RedVisor: Reasoning-Aware Prompt Injection Defense via Zero-Copy KV Cache Reuse

TL;DR

RedVisor is a novel defense framework for Large Language Models that combines explainability and prevention to detect and reject prompt injection attacks efficiently without degrading model utility.

Contribution

RedVisor introduces a reasoning-aware, fine-grained detection and prevention approach with a lightweight adapter and KV Cache Reuse, improving detection accuracy and throughput.

Findings

Outperforms state-of-the-art defenses in detection accuracy.

Achieves high throughput with negligible utility loss.

Successfully integrated into vLLM serving engine.

Abstract

Large Language Models (LLMs) are increasingly vulnerable to Prompt Injection (PI) attacks, where adversarial instructions hidden within retrieved contexts hijack the model's execution flow. Current defenses typically face a critical trade-off: prevention-based fine-tuning often degrades general utility via the "alignment tax", while detection-based filtering incurs prohibitive latency and memory costs. To bridge this gap, we propose RedVisor, a unified framework that synthesizes the explainability of detection systems with the seamless integration of prevention strategies. To the best of our knowledge, RedVisor is the first approach to leverage fine-grained reasoning paths to simultaneously detect attacks and guide the model's safe response. We implement this via a lightweight, removable adapter positioned atop the frozen backbone. This adapter serves a dual function: it first generates an explainable analysis that precisely localizes the injection and articulates the threat, which then explicitly conditions the model to reject the malicious command. Uniquely, the adapter is active only during this reasoning phase and is effectively muted during the subsequent response generation. This architecture yields two distinct advantages: (1) it mathematically preserves the backbone's original utility on benign inputs; and (2) it enables a novel KV Cache Reuse strategy, eliminating the redundant prefill computation inherent to decoupled pipelines. We further pioneer the integration of this defense into the vLLM serving engine with custom kernels. Experiments demonstrate that RedVisor outperforms state-of-the-art defenses in detection accuracy and throughput while incurring negligible utility loss.