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Disabling SSH CBC cipher on Cisco routers/switches

Hello,

Our client ordered PenTest, and as a feedback they got recommendation to "Disable SSH CBC Mode Ciphers, and allow only CTR ciphers" and "Disable weak SSH  MD5 and 96-bit MAC algorithms" on their Cisco 4506-E switches with CIsco IOS 15.0

I have gone through Cisco documentation that i could find, also tried to find the commands on the switches itself but i found no way manipulate these SSH options. (SSH v2 only is already configured) 

 

Is there any way to do   this on Cisco IOS? If not what are my alternatives?

2 ACCEPTED SOLUTIONS

Accepted Solutions
VIP Purple

You can use an external

You can use an external server for authentication. But that won't change anything in encryption.

RADIUS will be fine for Authentication, if you are also looking for strong authorization, you should look into TACACS+.

Back to you original problem:

Some/long time ago a customer had a similar problem and solved it the following way:

  1. All routers and switches had an access-class that only allowed two Linux-servers to access the devices by SSH.
  2. The SSH-server was accessed by the admins and used as a jump-point to access the routers/switches
  3. The linux-servers had a modified ssh-server config to only allow strong crypto from the Admins and also to audit the admin-work.

With that, there was strong crypto from the admin-workstations to the linux-server and quite weak crypto from the linux box to the routers switches (that was at the 2950/3900XL-times). But as the linux-boxes were positioned in the management-network, the all over risk was reduced.

Bronze

This is finally available in

This is finally available in Cisco ASA as of 9.1(7), 9.4(3), 9.5(3), and 9.6(1) with a basic hardened config such as:

ssh version 2
ssh cipher encryption high
ssh cipher integrity high
ssh key-exchange group dh-group14-sha1
show ssh ciphers

TLS1.2 can also be configured similarly, although the "high" template doesn't remove weak DHE, so you have to make it custom:

ssl server-version tlsv1.2
ssl client-version tlsv1.2
ssl cipher default custom "AES256-SHA:AES128-SHA"
ssl cipher tlsv1 custom "AES256-SHA:AES128-SHA"
ssl cipher tlsv1.1 custom "AES256-SHA:AES128-SHA"
ssl cipher tlsv1.2 custom "ECDHE-RSA-AES256-GCM-SHA384:AES256-GCM-SHA384:ECDHE-RSA-AES256-SHA384:AES256-SHA256:ECDHE-RSA-AES128-GCM-SHA256:AES128-GCM-SHA256:ECDHE-RSA-AES128-SHA256:AES128-SHA256"
ssl cipher dtlsv1 custom "AES256-SHA:AES128-SHA"
ssl dh-group group14

IOS-based devices starting with 15.5(2)T can use:

ip ssh server algorithm mac hmac-sha1 hmac-sha1-96
ip ssh server algorithm encryption aes128-ctr aes256-ctr

I read Karsten's link after I wrote this and it has many more details, so consider the above cliff notes.

7 REPLIES
VIP Purple

Based on your version you are

Based on your version you are quite limited in what you can configure. I wrote a little guide for the SSH-config:

https://supportforums.cisco.com/document/12338141/guide-better-ssh-security

Community Member

Thanks for the reply.So

Thanks for the reply.

So theres no way i can configure this on Cisco... thought so

Is there any other way, like using some kind of server and redirecting SSH authentication to it? Our client already has RADIUS server, can we use that? 

VIP Purple

You can use an external

You can use an external server for authentication. But that won't change anything in encryption.

RADIUS will be fine for Authentication, if you are also looking for strong authorization, you should look into TACACS+.

Back to you original problem:

Some/long time ago a customer had a similar problem and solved it the following way:

  1. All routers and switches had an access-class that only allowed two Linux-servers to access the devices by SSH.
  2. The SSH-server was accessed by the admins and used as a jump-point to access the routers/switches
  3. The linux-servers had a modified ssh-server config to only allow strong crypto from the Admins and also to audit the admin-work.

With that, there was strong crypto from the admin-workstations to the linux-server and quite weak crypto from the linux box to the routers switches (that was at the 2950/3900XL-times). But as the linux-boxes were positioned in the management-network, the all over risk was reduced.

Community Member

Thank you, we will probably

Thank you, we will probably try something similar, although i doubt it will help with future PenTests as CBC will still be enabled.

 

Thanks again

VIP Purple

Well, when the device doesn't

Well, when the device doesn't answer a SSH-request, there will be no cipher to negotiate. At least not from the general network. ;-)

Bronze

This is finally available in

This is finally available in Cisco ASA as of 9.1(7), 9.4(3), 9.5(3), and 9.6(1) with a basic hardened config such as:

ssh version 2
ssh cipher encryption high
ssh cipher integrity high
ssh key-exchange group dh-group14-sha1
show ssh ciphers

TLS1.2 can also be configured similarly, although the "high" template doesn't remove weak DHE, so you have to make it custom:

ssl server-version tlsv1.2
ssl client-version tlsv1.2
ssl cipher default custom "AES256-SHA:AES128-SHA"
ssl cipher tlsv1 custom "AES256-SHA:AES128-SHA"
ssl cipher tlsv1.1 custom "AES256-SHA:AES128-SHA"
ssl cipher tlsv1.2 custom "ECDHE-RSA-AES256-GCM-SHA384:AES256-GCM-SHA384:ECDHE-RSA-AES256-SHA384:AES256-SHA256:ECDHE-RSA-AES128-GCM-SHA256:AES128-GCM-SHA256:ECDHE-RSA-AES128-SHA256:AES128-SHA256"
ssl cipher dtlsv1 custom "AES256-SHA:AES128-SHA"
ssl dh-group group14

IOS-based devices starting with 15.5(2)T can use:

ip ssh server algorithm mac hmac-sha1 hmac-sha1-96
ip ssh server algorithm encryption aes128-ctr aes256-ctr

I read Karsten's link after I wrote this and it has many more details, so consider the above cliff notes.

Community Member

Re: This is finally available in

Hi,

 

Is there any chance of disaling SSL in version 12.2 on Cisco 3750 series switch.

 

Thank You,


Raja.

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