Ubuntu implements so much security one way or another. So much defenses against network level man in the middle or malicious proxies or wifi hotspots. Cryptographic verification generally works well but there is one big drawback: it requires correct date/time.
NTP in Ubuntu does not use any authentication by default, although it is supported by NTP.
I conclude, that almost no one is using authenticated NTP, because there are no instructions in a forum or blog how to enable NTP authentication. Therefore almost everyone uses standard configuration and is at risk.
An adversary can tamper with the unauthenticated NTP replies and put the users time several years back, especially, but not limited, if the bios battery or hardware clock is defect. That issue becomes more relevant with new devices like RP, which do not even have a hardware clock.
Putting the clock several years back allows an adversary to use already revoked, broken, expired certificates; replay old, broken, outdated, known vulnerable updates etc.
No need to keep this private. Has been publicly discussed but without proper bug report and the discussion felt into oblivion.
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