Saturday, 25 April 2015

Firefox 37 and corporate intranet

After a recent update on Ubuntu 14.04.2 LTS, no connection to the beloved corporate expense reimbursement application could be established any more. Of course I realised this situation at a time when I urgently needed to use that application.


Long time ago I have installed the Firefox extension SSL Version Control which allows me to use stronger encryption by defaults, while enables lowering the security level easily if needed. I quickly realised that requiring TLS 1.2 everywhere was not be a workable practice, so I settled with TLS 1.0 back then to get rid of at least SSLv3. Google for POODLE to learn more about the reasons.

I still needed to occasionally allow SSLv3 from time to time in order to be able to use corporate web applications hosted on improperly configured servers - this is the reality of a huge US based enterprise in 2015, no comment.

In Firefox 34 (released in December 2014), SSLv3 was disabled by default, but could be easily enabled by end user either via the extension referred above or changing property security.tls.version.min in about:config as explained here.

Firefox 37, released right before April Fool's Day 2015, disabled insecure TLS version fallback as a security improvement. This means that nor the extension, nor setting the above mentioned configuration property to 0 would allow me any more connect to sites via SSLv3. It is suggested to set another property, security.tls.version.fallback-limit to 1 and file a bug noting the URL of the site where this is needed.

Obviously, I did not submit a but report to Mozilla with a link to our intranet expense reimbursement application. Apparently, setting fallback limit to 1 or 0 did do the magic. Needless to say at that point I was a bit annoyed and under time pressure.

The solution

To make the long story short, Firefox 37 by default does not even respect user-configurable whitelisting, but uses a hardcoded list of hosts. Disabling the use of the compiled-in whitelist allows the end user to provide a custom list of hosts where insecure TLS version fallback should be allowed.See bugs 1114816 and 1084025 for details.

I ended up with disabling the SSL Version Control extension and manually tweaking TLS fallback related properties as shown below.

security.tls.version.max = 3 # default value
security.tls.version.min = 1
security.tls.insecure_fallback_hosts = w3.***.com
security.tls.insecure_fallback_hosts.use_static_list = false
security.tls.version.fallback-limit = 1

Update: Two days ago, the host running the corporate expense reimbursement application was reconfigured to support TLS 1.0 (but not any newer version). The configuration properties above have been updated to reflect this change.

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