A Failure-Mode Benchmark for Polymorphic Sybil Poisoning in RAG

Donghyun Lee; Juntae Kim

cs.CRJul 4, 2026

A Failure-Mode Benchmark for Polymorphic Sybil Poisoning in RAG

Authors: Donghyun Lee, Juntae Kim

Organizations: Dongguk University

Abstract

We release a benchmark and failure-mode-aware evaluation framework for grounded QA under coordinated retrieval poisoning. The framework partitions reader outputs into four mutually exclusive categories (\emph{gold}, \emph{hijack}, \emph{abstention}, \emph{drift}), with instance-level paired clean-to-poison transition matrices and a Forced Exposure protocol isolating reader-side conflict resolution from retrieval variance. We introduce \emph{polymorphic sybil poisoning}, a coordinated attack class in which $S$ lexically diverse passages jointly support an attacker-chosen target while evading lexical near-duplicate filters that fully detect monomorphic baselines (capturing the residual 14.2% with E5 cosine raises false-positive rate 9 $\times$ on legitimate same-topic pairs). A monomorphic-polymorphic ablation under Forced Exposure isolates the diversity dimension and reveals a $+$ 18.8pp hijack amplification (95% paired bootstrap CI $[+15.4, +22.4]$ , $B{=}5{,}000$ ): monomorphic copies register only 4.0% as hijack while polymorphic surface diversity recovers 22.8% -- a 5.7 $\times$ amplification of the ASR-visible attack channel. ASR alone treats every non-target output identically; under attack, abstention and drift together hold 47-66% of output mass, unmonitored by ASR+ACC, and two readers at nearly identical ASR (within 0.2pp) differ by 16.5pp on abstention and 17.2pp on drift -- failure profiles invisible to ASR. We release the frozen benchmark (3{,}145 questions, 2{,}982 retained sybil groups; $S{=}6$ chosen to dominate top-10 retrieval slots, §\ref{sec:setup}), the official four-way evaluator, paired-transition utilities, and the Forced Exposure harness across five readers (7B-120B), two retrievers, and two cross-validation datasets (TriviaQA, 2Wiki), under CC~~BY-SA~~4.0 (data) and MIT (software); release information in §\ref{sec:release}.

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