Defenses at Odds: Measuring and Explaining Defense Conflicts in Large Language Models
About
Large Language Models (LLMs) deployed in high-stakes applications must simultaneously manage multiple risks, yet existing defenses are almost exclusively evaluated in isolation under a one-shot deployment assumption. In practice, providers patch models incrementally throughout their lifecycle-responding to newly exposed vulnerabilities or targeted data-removal requests without retraining from scratch. This raises a fundamental but underexplored question: does a later defense preserve the protections established by an earlier one? We present the first systematic study of cross-defense interactions under sequential deployment. Evaluating 144 ordered sequences across three risk dimensions and three model families, we find that 38.9% exhibit measurable risk exacerbation on the originally defended dimension. These interactions are highly asymmetric and order-dependent. To explain these phenomena, we conduct a mechanistic analysis on representative deployment sequences. Using layer-wise representational divergence and activation patching, we localize each defense to a compact set of critical layers. In conflicting sequences, the overlapping critical layers exhibit strongly anti-aligned parameter updates, whereas benign orderings maintain near-orthogonal updates. PCA trajectory analysis reveals that defense collapse stems from activation pattern reversals in these shared layers. We further introduce a layer-wise conflict score that quantifies the geometric tension between defense-induced activation subspaces, offering mechanistic insight into the observed reversals. Guided by this diagnosis, we propose conflict-guided layer freezing, a lightweight mitigation that selectively freezes high-conflict layers during sequential deployment, preserving prior protections without degrading secondary defense performance.