What you're monitoring is "Did my system request a renewed cert?" but what most people's customers care about is instead, "Did our HTTPS endpoint use an in-date certificate?"

For example say you've got an internal test endpoint, two US endpoints and a rest-of-world endpoint, physically located in four places. Maybe your renewal process works with a month left - but the code to replace working certificates in a running instance is bugged. So, maybe Monday that renewal happens, your "CT log monitor" approach is green, but nobody gets new certs.

On Wednesday engineers ship a new test release to the test endpoint, restarting and thus grabbing the renewed cert, for them everything seems great. Then on Friday afternoon a weird glitch happens for some US customers, restarting both US servers seems to fix the glitch and now US customers also see a renewed cert. But a month later the Asian customers complain everything is broken - because their endpoint is still using the old certificate.

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xorcist 3 hours ago | parent | next [-]

> Did our HTTPS endpoint use an in-date certificate?

For any non-trivial organization, you want to know when client certificates expire too.

In my experience, the easiest way is to export anything that remotely looks like a certificate to the monitoring system, and let people exclude the false positives. Of course, that requires you to have a monitoring system in the first place. That is no longer a given.

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tialaramex 2 hours ago | parent [-]

So, I've worked for both startups and large entities, including both an international corporation and a major university, and in all that time I've worked with exactly one system that used client TLS certificates. They mostly weren't from the Web PKI (and so none of these technologies are relevant, Let's Encrypt for example has announced and maybe even implemented choices to explicitly not issue client certs) and they were handled by a handful of people who I'd say were... not experts.

It's true that you could use client certs with say, Entra ID, and one day I will work somewhere that does that. Or maybe I won't, I'm an old man and "We should use client certs" is an ambition I've heard from management several times but never seen enacted, so the renaming of Azure AD to Entra ID doesn't seem likely to change that.

Once you're not using the Web PKI cert expiry lifetimes are much more purpose specific. It might well make sense for your Entra ID apps to have 10 year certs because eh, if you need to kill a cert you can explicitly do that, it's not a vast global system where only expiry is realistically useful. If you're minting your own ten year certs, now expiry alerting is a very small part of your risk profile.

	▲	lucidnonsense an hour ago \| parent [-]
		Client certificates aren't as esoteric as you think. They're not always used for web authentication, but many enterprises use them for WiFi/LAN authentication (EAP-TLS) and securing confidential APIs. Shops that run Kubernetes use mTLS for securing pod to pod traffic, etc. I've also seen them used for VPN authentication.

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machinationu an hour ago | parent | prev [-]

sure, I was just giving parent another way of finding all the certificates besides scanning the network