While generative artificial intelligence (GenAI) models have achieved significant success, their misuse for generating deceptive content raises growing concerns about online information security. Out-of-context (OOC) multimodal misinformation detection systems typically rely on Web-retrieved evidence to identify images repurposed in false contexts, but they are increasingly challenged by the presence of GenAI-polluted evidence. Existing work mainly focuses on verifying claims that have undergone stylistic rewriting at the claim level and assume a clean evidence corpus. In this work, we remove this assumption and systematically study the impact of GenAI-driven evidence pollution threat on OOC detection. We show that polluted evidence can degrade the performance of state-of-the-art detectors by more than 9 percentage points. We propose two mitigating strategies, cross-modal evidence reranking and cross-modal claim-evidence reasoning, to address the challenge posed by polluted evidence. Extensive experiments on two benchmark datasets demonstrate that our approaches effectively enhance the robustness of existing OOC detectors amidst polluted evidence. The source code and data are publicly available at https://github.com/YanZehong/GenAI-Evidence-Pollution.

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