AI agents call browser_get_cookies to retrieve information from Byob without modifying anything — typically the context-gathering step in research, monitoring, and reporting workflows, before the agent takes action elsewhere.
This tool retrieves sensitive authentication and session data (cookies with their values) from the browser. While it is a read operation (no data is modified or deleted), the severity is high because cookies often contain session tokens, authentication credentials, and personal tracking data that could be exploited if misused by a rogue agent.
From the tool's definition Tool name is 'browser_get_cookies' and description states 'Returns cookies (incl. value) for a given domain or URL.' The verb 'Returns' indicates a read/retrieval operation with no modification.
Attacks that exploit this kind of access
Returns cookies (incl. value) for a given domain or URL. Useful for. It is categorised as a Read tool in the Byob MCP Server, which means it retrieves data without modifying state.
Register the Byob MCP server in PolicyLayer and add a rule for browser_get_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_get_cookies is a Read tool with low risk. Read-only tools are generally safe to allow by default.
Yes. Add a rate_limit block to the browser_get_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_get_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_get_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.