SEL4 is proven correct. Formally verified. No security holes in the default install, of any sort, ever. I mean, it doesn't do anything. But it has no security holes, with almost mathematical certainty.
As an example, CompCERT is a formally verified C compiler, and it's had a couple bugs as a result of their specification of the underlying hardware being wrong.
Also, doesn't SEL4 have widespread, practical application? IIRC as the microkernal (maybe under Minix?) on the baseband hardware on cell phones? Maybe I'm confusing it with something else.