CacheTracer Uses Prefix-Cache Reuse as a Side Channel and Finds 37.1% of LLM API Reseller Pairs Share Hidden Upstreams

arXiv 2608.20732·high signalXBlueskyLinkedInCopy link

CacheTracer (arXiv 2608.20732, Aug 21) sent 1.1 million API requests across 636 endpoint pairs on 39 LLM API reseller endpoints, using a Flood-then-Prove primitive that populates cache state through one endpoint and tests whether another can reuse it. It found 37.1% of measured pairs share cache reach, a containment order spanning seven layers, and one cache reach contained within at least 31 other nodes. For anyone routing production traffic through a cheap reseller, this is a measurable blast-radius argument: a single upstream confidentiality or integrity failure propagates to users of seemingly independent providers.