Restrict a Snap's RPC API methods

If the dapps caveat of the endowment:rpc permission is set to true, any dapp can call the Snap's RPC API by default. You can restrict the whole API to specific dapp origins or restrict the API by method and origin.

Restrict the whole API

The endowment:rpc permission has an optional allowedOrigins caveat. You can use this to restrict the domains that are allowed to make calls to the Snap's RPC API.

Restrict by method and origin

Sometimes a more granular control is required, such as filtering by method and caller origin.

You can restrict by method and origin using the origin parameter of the onRpcRequest entry point. For example:

index.ts

import type { OnRpcRequestHandler, UnauthorizedError } from "@metamask/snaps-sdk";

type MethodPermission = "*" | string[];

const RPC_PERMISSIONS: Record<string, MethodPermission> = {
  hello: "*",
  secureMethod: [
    "https://metamask.io",
    "https://www.mydomain.com",
  ]
};

const isAllowed = (method: string, origin: string) => {
  return RPC_PERMISSIONS[method] === "*" || RPC_PERMISSIONS[method].includes(origin);
};

export const onRpcRequest: OnRpcRequestHandler = async ({
  origin,
  request,
}) => {
  // Check permissions.
  if (!isAllowed(request.method, origin)) {
    throw new UnauthorizedError(`Method ${request.method} not authorized for origin ${origin}.`);
  }

  switch (request.method) {
    case "hello":
      return "world!";

    case "secureMethod":
      return "The secret is: 42";

    default:
      throw new Error("Method not found.");
  }
};

You can construct more powerful filtering methods using regular expressions or any other logic of your choice.