add_regex · Firegex MCP: Risk & Policy

WHAT IT DOES

What add_regex does on Firegex

AI agents use add_regex to create or update resources in Firegex — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your Firegex environment.

WHY IT NEEDS A POLICY

Why add_regex needs a policy

This tool creates and persists a new regex pattern in a firewall service configuration. While it is reversible (can be deleted via delete_regex), it modifies security-critical firewall rules that control traffic filtering. The severity is medium because misconfiguration could inadvertently block legitimate traffic or fail to block malicious traffic, but it doesn't directly delete data or execute arbitrary commands.

From the tool's definition Tool name 'add_regex' and description 'Add a PCRE2 regex to a service' indicate a create/add operation that modifies service configuration by inserting a new regex rule.

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

add_regex Rate-limited

add_regex stays usable, but capped — an agent stuck in a loop can't make hundreds of changes a minute. Everything else on the server is denied unless you say otherwise.

View as policy code

policy.json

{
  "version": "1",
  "default": "deny",
  "tools": {
    "add_regex": {
      "limits": [
        {
          "counter": "add_regex_rate",
          "window": "minute",
          "max": 30,
          "scope": "grant"
        }
      ]
    }
  }
}

LIMIT THIS TOOL →

Instant setup, no code required.

PolicyLayer is an MCP gateway: every tool call is checked against your rule before it reaches Firegex, so you can run agents against it safely.

FAQ

Questions about add_regex

What does the add_regex tool do? +

Add a PCRE2 regex to a service. It is categorised as a Write tool in the Firegex MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.

How do I enforce a policy on add_regex? +

Register the Firegex MCP server in PolicyLayer and add a rule for add_regex: 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 Firegex. Nothing to install.

What risk level is add_regex? +

add_regex is a Write tool with medium risk. Write tools should be rate-limited to prevent accidental bulk modifications.

Can I rate-limit add_regex? +

Yes. Add a rate_limit block to the add_regex 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.

How do I block add_regex completely? +

Set action: deny in the PolicyLayer policy for add_regex. 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.

What MCP server provides add_regex? +

add_regex is provided by the Firegex MCP server (umbra2728/firegex-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.