CUCM: TLS 1.2 and Legacy Phones

Hello world!

Today’s post will be quick and dirty but hopefully useful.

In Cisco Call Manager (CUCM) version 11.5(1)SU3 support was introduced for Transport Layer Security (TLS) versions 1.1 and 1.2. Prior to this release 1.0 was the only supported version. With security being the driving factor in much of the IT world these days, there is a push to secure everything to the highest available level, that includes the Collaboration environment. I have many customers that want to take advantage of the higher TLS version levels and that is a good thing, but there are gotchas.

If you know anything about Cisco IP phone communications you know that several services; and specifically the Corporate and Personal directory service are pre-configured applets that run between the phone and CUCM. In the case of the directory service(s) they talk using 8443 which is a secure port and thus uses a certificate to communicate (along with the Trust Verification Service (TVS)). That certificate is directly encrypted with the help of TLS. Because support for the newer/higher TLS versions has only recently come into play there are several generations of IP phones that do not support anything above TLS 1.0 (or 1.1 in some cases). This list of legacy endpoints includes the 7900 series and Cisco’s previous “Cadillac” the 9900 series as well as others.

If you have legacy endpoints and change your TLS version to something above 1.0 you will notice that the directory services on those endpoints will fail with a “Host not found” error. What is actually happening involves a failed TLS handshake between the phone and the Trust Verification Service (TVS) on CUCM. Because the phone cannot communicate with the TVS using TLS 1.2 the handshake fails and the directory service cannot be accessed.

So what are your options?

  1. Replace your legacy phones, there is always a financial fix and this is it.
  2. You can manually create a directory service that talks HTTP and assign it to the phones. You could make this an Enterprise Subscription if you want everyone to use it. The URL that you should use is: http://YOURCUCMFQDNorIPHere:8080/ccmcip/xmldirectory.jsp. Since this is an HTTP service you can use the IP instead of the FQDN. This will allow the service to function whether name services are functional or not. This will require user training but may make the most sense depending on the environment.
  3. Revert to TLS 1.0. This sounds easy, but there are gotchas here too.

The gotchas of going back…

Switching from TLS 1.0 to TLS 1.1 or 1.2 (If you are going to change, go to 1.2) is a relatively straight forward process. You log into the platform CLI of your CUCM publisher and any subscribers and issue the following command set tls min-version 1.2. When you enter this command you will be asked to confirm and once confirmed (with yes) the system will restart. This happens immediately and should only be done during a maintenance window.

Switching back from TLS 1.2 to 1.0 follows the same steps with the same command and the same reboot process. However, when you switch back from a higher TLS version to a lower TLS version, anything that was encrypted using that higher TLS version becomes invalid. This includes certificates (which automatically regenerate) and also the application UI password store  (including your CCM Administration credential). This password must be reset from the CLI and changed to something different before you can log into the system again. Note that this does not change your Prime License Manager (PLM) admin password if co-resident with your CUCM.

Security is important and continually making our security better is the only way to prevent incidents. With that said, proper planning will stop security changes from turning into outages.

Cisco’s TLS 1.2 Compatibility Matrix: https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/uc_system/unified/communications/system/Compatibility/TLS/TLS1-2-Compatibility-Matrix.html

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