Friday, 8 January 2016

Android "This certificate is not from a trusted authority" or "no peer certificate" errors - fine on desktop!

Moving from Azure VMs back to Cloud Services

Sooooo, I am running a cloud service in a pair of VMs. It started life as a cloud service (PaaS) on Azure but after someone broke the Azure Powershell tools, it no longer deployed and so I went traditional and installed it manually.

Recently, I decided to change it back. VMs are OK in that you can deploy very quickly using Powershell/SVN etc. but they also require regular maintenance and monitoring and they are also slightly more expensive than the equivalent cloud services.

The Problem

Anyway, deployed it all, tested it in Chrome and it looked fine so I changed over the DNS to point to the new cloud service. Our SSO service seemed to work OK but the Android app didn't so after a hasty swap back, I opened the Android app in the debugger to find out what was happening.

Getting to the web service call showed the exception "No peer certificate", which I understand but which didn't make sense. I visited the site in the browser and even ran a couple of SSL tests like the Qualys one and they reported no problems. Clearly there was a chain problem. As a quick check, I also tried to visit the same URL in the Android browser and got another error, which was more useful: "This certificate is not from a trusted authority", it also showed the certificate chain and the fact that the chain was somehow broken - again, I knew what this meant in theory but didn't understand why it was OK from the desktop and from the online test tools.

A clue was that the Qualys test showed 2 certificate paths, one that pointed to a new CA root certificate and another longer chain that used something called a cross-root to point to an older root certificate, something done for backward compatibility reasons (but one which causes problems!).

The Cause

Windows (and other servers?) use the issuer and subject names to match certificate chains up, it turns out that although I had my own "COMODO RSA Certification Authority" intermediate certificate (which used the cross-root and old root cert), it was also the name of a trusted root certificate in Windows - a newer cert.

Windows scores the paths (apparently) and all things being equal, chooses the shorter one as the standard certificate path to use in the SSL handshake - or at least, the validation process on the client does this.

In this case, the shorter path used a newer root certificate that simply isn't present on Android (not sure how often these are updated). You can see what is supported under settings -> security -> trusted credentials.

For some reason, desktops can handle this, probably because their root certs are more up to date but they also cache intermediate certificates, which might make a site work because of a previously visited site.

The Solution

You have to break the path you don't want by deleting the relevant certificates (the ones whose names conflict). In this case, I deleted the newer root cert by logging in with remote desktop. I also REBOOTED and then only a single path gets returned and it all works again.

Clearly, I have to be aware that there is a chance this problem will rear its head again if the cloud services are every deployed again from scratch. I should probably write a script to delete the offending certificate but for now I will add it to the checklist for deployment so a quick check can ascertain if the problem is still resolved or not.
Post a Comment