Prefix caching was leaking other tenants' prompts through timing: a selective-isolation defence kept 70% more cache reuse
AspireAI community · arXiv (Middleware 2026)
Automatic Prefix Caching creates a timing side channel because a cache hit returns faster than a miss, letting an attacker in a multi-tenant deployment infer another user's request from hit/miss patterns; existing defences respond by disabling cache sharing and isolating users wholesale. CacheSolidarity instead monitors cache reuse across users, flags suspicious sharing and isolates only the prefixes that need it, and the authors report up to 70% higher cache reuse and 30% lower inference latency than user-isolation defences. The paper's v3 is the Middleware 2026 camera-ready and adds a detailed security analysis, though the evaluation runs on specific model and GPU configurations.
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Two sparse-autoencoder analyses have resisted scaling: matching semantically similar features across layers, and compressing large feature circuits into interpretable supernodes. This paper argues both are the same underlying problem, estimating semantic distances between SAE features that sit on different activation manifolds. Its representation treats each feature not as a single decoder vector but as an activation-weighted distribution over the hidden states expressing it, projects those distributions into a shared reference space, and compares them with Wasserstein distance. The representation is proved invariant to activation rescaling and stable under perturbation, and recovers true matches under finite-sample margin conditions. Empirically it outperformed decoder-vector and LLM-based baselines and captured subtle functional distinctions between related features, and it compressed large feature circuits into interpretable supernodes automatically. The limitation is that this is interpretability tooling: the paper demonstrates better feature matching and circuit compression, not that downstream interpretability claims become more accurate.