Search Apart, Err Together: Exploiting False Corroboration in Multi-Agent Web Search

Multi-agent Web retrieval systems increasingly rely on independent search paths for factual reliability. Yet independent retrieval does not guarantee independent evidence: a single factual origin may propagate across publishers, domains, and content forms, allowing agents to retrieve superficially distinct but provenance-dependent evidence. We term this failure mode cross-agent corroboration amplification. In this paper, we introduce SourceSplit, a Web-level source expansion mechanism that transforms a single factual origin into heterogeneous descendant pages while preserving their shared evidential basis. By altering only its Web exposure, SourceSplit increases its reach across independent agent retrieval paths without introducing additional factual support. We construct a controlled Web retrieval benchmark with 100 paired counterfactual cases. Across cases, SourceSplit increases the Coordinator's selection of the target-origin answer from 6.6% to 34.0%, showing that changing how the same origin is represented and exposed—without adding independent evidential support—can substantially influence the final multi-agent decision. To mitigate this effect, we develop a candidate selection strategy based on cross-agent evidential dependence, reducing the attack-induced gain from 27.4 to 7.2 percentage points. These results show that retrieval diversity does not imply evidence independence and highlight the need to account for provenance and evidential dependence in multi-agent Web retrieval.