A Microsoft engineer noticed something was off on a piece of software he worked on. He soon discovered someone was probably trying to gain access to computers all over the world.
I suspect this was just a lucky catch of shit that happens all the time. Supply chain attacks are super scary and effectively impossible to eliminate in modern software development.
It's almost impossible to spot by people looking directly at the code. I'm honestly surprised this one was discovered at all. People are still trying to deconstruct this exploit to figure out how the RCE worked.
And supply chain attacks are effectively impossible to eliminate as an attack vector by a developer-user of a N-level dependency. Not having dependencies or auditing every dependency is unreasonable in most cases.
Right now the greatest level of supply chain secuirty that I know of is formal verification, source reproducible builds, and full source bootstrapping build systems. There was a neat FPGA bootstrapping proj3ct (the whole toolchain to program the fpga could be built on the FPGA) at last years FOSDEMs conference, and I have to admit the idea of a physically verifiable root of trust is super exciting to me, but also out of reach for 98% of projects (though more possible by the day).