I cannot fathom how complicated arbitrary code execution could get with multi byte characters that could use shift registers, null bytes and byte order marks with determinism in a NOP slide on a heap spray.

Filtering only printable user input helps but even bit map images can expose a heap to a sensitive registers that will execute some target specific generated shell code.

https://en.m.wikipedia.org/wiki/NOP_slide.

https://en.m.wikipedia.org/wiki/Heap_spraying