SSH Terrapin Prefix Truncation Weakness

[dacruzer1]

Good Day All,

I found a vulnerability on my 4321 router regarding this:

"The remote SSH server is vulnerable to a man-in-the-middle prefix truncation weakness known as Terrapin. This can allow a remote, man-in-the-middle attacker to bypass integrity checks and downgrade the connection's security."

Anyone information on how to resolve this is greatly appreciated.

-Alfred
 

[Rob Ingram]

@dacruzer1 use unaffected algorithms such as AES-GCM

ip ssh server algorithm encryption aes256-gcm aes128-gcm

[ashishkumar26]

Hi Rob, We have received SSH Terrapin Prefix Truncation Weakness (CVE-2023-48795) vulnerability on AIR-CT5508-K9 IOS version AireOS 8.5.140.0. This vulnerability is associated with bug CSCwi61646. Please advise if we can ignore this vulnerability as on below link it says workaround is Not Available or Not Applicable. Or kindly suggest for the FIX.

https://bst.cisco.com/bugsearch/bug/CSCwi61646?rfs=qvlogin

[balaji.bandi]

adding to other comment check also below CVE :

https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-iosxe-webui-privesc-j22SaA4z

[dacruzer1]

HI Rob,

I configured that line and now I am getting this error when I tried to SSH:

couldn't agree a client-to-server cipher (available: aes256-gcm, aes128-gcm)

I am using putty to ssh

[Rob Ingram]

@dacruzer1 where do you receive this error on the device when configuring or in putty when connecting? If in putty then putty might not support GCM, what version of putty? And what ciphers does it support?

[dacruzer1]

Hi Rob,

In putty, when connecting.  Putty release version 0.70  

It supports:  AES (SSH-2 only), ChaCHa20 (SSH-2 only), Blowfish, 3DES

Is there an SSH tool that support GCM ?

Thanks!

[Rob Ingram]

@dacruzer1 0.70 is very old, 0.80 is the latest, download - https://www.chiark.greenend.org.uk/~sgtatham/putty/releases/0.80.html

[dacruzer1]

Hi Rob,

Downloaded and installed the latest version of putty, but still not connecting.  

Putty Fatal Error:

"Selected client-to-server cipher "aes256-gcm, aes128-gcm" does not correspond to any supported algorithm"

[Rob Ingram]

@dacruzer1 I can connect using GCM, try changing the priority cipher selection.

[dacruzer1]

Hi Rob,

It did not work, the same error...

I will need to console in and remove that line that I configured.

[balaji.bandi]

Can you post the show logging and your show ssh output ?

[dacruzer1]

2024-01-12 13:46:07 Looking up host "204.62.69.1" for SSH connection
2024-01-12 13:46:07 Connecting to 204.62.69.1 port 22
2024-01-12 13:46:07 We claim version: SSH-2.0-PuTTY_Release_0.80
2024-01-12 13:46:07 Connected to 204.62.69.1
2024-01-12 13:46:07 Remote version: SSH-2.0-Cisco-1.25
2024-01-12 13:46:07 We believe remote version has SSH-1 ignore bug
2024-01-12 13:46:07 We believe remote version needs a plain SSH-1 password
2024-01-12 13:46:07 We believe remote version can't handle SSH-1 RSA authentication
2024-01-12 13:46:07 Using SSH protocol version 2
2024-01-12 13:46:07 No GSSAPI security context available
2024-01-12 13:46:07 Selected client-to-server cipher "aes256-gcm,aes128-gcm" does not correspond to any supported algorithm

[balaji.bandi]

try latest putty 0.80 and check

[mmsabaeai]

Hi everyone,

I ran into the same issue as @dacruzer1 has with trying to SSH to the switch after using unaffected algorithms that @Rob Ingram listed above, even with the latest version of Putty 0.80. I was able to fix that issue by using the following on the switch:

ip ssh server algorithm encryption aes256-gcm@openssh.com aes128-gcm@openssh.com

I still needed to use Putty 0.80 even after I did the change.

Please let me know if what I did is the correct way or not.

Thank you,

Manwar