LLM Safety Circuits Found in Just 50 Neurons by Unit 42
Palo Alto Unit 42 researchers have developed a technique called perturbation probing that identifies the precise feed-forward neurons responsible for LLM safety refusal behaviour, finding that as few as 50 neurons out of 350,208 control safety guardrails in Qwen3-4B. Disabling those neurons altered responses on 80% of tested harmful prompts, demonstrating that RLHF-aligned safety is structurally fragile rather than distributed. The research also introduces an FFN/Skip ratio metric that predicts model safety fragility across 13 models with 81% explanatory power, giving defenders a rapid quantitative tool for comparing alignment robustness.
