MechAudit-40: White-Box Auditing across 40 LLM Attack Mechanisms

While LLM attacks span prompt optimization, multi-turn context manipulation, retrieval poisoning, and model backdoors, white-box defenses are typically evaluated on isolated attack families. Consequently, whether heterogeneous attacks leave internal representation shifts that generalize to unseen threat mechanisms remains unknown. We present MechAudit-40, a systematic evaluation of 40 attack mechanisms across five open-weight model architectures. Threat-specific success criteria, 100,000 matched clean-attack representation pairs, predefined categories, and grouped holdouts isolate genuine attack-induced displacement from target scale, corpus bias, and data-leakage shortcuts. Across this testbed, attacks induce structured multi-depth trajectories rather than isolated layer spikes. While raw peaks are non-portable across architectures, target-calibrated profiles preserve transferable geometric signatures: under complete mechanism holdout, hidden states alone recover the threat category of unseen attacks with 82.5% accuracy. Guided by this finding, we design MechAudit, a runtime auditor that operates under strict zero-oracle constraints without requiring clean baseline traces or attack metadata. MechAudit detects 81.1% of held-out attack executions at a 0.70% false-positive rate and maintains 78.1% recall when an entire functional category is withheld. In matched comparisons, MechAudit is the only detector that avoids mechanism-level coverage collapse, maintaining over 50% recall across all 40 mechanisms. Internal representations thus support cross-mechanism attack-exposure auditing against calibrated benign references, but decouple from downstream task compromise and parameter integrity.