Such LLM would be susceptible to injections itself, even if it's not instruction-tuned (or it would be too dumb to work as a reliable guardrail). Chain injections are trivial enough, current black box style agentic systems are easily reverse engineered in practice if you have any understanding. You can mitigate it in a way similar to the security of any human organization, but fundamentally it's a cat and mouse game, just like in any human organization.