AI agents use browser_set_cookies to create or update resources in Byob — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your Byob environment.
This tool creates or modifies cookies in the browser, which are reversible data writes. While cookies can influence authentication state and session management (raising severity to high due to potential for session hijacking or privilege escalation if misused), the action itself is not destructive, financial, or code execution. It fits the Write category as it creates/modifies client-side data.
From the tool's definition Tool name is 'browser_set_cookies' and description states 'Write a cookie to the user', explicitly indicating data modification capability.
Attacks that exploit this kind of access
Write a cookie to the user. It is categorised as a Write tool in the Byob MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.
Register the Byob MCP server in PolicyLayer and add a rule for browser_set_cookies: 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 Byob. Nothing to install.
browser_set_cookies is a Write tool with medium risk. Write tools should be rate-limited to prevent accidental bulk modifications.
Yes. Add a rate_limit block to the browser_set_cookies 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 browser_set_cookies. 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.
browser_set_cookies is provided by the Byob MCP server (wxtsky/byob). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.