browser_set_local_storage · MCP: Risk & Policy

WHAT IT DOES

What browser_set_local_storage does on Mcp

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

WHY IT NEEDS A POLICY

Why browser_set_local_storage needs a policy

This tool modifies browser localStorage by setting key-value pairs. While the changes are reversible (keys can be overwritten or deleted), the tool enables an agent to persistently alter client-side state for a specific origin, potentially affecting application behavior, authentication tokens, or user preferences.

From the tool's definition Tool "Set a localStorage key to a JSON-serializable value for a specific origin" - the term 'Set' combined with 'key to value' indicates data modification. localStorage is a client-side persistence mechanism that stores data reversibly.

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

browser_set_local_storage Rate-limited

browser_set_local_storage 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": {
    "browser_set_local_storage": {
      "limits": [
        {
          "counter": "browser_set_local_storage_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 Mcp, so you can run agents against it safely.

FAQ

Questions about browser_set_local_storage

What does the browser_set_local_storage tool do? +

Set a localStorage key to a JSON-serializable value for a specific origin inside an existing browser session. It is categorised as a Write tool in the Mcp MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.

How do I enforce a policy on browser_set_local_storage? +

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

What risk level is browser_set_local_storage? +

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

Can I rate-limit browser_set_local_storage? +

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

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

browser_set_local_storage is provided by the MCP server (victormyschik/mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.