistp521,ecdsa-sha2-nistp521-cert-v01@openssh.com,ssh-ed25519,ssh-ed25519-cert-v01@openssh.com,rsa-sha2-256,rsa-sha2-256-cert-v01@openssh.com,rsa-sha2-512,rsa-sha2-512-cert-v01@openssh.com,ssh-rsa,ssh-rsa-cert-v01@openssh.com -oPubkeyAcceptedKeyTypes=ecdsa-sha2-nistp256,ecdsa-sha2-nistp256-cert-v01@openssh.com,ecdsa-sha2-nistp384,ecdsa-sha2-nistp384-cert-v01@openssh.com,ecdsa-sha2-nistp521,ecdsa-sha2-nistp521-cert-v01@openssh.com,ssh-ed25519,ssh-ed25519-cert-v01@openssh.com,rsa-sha2-256,rsa-sha2-256-cert-v01@openssh.com,rsa-sha2-512,rsa-sha2-512-cert-v01@openssh.com,ssh-rsa,ssh-rsa-cert-v01@openssh.com -oCASignatureAlgorithms=ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,ssh-ed25519,rsa-sha2-256,rsa-sha2-512,ssh-rsa'

Then restart SSHD:

/scripts/restartsrv_sshd

That's it! After confirming the hashing algorithms are secure, the PCI vendor will need to perform a new scan.