So it is with that perennial nuisance, the injection vulnerability. All the fuzzers in the world cannot hope to find all of them.
There are two paths.
One, keep looking. Markets spring up for discoveries. Make measurements of density, value, and expected loss. Acknowledge they remain forever and keep getting hacked.
Two, apply the principles we have known since at least the development of Signalling System No. 7 in fucking 1984, and separate the control and data channels properly.