Trusting trust

So how likely is it that there are hidden trojan horse-like vulnerabilities in common open source software that we haven't noticed? How valuable is the idea that we should start over and build our systems the right way to prevent that possibility at every step?

>>6
But I'd have to write it in machine code, because my assembler might be compromised.

In fact, my editor might also be compromised, and detect that I'm trying to bootstrap a compiler. I'd have to flip bits on the machine itself (with switches?) to program a basic hex editor first