The latest research seems to indicate that TLS 1.3 completely breaks the MITM/proxy model of many current security tools.
I don't fully understand how it does that and if there are ways around this. Are SSL/TLS proxy vendors going to face existential challenges as adoption of 1.3 becomes mainstream? What is the actual mechanism that blocks the MITM middleboxes?
I see a couple excellent responses from Steffan Ulrich on two threads from 2015, but given the link above, it seems the 1.3 spec may have changed since those answers: