arcstone-mcp-sidecar: A Rust Sidecar That Blocks MCP Tool Call Replay Attacks with Atomic File Locks
AdmissibilityScience · reddit · 2026-09-22
A developer released arcstone-mcp-sidecar, a lightweight Rust capability gate addressing a real security gap: agent loops that auto-retry or mutate parameters to bypass MCP tool restrictions.
How it works:
- Separates proposal reasoning from physical execution — a trusted issuer creates a single-use claim bound to action + target + payload SHA-256;
- The Rust sidecar consumes the claim atomically via file locks (OEXCL / createnew) on first run;
- Any replay or mutated retry returns DENY [CONSUMEDAUTHORIZATION].
Implementation details:
- Validation core is #![nostd] with zero dynamic heap allocations;
- Registered in the official MCP Registry;
- Cold-start testable locally via cargo test.
Open-sourced on GitHub; the author is soliciting feedback on atomic file locks vs. stateful daemons for claim tracking.
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