Change the (S)Channel! Deconstructing the Microsoft TLS Session Resumption bug
Initial Problem Report
Several months ago we started hearing occasional reports from .NET developers that they were having trouble maintaining HTTPS sessions with one of our customer’s websites. Establishing connections worked just fine but they would periodically get disconnected, resulting in an exception that crashed their application. Around the same time, we also started hearing reports that two other Microsoft products—Internet Explorer and its heir-apparent, Edge—were also having trouble with our edge.
Just a few weeks prior, we had updated our handling of TLS session tickets to be more performant and more secure. We suspected these improvements were the trigger and focused our investigation there. What we learned was that the problem ran much deeper than .NET or IE. It went all the way down to the SChannel security package, which provides TLS functionality for a vast array of Microsoft applications.
TLS Session Tickets
Before diving into the story further, however, it’s helpful to understand exactly what TLS session tickets are, how they’re beneficial to HTTPS, and what optimizations CloudFlare does to use them at scale. (If you’d like to skip over the primer and jump right to the punchline, go ahead and click here.)
Overview
First introduced in Continue reading
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