Consistency Is the Attack Surface: Detecting Coordinated Retrieval Poisoning with Embedding Geometry

Overview & Synopsis

Consistency-based defenses for retrieval-augmented generation (RAG) rest on an assumption that a sparse adversary breaks: a poisoning campaign may modify a negligible fraction of the corpus, yet retrieval concentrates its passages into the reader's context. Across four Natural Questions retrieval seeds, at least one injection reaches the top-5 in 94.1% of targeted query--seed rows, and all three form a retrieved majority in 52.9%. Under our short-answer protocol, RobustRAG and ReliabilityRAG reach exposure-conditional attack success 0.512, against 0.576 for undefended RAG, while lowering clean accuracy from 0.328 to 0.216. Coordination, however, leaves a fingerprint: a dense, query-anchored component in the evidence embedding geometry. CoordGuard audits this structure from robustly normalized query-weighted affinities, without poison labels, correct answers, or reader gradients. Its always-filter mode suppresses the selected component whenever one forms; a benign-calibrated gate suppresses it only at unusually high scores and traces a robustness--utility frontier. Pooling four retrieval seeds, e5 always-filtering reduces $\mathrm{ASR}_{\mathrm{exp}}$ to 0.167 overall and 0.138 on parametrically unknown queries, at a 13.6-point clean cost, while a Contriever gate retains clean accuracy 0.315 at $\mathrm{ASR}_{\mathrm{exp}}=0.331$. The reduction transfers across two readers, three tasks, and multiple retriever--auditor pairs, and count-matched controls attribute it to component-targeted removal rather than deletion volume. These findings recast poison detection from judging truth to auditing the structure of retrieved evidence.