harden_kernel
Kernel hardening: sysctl, kernel security, bootloader, memory protections
This record as markdown: /tools/defense/harden-kernel.md
What harden_kernel does on Defense
AI agents invoke harden_kernel 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 |
|---|---|---|---|
all | boolean | — | Return all sysctl values |
key | string | — | Sysctl key |
value | string | — | Value to set |
action | string | Yes | Action to perform |
dry_run | boolean | — | Preview changes |
pattern | string | — | Filter keys matching substring |
binaries | array | — | Binary paths to check |
category | string | — | Settings category to audit |
check_type | string | — | Kernel security check type |
persistent | boolean | — | Persist to /etc/sysctl.d/99-defense-mcp.conf |
grub_password | string | — | GRUB bootloader password |
kernel_params | string | — | Space-separated kernel parameters to add |
Parameters from the server's own tool schema.
Why harden_kernel is rated High
This tool modifies kernel-level parameters (sysctl), bootloader configuration, and memory protections. These are system-wide configuration changes that affect the operating system's security posture. While they are generally reversible in principle, applying incorrect sysctl or bootloader settings can render a system unbootable or destabilize critical services, making misuse high-severity.
From the tool's definition Kernel hardening: sysctl, kernel security, bootloader, memory protections
Risk signalsHigh parameter count (15 properties)
Attacks that exploit this kind of access
The rule that runs harden_kernel 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 harden_kernel, this is the rule to start with:
harden_kernel 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 harden_kernel call is checked against it from then on.
Questions about harden_kernel
Kernel hardening: sysctl, kernel security, bootloader, memory protections. 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.
harden_kernel accepts 12 parameters: all, key, value, action, dry_run, pattern, binaries, category, check_type, persistent, grub_password, kernel_params. 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 harden_kernel: 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.
harden_kernel 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 harden_kernel 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 harden_kernel. 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.
harden_kernel 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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