SEAL: Sealed Queries for Local Sanitization against Indirect Prompt Injection

Indirect prompt injection threatens local LLM agents through untrusted tool outputs, motivating defenses that suppress attacks while preserving task evidence under limited inference budgets. We introduce SEAL, which learns a soft prompt and scoring readout offline to sanitize tool outputs using a frozen task model. SEAL constructs scoring queries from trusted initial context and the learned prompt, keeping the reference independent of inspected tool text. It scores tokens without normalization across context tokens and uses preservation regularization to encourage retention of useful evidence during localized deletion. This design requires neither defense-generated tokens nor a separate verifier model. Across AgentDojo and AgentDyn, SEAL suppresses attacks while improving task completion under attack. Specifically, on AgentDojo, SEAL reduces attack success from 34.6\% to 0.1\% and increases Joint---task completion without attack success---from 51.4\% to 77.5\%, while using approximately $127\times$ fewer defense-related tokens than PISanitizer, a state-of-the-art prompt injection defense. Replication on a second backbone with the same training recipe supports applicability across model families.

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19 pages