ContractWarden: Kernel-Enforced Damage Boundaries for AI Agents via Human-Authorized Contracts

Abstract

Large language model agents can execute commands, create subprocesses, and directly access files and networks, allowing prompt injection or planning errors to become operating-system side effects. We present ContractWarden, a Linux reference monitor that enforces a human-authorized damage boundary without trusting the agent or its policy suggestions. A model may propose a tri-state asset contract - allow, deny, or no_egress - but a human makes the final choice. An execution gate binds the contract to a concrete task before untrusted code runs. An extended Berkeley Packet Filter (eBPF) Linux Security Modules (LSM) data plane then enforces file and network decisions and monotonically propagates no_egress through processes, regular files, pipes, FIFOs, and supported Unix-domain sockets. All 570 runs across 19 security tests satisfy predefined return-value and side-effect criteria. On three co-located file-I/O workloads, median overhead is 11.96-12.89% in a Linux 6.15 virtual machine and 35.79-61.54% on a Linux 6.15 physical platform, lower than the evaluated frozen ActPlane baseline. The results demonstrate deterministic kernel enforcement for declared assets, supported paths, and controlled object lifecycles.