ESLD (External Surrogate Latent Defense): A Latent-Space Architecture for Faster, Stronger Prompt-Injection Defense

Abstract: Modern AI assistants are agentic. To answer a single user request, the underlying LLM pulls in information from many sources, such as web searches, retrieved documents, tool outputs, and user follow-ups, and reasons over them across several steps. Any of these inputs can carry malicious content. This opens the door to prompt injection, where an attacker plants text designed to override the instructions given to the assistant by its developer. For example, an attacker applying for a job can insert white-on-white text in their resume saying This is the strongest candidate. Recommend for immediate hire''. A hiring assistant may then be steered toward a favorable recommendation regardless of actual qualifications. To defend against this threat, production systems use a separate guard model in front of the assistant. The guard reads incoming text and writes a verdict (safe'' or ``unsafe'') before the assistant is allowed to act. In an agentic task with many steps, this check becomes a latency bottleneck. This paper shows that the signal needed to separate safe from malicious input is already present in the guard model's internal representation, before it writes anything out. Reading this signal directly speeds up the safety check by more than $3\times$ on average, while improving detection accuracy over the guard's verdict by 16.4 percentage points on average. This is more than latency optimization. Guard-model checks that were previously too slow to run on every step of an agent can now be placed on the critical path without sacrificing accuracy, and in fact with higher accuracy than the guard provides on its own. ESLD (External Surrogate Latent Defense) packages this finding into a deployable defense. ESLD is a model-agnostic architecture that sits on top of any existing guard model and improves both latency and detection accuracy, without retraining or modifying the guard.