The Hardening Gradient: How DeFi Security Inequality Is Reshaping the Attack Surface (2017-2026)

We challenge the conventional narrative that DeFi security is deteriorating. Across 824 confirmed incidents (2017-2026), the DeFi Risk Index (annual loss / TVL) has declined 30% from 3.33% to 2.33%. However, this improvement is non-uniform: large protocols ($1B+ TVL) reduced incidents by 75%, while small protocols ($1M TVL) experienced a 178% increase. We term this divergence the hardening gradient — a structural bifurcation where well-resourced protocols harden while under-resourced protocols remain vulnerable to basic, preventable bugs. Consequences include attack surface fragmentation, a 300x collapse in median loss magnitude, and a vulnerability floor below ~$5M TVL. We develop a security elasticity model, decompose gradient drivers, and propose policy interventions including pooled audit subsidies and automated security infrastructure.