MURMUR: Using cross-user chatter to break collaborative language agents in groups
This addresses a critical security vulnerability for users of collaborative AI systems, highlighting a new attack vector in multi-user deployments.
The paper tackles the problem of cross-user poisoning attacks in multi-user language agents, where adversaries inject messages to manipulate shared state and trigger unintended actions, and demonstrates successful attacks on real systems with high success rates and persistent effects.
Language agents are rapidly expanding from single-user assistants to multi-user collaborators in shared workspaces and groups. However, today's language models lack a mechanism for isolating user interactions and concurrent tasks, creating a new attack vector inherent to this new setting: cross-user poisoning (CUP). In a CUP attack, an adversary injects ordinary-looking messages that poison the persistent, shared state, which later triggers the agent to execute unintended, attacker-specified actions on behalf of benign users. We validate CUP on real systems, successfully attacking popular multi-user agents. To study the phenomenon systematically, we present MURMUR, a framework that composes single-user tasks into concurrent, group-based scenarios using an LLM to generate realistic, history-aware user interactions. We observe that CUP attacks succeed at high rates and their effects persist across multiple tasks, thus posing fundamental risks to multi-user LLM deployments. Finally, we introduce a first-step defense with task-based clustering to mitigate this new class of vulnerability