Sunday, June 21, 2020

Disable Weak SSH/SSL Ciphers in Cisco IOS

For backward compatibility, most companies still ship deprecated, weak SSH, and SSL ciphers. Cisco is no exception. For the security of your network and to pass a penetration test you need to disable the weak ciphers, disable SSH v1 and disable TLS version 1.0 and 1.1.

Firefox, Chrome and Microsoft all have committed to dropping support for TLS1.1. Firefox had actually done it in May 2020 but so many US Government sites quit working (during the Covid19 Hysteria) that they rolled back. Microsoft has set July 2020 to remove TLS 1.0/1.1 from IE, Edge Legacy, and Edge Chromium.

This blog covers Cisco IOS software. I plan to do another blog on IOS-XE and Nexus in the future.

SSH

Network device manufacturers (all of them I think) enabling SSH v1 by default really bothers me. Most Windows users connect with Putty which supports SSH v2. You should set Putty to default to SSH V2:




MAC/Linux users will be using OpenSSh which also supports SSH V2. You may run into situations on MAC/Linux where the weak ciphers are used and OpenSSH won't connect.

You will see a message similar to
ssh mhubbard@10.20.1.7
Unable to negotiate with 10.20.1.7 port 22: no matching key exchange method found. Their offer: diffie-hellman-group1-sha1
 
This is easy to resolve:

1. Open the SSH config file - gedit ~/.ssh/config
2. Add the necessary host IP and ciphers. KEX is Key Exchange:
        host 10.20.1.7
              KexAlgorithms +diffie-hellman-group1-sha1
              Ciphers 3des-cbc

On a really old switch, I ran into a host key exchange algorithm that I had never even heard of "ssh-dss". I had to add HostKeyAlgorithms=+ssh-dss to connect.

If you will only log into this device once or twice you can use the following without modifying the SSH config file:
ssh -oKexAlgorithms=+diffie-hellman-group1-sha1 10.20.1.7
You can use the "-G" switch and SSH will show you the ciphers that SSH is offering:
ssh -G mhubbard@10.20.1.7


The OpenSSH site has a page dedicated to legacy ciphers
openssh legacy ciphers

Removing weak SSH algorithms


All of the commands shown are from a 2960x running:
Version 15.2(4)E8 - Mainstream deployment (MD) from 18-Mar-2019

First, let's look at the default SSH setup
show ip ssh
SSH Enabled - version 1.99
Authentication methods:publickey,keyboard-interactive,password
Authentication Publickey Algorithms:x509v3-ssh-rsa,ssh-rsa
Hostkey Algorithms:x509v3-ssh-rsa,ssh-rsa
Encryption Algorithms:aes128-ctr,aes192-ctr,aes256-ctr,aes128-cbc,3des-cbc,aes192-cbcc
MAC Algorithms:hmac-sha1,hmac-sha1-96
Authentication timeout: 120 secs; Authentication retries: 3
Minimum expected Diffie Hellman key size : 1024 bits
IOS Keys in SECSH format(ssh-rsa, base64 encoded): TP-self-signed-1676064512
ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAAAgQCjsPhP/zpPgra0d3wzzt8fDZnKL4sUtCh0DVmV0fH6
m+/Xke7IRMvxg2OEk333uHlKD+Ww6w8D2eMOzY7/R6edHA4UtKXwohJN1OZKS1ltL4tDSZSIeLO3juOL
GfxKBtvGd30Y2jzYYMmTQGP9u1VrKdQRKAU13/c+iOiQPi3Q4w==          

The "version 1.99" means that it supports SSH v1 and v2. We want to disable v1 and remove the cbc and 3Des ciphers. These are "Cipher Block Chain" algorithms and will cause a failure during a penetration test.

From global configuration mode enter the following:
ip ssh version 2 !disable V1

ip ssh server algorithm encryption aes256-ctr aes128-ctr

ip ssh server algorithm mac hmac-sha1

no ip ssh server algorithm mac hmac-sha1-96

You should also perform the following to harden SSH

crypto key generate rsa modulus 4096 label SSH-KEYS

!Note that generating 4096 bit keys can take up to 3 minutes.

ip ssh rsa keypair-name SSH-KEYS !associate keys to SSH

!set minimum bit size for client connection                                

ip ssh dh min size 2048


Let's see what SSH looks like now:
show ip ssh
SSH Enabled - version 2.0
Authentication methods:publickey,keyboard-interactive,password
Authentication Publickey Algorithms:x509v3-ssh-rsa,ssh-rsa
Hostkey Algorithms:x509v3-ssh-rsa,ssh-rsa
ip ssh server algorithm encryption aes256-ctr aes128-ctr
MAC Algorithms:hmac-sha1
Authentication timeout: 120 secs; Authentication retries: 3
Minimum expected Diffie Hellman key size : 2048 bits
IOS Keys in SECSH format(ssh-rsa, base64 encoded): SSH-KEYS
ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAACAQC5KQxmPn8tyfK+9fq6NC75whEQD02POopz9SE/SKeP
ibO8KM7kSVdwy7anUhmgiX5jGmpecTFoP+txdA+KuEszAL5x8aeNZsPAykqBU6JClIz3fnMKjgoIqFlZ
mwhL0Qow4OGrd52EkRNRxAc2TYpBr5p0ICdaxeHd7etzgXjkwcZpQ1e2kqvV9XU94LBO1R93AgYYLCsT
nFsKga4tvvikXqKuwe3tfWKzNfO4LY1mZE9FXecoNW0Kb8p4U/pO/w69oEbHmmH7BfgWSHCCVZlgBhcf
DtJa+oVnqHrMwVza+ViTMQLghvt63zewvTN2I235K6W+GhgUmx6p+Q62Rsrfrc+4K5ECVKNf7fzmlg6X
Zs+P3WKgP8rh2z7ObTT917pp1VXw4pUkeqCCtMEmkiICO0TzU1dXyuoEPNGeES8wxYOSdaMA0DGEL34p
Ccb6hb1RQbHjSjQZfDOXaZ0UwXtVJ07v7PR7fOhFHem58w2P+qmCwnEYFZrZhizR1y1SUDxs6Z7vZV98
cyoTo98dWG4WDGiHM1loLq3SA3OMfceq5g2waPVBNmpZlzXitCTern1bZ15zdLvhxY1589A/TaSZuMeP
lhjQ1mlYp3qf0Jt7eoaWNPRV/i0VUaRfxNBefiNBI5pS8ybj3bhfWpZe8QOOMAHRahAPPI9PasOBuMHR

In 2020, this is still pretty lame, but keep reading! Cisco has been adding newer ciphers and removing some deprecated ciphers in newer IOS versions. You can check what's available on your version using:

test(config)#ip ssh server algorithm encryption ?
  3des-cbc    Three-key 3DES in CBC mode
  aes128-cbc  AES with 128-bit key in CBC mode
  aes128-ctr  AES with 128-bit key in CTR mode
  aes192-cbc  AES with 192-bit key in CBC mode
  aes192-ctr  AES with 192-bit key in CTR mode
  aes256-cbc  AES with 256-bit key in CBC mode
  aes256-ctr  AES with 256-bit key in CTR mode

test(config)#ip ssh server algorithm mac ?
  hmac-sha1     HMAC-SHA1 (digest length = key length = 160 bits)
  hmac-sha1-96  HMAC-SHA1-96 (digest length = 96 bits, key length = 160 bits)

If you look at Authentication in the output you notice that Public key is an option. I wrote a blog showing how to use SSH keys instead of passwords -
Authenticating to Cisco devices using SSH keys

 

Weak SSL ciphers


First, we will look at the current secure server settings. To see all possible secure server settings:
sh ip http server
  all             HTTP server all information
  connection      HTTP server connection information
  external        HTTP external registration
  history         HTTP server history information
  secure          HTTP secure server status information
  session-module  HTTP server application session module information
  statistics      HTTP server statistics information
  status          HTTP server status information

sh ip http server all        
HTTP server status: Disabled
HTTP server port: 80
HTTP server authentication method: local
HTTP server access class: 0
HTTP server base path: flash:/c2960x-universalk9-mz.152-4.E8/html
HTTP server help root:
Maximum number of concurrent server connections allowed: 16
Maximum number of secondary server connections allowed: 5
Server idle time-out: 180 seconds
Server life time-out: 180 seconds
Server session idle time-out: 180 seconds
Maximum number of requests allowed on a connection: 25
HTTP server active session modules: ALL
HTTP secure server capability: Present
HTTP secure server status: Enabled
HTTP secure server port: 443
HTTP secure server ciphersuite:  dhe-aes-128-cbc-sha dhe-aes-256-cbc-sha
        edche-rsa-aes-256-cbc-sha edche-rsa-rc4-128-sha

HTTP secure server client authentication: Disabled
HTTP secure server trustpoint:
HTTP secure server active session modules: ALL

To see who is connected to the switch over TLS:
sh ip http server connection

HTTP server current connections:
local-ipaddress:port  remote-ipaddress:port in-bytes   out-bytes
  192.168.10.31:443    192.168.10.211:55014 1394       586227


Viewing available current cipher suites


ip http secure-ciphersuite ?

  aes-128-cbc-sha            Encryption type tls_rsa_with_aes_cbc_128_sha
                             ciphersuite
  aes-256-cbc-sha            Encryption type tls_rsa_with_aes_cbc_256_sha
                             ciphersuite
  dhe-aes-128-cbc-sha        Encryption type tls_dhe_rsa_with_aes_128_cbc_sha
                             ciphersuite
  dhe-aes-256-cbc-sha        Encryption type tls_dhe_rsa_with_aes_256_cbc_sha
                             ciphersuite
  edche-rsa-aes-256-cbc-sha  Encryption type tls_ecdhe_rsa_aes_256_cbc_sha
                             ciphersuite
  edche-rsa-rc4-128-sha      Encryption type tls_ecdhe_rsa_rc4_128_sha
                             ciphersuite
  null-sha                   Encryption type tls_rsa_with_null_sha ciphersuite



Notice that rc4 and Null are supported!

To verify what was being offered by the switch I ran the nmap ssl-cert and ciphers script.
sudo nmap --script ssl-cert,ssl-enum-ciphers -p 443 192.168.10.31
Nmap scan report for 10.241.3.40
Host is up, received echo-reply ttl 254 (0.10s latency).
Scanned at 2020-06-18 15:28:06 PDT for 3s

PORT     STATE  SERVICE       REASON
443/tcp  open   https         syn-ack ttl 254
| ssl-cert: Subject: commonName=IOS-Self-Signed-Certificate-1302447744
| Issuer: commonName=IOS-Self-Signed-Certificate-1302447744
| Public Key type: rsa
| Public Key bits: 1024
| Signature Algorithm: sha1WithRSAEncryption
| Not valid before: 2020-06-16T22:55:16
| Not valid after:  2030-01-01T00:00:00
| MD5:   c522 61ff 31c4 c9aa 971d 7cfd 4eb7 14de
| SHA-1: 50fb 7c7d d6a8 86c0 ba67 1293 11d7 f529 058e e1de
| -----BEGIN CERTIFICATE-----
| MIICKzCCAZSgAwIBAgIBATANBgkqhkiG9w0BAQUFADAxMS8wLQYDVQQDEyZJT1Mt
| U2VsZi1TaWduZWQtQ2VydGlmaWNhdGUtMTMwMjQ0Nzc0NDAeFw0yMDA2MTYyMjU1
| MTZaFw0zMDAxMDEwMDAwMDBaMDExLzAtBgNVBAMTJklPUy1TZWxmLVNpZ25lZC1D
| ZXJ0aWZpY2F0ZS0xMzAyNDQ3NzQ0MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKB
| gQDCgxwOBYowFY7GgS3Q81u6CRTzcaEb2SwZvzSsjTLmHPqrB7OYgGukAgs19+Xa
| 8jRS3jY4Q492RtpyBAb4BU9naHXRKvD2zB5e9QDreeFOf73If6f8V/BtjqSozYZW
| N0RPpgqIWVbgQbkr1eBbnXgE1/TO7czYcjae/OTSZwQL1QIDAQABo1MwUTAPBgNV
| HRMBAf8EBTADAQH/MB8GA1UdIwQYMBaAFDL08Ihv1OFKYBqkbHJ5wpXt3G7IMB0G
| A1UdDgQWBBQy9PCIb9ThSmAapGxyecKV7dxuyDANBgkqhkiG9w0BAQUFAAOBgQCH
| GxSZ29CUBrvCkDU4knDw9WmdLKqgMl88+dpZmOO758+o4B8lMT0f+Ixny7drFIJ7
| rrkhrqpCHnLDJtXYcINiaKASs3tPIpQ21nQ1r5WTdW8GqaTVcOBIFG0KWlJGVmsF
| RepCnGblGV/3mrUWImNU8xwY+uZS2vAFKAVXYVLk5w==
|_-----END CERTIFICATE-----
| ssl-enum-ciphers:
|   TLSv1.1:
|     ciphers:
|       TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA (secp256r1) - A
|       TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA (secp256r1) - A
|     compressors:
|       NULL
|     cipher preference: client
|     warnings:
|       Weak certificate signature: SHA1
|   TLSv1.2:
|     ciphers:
|       TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA (secp256r1) - A
|       TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA (secp256r1) - A
|     compressors:
|       NULL
|     cipher preference: client
|     warnings:
|       Weak certificate signature: SHA1
|_  least strength: A
465/tcp  closed smtps         reset ttl 254
993/tcp  closed imaps         reset ttl 254
995/tcp  closed pop3s         reset ttl 254
3389/tcp closed ms-wbt-server reset ttl 254

NSE: Script Post-scanning.
NSE: Starting runlevel 1 (of 1) scan.
Initiating NSE at 15:28
Completed NSE at 15:28, 0.00s elapsed
Read data files from: /usr/local/bin/../share/nmap
Nmap done: 1 IP address (1 host up) scanned in 2.71 seconds
           Raw packets sent: 9 (372B) | Rcvd: 6 (232B)


To secure TLS I upgraded to 15.2.7E2. This release allows TLS 1.0 and 1.1 to be disabled. To pass a penetration test you will need to disable both. Once the upgrade is complete run the following:
test(config)#ip http secure-ciphersuite ?
  aes-128-cbc-sha            Encryption type tls_rsa_with_aes_cbc_128_sha ciphersuite
  aes-256-cbc-sha            Encryption type tls_rsa_with_aes_cbc_256_sha ciphersuite
  dhe-aes-128-cbc-sha        Encryption type tls_dhe_rsa_with_aes_128_cbc_sha ciphersuite
  edche-rsa-aes-256-cbc-sha  Encryption type tls_ecdhe_rsa_aes_256_cbc_sha ciphersuite


test(config)#ip http secure-ciphersuite edche-rsa-aes-256-cbc-sha aes-256-cbc-sha
test(config)#ip http tls-version ?
  TLSv1.0  Set TLSv1.0 version Only
  TLSv1.1  Set TLSv1.1 version Only
  TLSv1.2  Set TLSv1.2 version Only
test(config)#ip http tls-version tlsv1.2
To verify, I re-ran the nmap ssl-cert and ciphers scripts. This time only TLS 1.2 is enabled.
sudo nmap --script ssl-cert,ssl-enum-ciphers -p 443 192.168.10.31
Nmap scan report for 192.168.10.31
Host is up, received echo-reply ttl 254 (0.0072s latency).
Scanned at 2020-06-18 15:50:03 PDT for 3s

PORT    STATE SERVICE REASON
443/tcp open  https   syn-ack ttl 254
| ssl-cert: Subject: commonName=IOS-Self-Signed-Certificate-1302447744
| Issuer: commonName=IOS-Self-Signed-Certificate-1302447744
| Public Key type: rsa
| Public Key bits: 1024
| Signature Algorithm: sha1WithRSAEncryption
| Not valid before: 2020-06-16T22:55:16
| Not valid after:  2030-01-01T00:00:00
| MD5:   c522 61ff 31c4 c9aa 971d 7cfd 4eb7 14de
| SHA-1: 50fb 7c7d d6a8 86c0 ba67 1293 11d7 f529 058e e1de
| -----BEGIN CERTIFICATE-----
| MIICKzCCAZSgAwIBAgIBATANBgkqhkiG9w0BAQUFADAxMS8wLQYDVQQDEyZJT1Mt
| U2VsZi1TaWduZWQtQ2VydGlmaWNhdGUtMTMwMjQ0Nzc0NDAeFw0yMDA2MTYyMjU1
| MTZaFw0zMDAxMDEwMDAwMDBaMDExLzAtBgNVBAMTJklPUy1TZWxmLVNpZ25lZC1D
| ZXJ0aWZpY2F0ZS0xMzAyNDQ3NzQ0MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKB
| gQDCgxwOBYowFY7GgS3Q81u6CRTzcaEb2SwZvzSsjTLmHPqrB7OYgGukAgs19+Xa
| 8jRS3jY4Q492RtpyBAb4BU9naHXRKvD2zB5e9QDreeFOf73If6f8V/BtjqSozYZW
| N0RPpgqIWVbgQbkr1eBbnXgE1/TO7czYcjae/OTSZwQL1QIDAQABo1MwUTAPBgNV
| HRMBAf8EBTADAQH/MB8GA1UdIwQYMBaAFDL08Ihv1OFKYBqkbHJ5wpXt3G7IMB0G
| A1UdDgQWBBQy9PCIb9ThSmAapGxyecKV7dxuyDANBgkqhkiG9w0BAQUFAAOBgQCH
| GxSZ29CUBrvCkDU4knDw9WmdLKqgMl88+dpZmOO758+o4B8lMT0f+Ixny7drFIJ7
| rrkhrqpCHnLDJtXYcINiaKASs3tPIpQ21nQ1r5WTdW8GqaTVcOBIFG0KWlJGVmsF
| RepCnGblGV/3mrUWImNU8xwY+uZS2vAFKAVXYVLk5w==
|_-----END CERTIFICATE-----
| ssl-enum-ciphers:
|   TLSv1.2:
|     ciphers:
|       TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA (secp256r1) - A
|       TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA (secp256r1) - A
|       TLS_RSA_WITH_AES_256_CBC_SHA (rsa 1024) - A
|     compressors:
|       NULL
|     cipher preference: client
|     warnings:
|       Weak certificate signature: SHA1
|_  least strength: A

NSE: Script Post-scanning.
NSE: Starting runlevel 1 (of 1) scan.
Initiating NSE at 15:50
Completed NSE at 15:50, 0.00s elapsed
Read data files from: /usr/local/bin/../share/nmap
Nmap done: 1 IP address (1 host up) scanned in 2.99 seconds
           Raw packets sent: 5 (196B) | Rcvd: 2 (72B)

Results

You can see that it still uses SHA1 as the certificate signature. You can use  Ciphersuite Info to compare different ciphers.

What about SSH?

Let's see what's new for SSH in 15.7.2E2.
ip ssh server algorithm mac ?
  hmac-sha1      HMAC-SHA1 (digest length = key length = 160 bits)
  hmac-sha1-96   HMAC-SHA1-96 (digest length = 96 bits, key length = 160 bits)
  hmac-sha2-256  HMAC-SHA2-256 (digest length = 256 bits, key length = 256
                 bits)
  hmac-sha2-512  HMAC-SHA2-512 (digest length = 512 bits, key length = 512
                 bits)


ip ssh serv algorithm encryption ?


3des-cbc    Three-key 3DES in CBC mode
  aes128-cbc  AES with 128-bit key in CBC mode
  aes128-ctr  AES with 128-bit key in CTR mode
  aes192-cbc  AES with 192-bit key in CBC mode
  aes192-ctr  AES with 192-bit key in CTR mode
  aes256-cbc  AES with 256-bit key in CBC mode
  aes256-ctr  AES with 256-bit key in CTR mode


Now we can eliminate the ancient HMAC-SHA1 and CBC ciphers from our switch!

First, we will add the sha2 HMACs
ip ssh server algorithm mac hmac-sha2-256 hmac-sha2-512

Then remove the sha1 hmacs
no ip ssh server algorithm mac hmac-sha1
no ip ssh server algorithm mac hmac-sha1-96

And now the encryption
ip ssh server algorithm encryption aes256-ctr aes192-ctr aes128-ctr

The results

show ip ssh

SSH Enabled - version 2.0
Authentication methods:publickey,keyboard-interactive,password
Authentication Publickey Algorithms:x509v3-ssh-rsa,ssh-rsa
Hostkey Algorithms:x509v3-ssh-rsa,ssh-rsa
Encryption Algorithms:aes256-ctr,aes192-ctr,aes128-ctr
MAC Algorithms:hmac-sha2-256,hmac-sha2-512
KEX Algorithms:diffie-hellman-group-exchange-sha1,diffie-hellman-group14-sha1
Authentication timeout: 120 secs; Authentication retries: 3
Minimum expected Diffie Hellman key size : 2048 bits
IOS Keys in SECSH format(ssh-rsa, base64 encoded): SSH-KEYS
Modulus Size : 4096 bits
ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAACAQDCsyuZ8/lMCNHSLREb6vGQoBVehYQQI0+eJlanuyq5
f+iTqFcceR7vvXP14JhHmXe2lkygOZ8VIeilMJkpS8q748TaBL9QfmUAdDkbbk1wYPNKM2sLn/ACuerf
ImNa4vQFNaP28zqaCMhre/Z0DCRJvDnOXs2fepQnQZ6ZvbOgwMRw6rvTiLcPYlB46VlaS6T1ogEbsPLz
HG1e2UeGOnxyIU9j99+sUq3h5omoxtOd33c7ygyBgghBm+G4rHoD4EsJmejK2/Ai1PsjHIN16EaTAB0Y
MiIFByAYr4/Hr+6ANejxDrFpeY3DDBTvXIcES3S+C/Ch6JEoFVfHufc5ni8OReE7KQhrBctNfhoXvFRO
wITNNyyu/jk1LLDTaLFbL/auw/eXGXlXXerWRFY6HvmAbQannl9wryvy97Hm4LJVO+DtTspwvw4IKrQT
HDMdyXvTI6RMjIlGb/7hiUeFb33wx7sw/DwkgjyUCWh8R8nCEoLfpz7qOchW2/WSj+608m62Eh6WDy5q
qkDpstQRD7AbE2OBtiuYgYJaNJfZ1qhIQXlvtQCTgRRS2TvInnoGg+STD2+lWR5WufgKEO778tNDXt3H
YRSdD2N1YcjXG+y0hB/xjvWSoMkr+G2Btxtm8QPgvXQRe9aFU/kALMBKBJ6Q+rDXr2QbyA7zpDudkAn3

Security Header Enhancements

For IOS-XE devices, starting with 16.4.1, the Nginx/HTTP headers have the following settings for increased security:

Nginx – Web user interface - 
Nginx applications take care of the headers for their response. As Web UI is one of the NginX application, it adds the security headers. 

The three headers are the following:

  •     X-XSS-Protection: 1; mode=block
  •     X-Frame-Options: SAMEORIGIN
  •     X-Content-Type-Options: nosniff


Do the same thing to your Linux Servers

There is a good chance your organization is running some Linux servers. Out of the box, CentOS/Ubuntu will have several weak ciphers. It's very easy to correct that but you will need root privileges.

First, we will check what ciphers your server is offering. If the server has a public IP address you can go to https://sshcheck.com and enter the FQDN or the IP address. You will get back a comprehensive report back with suggestions on which items should be disabled.

If the server is internal you can use nmap's ssh-enum script:
sudo nmap --script ssh2-enum-algos 192.168.10.239

This will return a list of the crypto offered by your server.

Update the sshd config file

The sshd config file is located at /etc/ssh. We need to open it and add the suites we want. First we will make a backup copy.

sudo cd /etc/ssh
sudo cp sshd_config sshd_config.bak
sudo nano sshd_config

Add the following (Make sure these fit your company's security policies)
ciphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes128-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-ctr

macs umac-128-etm@openssh.com,hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,umac-128@openssh.com,hmac-sha2-256

KexAlgorithms diffie-hellman-group14-sha256,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,diffie-hellman-group-exchange-sha256,curve25519-sha256,curve25519-sh$

You can add these anywhere. I put them just below the section
"# Ciphers and keying".

Press ctrl+x, enter Y to save the file and enter to complete.

You can use
sudo sshd -t
to verify the changes. If there are no mistakes in the configuration file nothing will be displayed. If there are errors you get a message with the line number where the error occurred.

You can use
sshd -T
to dump the current ssh configuration.

Now we just need to restart the ssh daemon;
sudo systemctl restart sshd


Check your work

Refresh the sshcheck page or rerun nmap. You should see just the cipher suites you entered. Here is nmap against my server:
nmap --script ssh2-enum-algos -sV -p22 hubbardonnetworking.com

Starting Nmap 7.70 ( https://nmap.org ) at 2020-06-24 22:15 PDT
Nmap scan report for hubbardonnetworking.com (107.170.203.230)
Host is up (0.026s latency).

PORT   STATE SERVICE VERSION
22/tcp open  ssh     OpenSSH 7.4 (protocol 2.0)
| ssh2-enum-algos:
|   kex_algorithms: (6)
|       diffie-hellman-group14-sha256
|       diffie-hellman-group16-sha512
|       diffie-hellman-group18-sha512
|       diffie-hellman-group-exchange-sha256
|       curve25519-sha256
|       curve25519-sha256@libssh.org
|   server_host_key_algorithms: (5)
|       ssh-rsa
|       rsa-sha2-512
|       rsa-sha2-256
|       ecdsa-sha2-nistp256
|       ssh-ed25519
|   encryption_algorithms: (6)
|       chacha20-poly1305@openssh.com
|       aes256-gcm@openssh.com
|       aes128-gcm@openssh.com
|       aes256-ctr
|       aes192-ctr
|       aes128-ctr
|   mac_algorithms: (5)
|       umac-128-etm@openssh.com
|       hmac-sha2-256-etm@openssh.com
|       hmac-sha2-512-etm@openssh.com
|       umac-128@openssh.com
|       hmac-sha2-256
|   compression_algorithms: (2)
|       none
|_      zlib@openssh.com

Service detection performed. Please report any incorrect results at https://nmap.org/submit/ .
Nmap done: 1 IP address (1 host up) scanned in 0.70 seconds
 
I have a Python script that presents a menu with several nmap security scripts. If you haven't used nmap much it is worth a look.

Python tool for nmap scripts

References

Putty SSH V2
SSH Algorithms for Common Criteria Certification
Cisco IOS HTTP Services Command Reference

1 comment: