Linux

Cheeky answer:

Actual answer:
Theoretically anyway, open source software's guarantee of "no backdoor" is that the code is auditable, and you could study it and know if it has any holes and where. Of course, that presumes that you have the knowledge AND time to actually go and study thousands of lines of code. Unrealistic.
Slightly less guaranteed but still good enough to calm my mind, is the idea that there is a whole-ass community of people who do know their shit and who are constantly checking this.

Do note that like. Closed source software is known to be backdoored, only, the backdoors are mostly meant for either the owners of the software (check the fine print folks) or worse, the governments.

The biggest thing that you should note is that: It is unlikely that you (or I or most of the people here) are interesting enough that anyone will actually exploit those vulnerabilities to personally fuck you over. Your photos aren't interesting enough except as part of a mass database (which is why Google/Facebook want them). Same for your personal work data and shit.

Unless those backdoors could be used to turn your machine into a zombie for some money-making scheme (crypto or whatever) OR you're connected to people in power OR you personally piss off someone who is a hacker -- it is very unlikely you'll get screwed over due to those vulnerabilities :P

We dont.

I'm pretty sure most closed source software is already backdoored.

"We don't" is the short answer. It's unfortunate, but true.

How do you know there isn't a logic bug that spills server secrets through an uninitialized buffer? How do you know there isn't an enterprise login token signing key that accidentally works for any account in-or-out of that enterprise (hard mode: logging costs more than your org makes all year)? How do you know that your processor doesn't leak information across security contexts? How do you know that your NAS appliance doesn't have a master login?

This was a really, really close one that was averted by two things. A total fucking nerd looked way too hard into a trivial performance problem, and saw something a bit hinky. And, just as importantly, the systemd devs had no idea that anything was going on, but somebody got an itchy feeling about the size of systemd's dependencies and decided to clean it up. This completely blew up the attacker's timetable. Jia Tan had to ship too fast, with code that wasn't quite bulletproof (5.6.0 is what was detected, 5.6.1 would have gotten away with it).

Afaik, most phones are backdoored that can be abused using tools like "pegasus" which led to a huge indignation in Hungary. I don't belive PCs are exceptions. Intel ME is a proprietary software inside the CPU, often considered as a backdoor in Intel. AMD isn't an exception. It's even weirder that Intel produces chips with ME disabled for governments only.

The best you can do is use OSS software that has been battle tested. Stuff like OpenSSH and OpenVPN are very unlikely to have backdoors or major vulnerabilities currently being exploited. If you don't trust something to not be vulnerable, you're best to put it behind a more robust layer of authentication and access it only by those means.

We don’t.

Use software with an active community, don't install things you don't need, update regularly, and be thankful that you probably aren't worth using a zero-day backdoor on. Your telecom provider, on the other hand, might be - but there's not much you can do about that!

Self hosting personal photos doesn't generally require opening anything up to the internet, so most backdoors would not be accessible by anyone but you.

How do you know what you don't know?

That's basically what you're asking. If you have an answer to that general question, it will answer your specific question.

I would say you can't, but if you are using open source software, then somebody can and will find them eventually and they will be patched. Unlike with closed source software, you will never know if it has a backdoor or not. This whole episode shows both the problems with open source, being lack of funding for security audits, and the beauty of open source, being that eventually it will be detected and removed.

Good question. I have asking myself the same thing as well. In case of ssh it is possible to use 2FA with a security key, which is something I'd like to put in my todo.txt

Ah shit we are back to "Ken Thompson Compiler Hack" again

Even if there are nation state level backdoors, your personal server is not a valuable enough target to risk exposing them. Just use common sense, unattended-upgrades, and don't worry too much about it.

Check the source or pay someone to do it.

If you're using closed source software, its best to assume it has backdoors and there's no way to check.

This was likely a state-sponsored attack. If your SSH isn't exposed to the internet this probably wouldn't have effected you. Also most people run stable distros like Debian on their server, and this particular vulnerability never made it to the stable branch. I would guess that most of the computers you have ever used have backdoors. Even if you run Linux (which may itself have some) you might still have a proprietary UEFI on your motherboard. Something like xz wouldn't effect you because no government really cares what's on your server. Smaller attacks can be avoided through common sense. Some people will expose services that require zero authentication to the internet. Follow basic best practices and you will probably be fine.

Edit: Also remember, google photos once flagged a picture of a child that the child's father had taken for medical reasons as abuse, so self-hosted may not be completely private or secure, but it's better than the alternative.

You don't. Hackers often exploit things like this for ages before they are found. Every bit of non simple software also has bugs.

But chances are you won't be the target.

Just keep everything updated and you should be alright.

The main solace you can take is how quickly xz was caught: there is a lot of diverse scrutiny on it.

I do IT security for a living. It is quite complicated but not unrealistic for you to DIY.

Do a risk assessment first off - how important is your data to you and a hostile someone else? Outputs from the risk assessment might be fixing up backups first. Think about which data might be attractive to someone else and what you do not want to lose. Your photos are probably irreplaceable and your password spreadsheet should probably be a Keepass database. This is personal stuff, work out what is important.

After you've thought about what is important, then you start to look at technologies.

Decide how you need to access your data, when off site. I'll give you a clue: VPN always until you feel proficient to expose your services directly on the internet. IPSEC or OpenVPN or whatevs.

After sorting all that out, why not look into monitoring?

Reading the source code for everything running on your machine and then never updating is the only way to be absolutely 100% sure.

If backdoors exist, they’re probably enough to get your data no matter where it’s stored, so self hosting should be fine. Just keep it up to date and set up regular automatic backups.