Some of the best/easiest ways for someone with experience to find vulnerabilities is to throw automatic analysis tools at it. Many of which do work on binaries/disassembly, too.
Don't forget that in many cases it's not too hard to disassemble binaries.
Sure having the source can make it easier, but then things being open source also means that non evil actors might look at you code and help you find/fix problems before they become big. Which I believe somewhat balances it out. (for larger/widely used projects).
Like imagine log4j becoming known because of multiple supper wide spread attacks happening all over the internet, instead of getting known and then fixed and then mass attacks happened because updates take so long. And the log4j vulnerability is totally discoverable without source code. Idk. who found it but I wouldn't be surprised if someone ran into a non-security bug triggered by it and was, hey, wait a moment. Even if it didn't happen like that, it totally could have(1). Closed source would not have helped at all. At best it would delayed it and then made the effects worse due to coordinated wide spread attacks before there is a fix/mitigations.
On the contrary, log4j incident is an excellent example of how relying on open source for security completely failed. This vuln existed for years, all while being open to security researchers to find it. They didn't. Instead, there is evidence that black hats found the vuln first (perhaps because log4j is open source?).
Please, let's continue this back-and-forth of condescension. It's really productive. /s
Maybe it's not, maybe you guys have drastically improved things. What makes me strongly doubt it is that we still haven't seen substantial liability or other consequences for bad or negligent actors in this space.
FAANG security is just like everywhere else: mixed. There are mixed levels of knowledge & experience, and mixed views: i.e. even though there's a lot of consensus on high-level tenets and frameworks, like e.g. Google's Beyondcorp stuff (along with some certain security 101 stuff that's been discussed for the past 138 years), you'll always get engineers who don't like X or Y. Just like anywhere else.
Nothing is a continuum.
It seems the places where the manual digging occurs most are with devices where nobody has even thought or bothered to use such tools.
The vulns the people find are often incredibly laughable; unauthenticated actions leading to a clever path to higher privileges.
Honestly, anyone that invests the slightest amount of effort in researching how vulnerabilities are found could arrive here. I'm surprised there's skepticism, quite honestly.
It often comes down to the would-be attacker downloading a published update, looking around the PHP or whatever within, and finding unprotected or bugged APIs to leverage -- often to write their own modified version of that same update.
But the implication is that compromises were found because those firmwares were open and/or that closed firmwares suffer fewer similar compromises. Which is what I would question.
So many compromises against IoT devices or network equipment come from inspecting the open/recycled firmware.
There are a lot of interesting defcon videos about using this method
edit: To clarify, this isn't to imply that I think closed source would necessarily be better.
I think this quote puts it well. An all-too-common story about security in open source... of four people: Everybody, Somebody, Anybody, and Nobody:
"Anybody could have done it, but Nobody did it. Somebody got angry about that, because it was Everybody's job. Everybody thought Anybody could do it but Nobody realized that Everybody wouldn't do it.
It ended up that Everybody blamed Somebody when Nobody did what Anybody could have done."
Source: https://www.youtube.com/watch?v=OnJ18pyMncE