session_set_active

THE RISK

Medium Risk

Why session_set_active needs a policy

This tool modifies the active session state by designating a default target for future browser operations. It is a reversible state change (you can switch sessions again), qualifying it as Write. Misuse could redirect subsequent browser automation actions to unintended sessions, potentially causing medium-severity issues.

From the tool's definition Set which browser session should be the default target for subsequent operations

Documented attack patterns abuse exactly the kind of access session_set_active gives an agent:

POLICY

How to control session_set_active

PolicyLayer is an MCP gateway — it sits between your AI agents and Firefox MCP Server, and nothing reaches the server without passing your rules. This is the rule we recommend for session_set_active:

policy.json

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

session_set_active 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.

Create a free account and register Firefox MCP Server — nothing to install.
Add this policy — paste it, or build it visually.
Point your MCP client (Claude, Cursor, anything) at your gateway URL.

LIMIT THIS TOOL →

Free to start. No card required.

EXPLORE

Related tools and policies

More Firefox MCP Server tools

Destructive debug_buffers_clear Clear accumulated debug data buffers to free memory and start fresh monitoring. Destructive browser_close Close the entire browser instance and clean up all sessions and resources. Destructive session_close Close a specific browser session and clean up its resources. Execute browser_launch Launch Firefox browser with multi-session support and comprehensive debugging capabilities Execute debug_monitoring_start Start or restart comprehensive monitoring for console logs, errors, network activity, and Execute element_wait Wait for specific elements to appear or become visible. Essential for handling dynamic con Execute javascript_execute Execute custom JavaScript code in the browser context. Powerful for advanced DOM manipulat Execute page_navigate Navigate to a specific URL in the browser. Works with any web page or web application.

All 29 Firefox MCP Server tools →

Write tools on other servers

M-Team MCP Server download_torrent YouTube MCP Server generate_video_title Android Forensics ADB MCP Server combine_reports 0xarchive web3_signup

Go deeper

The MCP Attack Database → Documented attack patterns against MCP deployments — and the policies that stop them.
Argument validation → Checking tool call arguments against schema and policy before they run.
Scoped token → Per-person credentials that grant a defined subset of servers and tools.
Rate Limiting MCP Tool Calls: A Practical Guide →
MCP Security: Why Prompt Guardrails Aren't Enough →

FAQ

Questions about session_set_active

What does the session_set_active tool do? +

Set which browser session should be the default target for subsequent operations. It is categorised as a Write tool in the Firefox MCP Server MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.

How do I enforce a policy on session_set_active? +

Register the Firefox MCP Server MCP server in PolicyLayer and add a rule for session_set_active: 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 Firefox MCP Server. Nothing to install.

What risk level is session_set_active? +

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

Can I rate-limit session_set_active? +

Yes. Add a rate_limit block to the session_set_active 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 session_set_active completely? +

Set action: deny in the PolicyLayer policy for session_set_active. 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 session_set_active? +

session_set_active is provided by the Firefox MCP Server MCP server (jediluke/firefox-mcp-server). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.

Enforce policy on every Firefox MCP Server tool call.

Start from Firefox MCP Server, add the rest of your stack, and see everything your agents can call. Then put policy on all of it.

CHECK YOUR STACK →