app_harden
App hardening: audit running apps, recommendations, firewall rules, systemd sandboxing
This record as markdown: /tools/defense/app-harden.md
What app_harden does on Defense
AI agents invoke app_harden to trigger actions in Defense. What it does depends on the arguments the agent supplies, and its effects often reach beyond the immediate call: builds kicked off, notifications sent, workflows started.
| Parameter | Type | Required | Description |
|---|---|---|---|
action | string | Yes | Application hardening action |
dry_run | boolean | — | Preview changes without applying |
app_name | string | — | Application name. Available: qbittorrent, nginx, sshd, postgresql, mysql, redis, cups, avahi, mongodb, exim, samba |
lan_cidr | string | — | LAN CIDR for localhost-only ports |
service_name | string | — | Override systemd service name |
Parameters from the server's own tool schema.
Why app_harden is rated High
This tool actively applies firewall rules and systemd sandboxing configurations — both of which modify live system security policies and service isolation settings. While it also performs auditing and provides recommendations (Read/Write), the application of firewall rules and systemd sandbox configurations constitutes executing system-level changes that affect how processes run and communicate.
From the tool's definition app hardening: audit running apps, recommendations, firewall rules, systemd sandboxing
Attacks that exploit this kind of access
The rule that runs app_harden safely
PolicyLayer is an MCP gateway: it sits between your AI agents and Defense, and checks every tool call against a rule you set before the call runs. Nothing changes on the server itself. For app_harden, this is the rule to start with:
app_harden stays usable, but rate-capped: a runaway agent can't fire it dozens of times a minute. Everything else on the server is denied unless you say otherwise.
The button opens the PolicyLayer dashboard: create your workspace, connect Defense, apply this rule, and every app_harden call is checked against it from then on.
Questions about app_harden
App hardening: audit running apps, recommendations, firewall rules, systemd sandboxing. It is categorised as a Execute tool in the Defense MCP Server, which means it can trigger actions or run processes. Use rate limits and argument validation.
app_harden accepts 5 parameters: action, dry_run, app_name, lan_cidr, service_name. Required: action. The full parameter table on this page comes from the server's own tool schema.
Register the Defense MCP server in PolicyLayer and add a rule for app_harden: allow, deny, rate-limit, or require approval. Point your MCP client at the PolicyLayer proxy URL and the rule is enforced on every call, before it reaches Defense. Nothing to install.
app_harden is a Execute tool with high risk. Execute tools should be rate-limited and have argument validation enabled.
Yes. Add a rate_limit block to the app_harden rule in your PolicyLayer policy. For example, setting max: 10 and window: 60 limits the tool to 10 calls per minute. Rate limits are tracked per agent session and reset automatically.
Set action: deny in the PolicyLayer policy for app_harden. The AI agent will receive a policy violation error and cannot call the tool. You can also include a reason field to explain why the tool is blocked.
app_harden is provided by the Defense MCP server (defense-mcp-server). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
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