SEAL: Towards Persistent Tool Poisoning Attack against LLM Agents via Prefix-Eliciting Trigger

Tool poisoning has emerged as a critical security threat to LLM agents, where adversaries embed malicious instructions in tool descriptions or external resources that enter the agent's context, allowing adversaries to manipulate agent's tool selection process. However, existing tool poisoning attacks primarily rely on an idealized assumption that the attacks task effect within a single interaction round. We show that their effectiveness degrades sharply in realistic multi-round agentic settings. We further reveal that this degradation stems from the fact that agents do not consistently surface attack-relevant signals in their reasoning as the planning context evolves, thereby weakening the attack's influence across rounds. To investigate whether tool poisoning remains a practical threat in such realistic settings, we propose SEAL, a two-stage method for persistent tool poisoning. Specifically, SEAL first performs Prefix-Eliciting Trigger Optimization (PETO) to optimize context-specific triggers that elicit attack-relevant reasoning prefixes, improving effectiveness within individual contexts. It then applies Local-to-Universal Trigger Aggregation (L2U) to combine these local triggers into a single universal trigger, improving generalization across different contexts and execution rounds. Extensive experiments across seven representative multi-round scenarios and different models demonstrate the attack effectiveness of SEAL, its transferability across target models, and its resistance to potential defenses.