The mcp-router CLI served its MCP aggregator on every interface and enforced authentication only when the operator asked for it. The serve command in apps/cli/src/commands/serve.ts defaulted its host to the all-interfaces address on a fixed port, and required a token only when the corresponding flag was supplied, so a default invocation exposed the aggregator, and every MCP server it fronted, to anyone able to reach the port. Release 0.6.3 defaults the host to the loopback address and refuses to start without a token whenever the host it is given is not a loopback address; no earlier release carries either check.

The mcp-router CLI tool defaulted to binding its MCP aggregator service on all network interfaces (0.0.0.0) rather than the loopback address, making it accessible to any host that could reach the port. Authentication via token was only enforced when explicitly requested by the operator via a flag, meaning default deployments had no authentication. This exposed the aggregator and all MCP servers it fronted to unauthenticated access. The vulnerability existed in apps/cli/src/commands/serve.ts. Version 0.6.3 addresses both issues by defaulting the host binding to the loopback address and requiring a token whenever a non-loopback address is specified. No earlier release includes either of these protective checks.