Field Notice: FN – 72511 – RSA Keys Less Than 2048 Bits Are Not Supported for SSH in Cisco IOS XE Release 17.11.1 and Later – Workaround Provided
In releases earlier than Cisco IOS XE Release 17.11.1, RSA keys less than 2048 bits can be used for the SSH server on the device.
In Cisco IOS XE Release 17.11.1 and later, RSA keys less than 2048 bits are denied for use with SSH by default due to its weak cryptographic properties. Cisco recommends to use stronger RSA keys that are at least 2048 bits. In order to continue to use RSA keys less than 2048 bits for SSH, explicit configuration is required. Without such a configuration change, SSH service on the device is disabled and SSH sessions to the device will fail. This results in loss of remote access to the device through SSH.
In Cisco IOS XE Release Bengaluru 17.6.1 and later, configuration of RSA keys less than 2048 bits for SSH generates a warning about a RSA key size compliance violation, but it does not impact SSH operations to the device. This warning message is displayed when a weak RSA key pair is used for SSH.
%SSH-5-SSH_COMPLIANCE_VIOLATION_RSA_KEY_SIZE: SSH RSA Key Size compliance violation detected. Kindly note that the usage of keys smaller than 2048 bits will be deprecated in the upcoming releases. Please revise your key configuration accordingly to avoid service impact.
In Cisco IOS XE Release 17.11.1 and later, RSA keys less than 2048 bits are denied by default and require explicit configuration to be allowed.
This advisory can be found at the following link: