Synology DSM 6.2 SSH Terrapin fix

I wrote this post half a year ago, but forgot to publish it. Nevertheless, I think it can still be useful.

As you may already know, Synology still hasn’t released a fix for this vulnerability, even though it was discovered almost a year ago. I use an old Synology RS3412RPxs server. The latest DSM version is still the 6.2.4-25556 Update 7 which came out on 2023-05-02. Okay, it’s a 12 year old server, so no surprise it’s EoL. According to this forum post there’s an alternative solution to the problem: turning off the affected ciphers.

Many security sites claim that we have to disable the following options:

• chacha20-poly1305
• any AES(128|192|256)-cbc ciphers
• any MACs that use Encrypt-then-MAC (EtM)

The investigation showed that my server had the following vulnerable algorithms:

• chacha20-poly1305@openssh.com
• umac-64-etm@openssh.com
• umac-128-etm@openssh.com
• hmac-sha2-256-etm@openssh.com
• hmac-sha2-512-etm@openssh.com
• hmac-sha1-etm@openssh.com

So I ended up with these options in /etc/ssh/sshd_config:

Ciphers aes256-gcm@openssh.com,aes128-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-ctr
Macs umac-64@openssh.com,umac-128@openssh.com,hmac-sha2-256,hmac-sha2-512

Then I restarted the sshd service: synoservicectl –reload sshd

I rechecked the active ciphers with: nmap –script ssh2-enum-algos -sV -Pn -p 22 ipaddress

encryption_algorithms: (5)
    aes256-gcm@openssh.com
    aes128-gcm@openssh.com
    aes256-ctr
    aes192-ctr
    aes128-ctr
mac_algorithms: (4)
    umac-64@openssh.com
    umac-128@openssh.com
    hmac-sha2-256
    hmac-sha2-512

This solution will do until I discard this old server.