Blast Radius - What Happens When Your AI Agent Gets Compromised
Smaller Blast Radius by Design
An MCP server that controls your desktop through the accessibility API has a naturally smaller blast radius than one with shell access. It can click buttons, read text, and navigate menus - but it cannot run arbitrary commands, delete files, or exfiltrate data through a terminal.
In theory, this is a meaningful security boundary. An agent that can only interact through the UI is limited to what the UI exposes. It cannot bypass application-level permissions or access files the current app does not show.
The Practice Problem
In practice, the security story gets complicated. Most desktop agent setups run the MCP server and other tools - including shell access through Claude Code - in the same session. The agent that is "limited to UI access" also has a tool that can run rm -rf / if it decides to.
This is not a bug in any individual tool. It is an architecture problem. When you give an agent multiple tools with different permission levels, the effective blast radius is the union of all tools, not the intersection.
Assessing Real Blast Radius
To honestly evaluate your agent's blast radius, ask these questions:
- What is the worst single action the agent could take? Not the intended action - the worst possible one given its full toolset.
- Are permission boundaries enforced or behavioral? If the only thing stopping the agent from running shell commands is the system prompt saying "do not use the shell tool," that is a behavioral constraint, not a technical one.
- What data can the agent access through the UI? The accessibility API exposes every text field, every label, every value on screen. Banking apps, password managers, email - all readable.
Practical Mitigations
Run agents with the minimum toolset needed for the task. If a workflow only needs UI automation, do not attach a shell tool. Use separate sessions for different permission levels. Log every action for audit.
The goal is not zero risk - it is honest risk assessment. Know your actual blast radius, not your theoretical one.
Fazm is an open source macOS AI agent. Open source on GitHub.