strapi/packages/strapi-plugin-users-permissions/config/policies/permissions.js

module.exports = async (ctx, next) => {
  let role;

  if (ctx.request && ctx.request.header && ctx.request.header.authorization) {
    try {
      const { _id, id } = await strapi.plugins['users-permissions'].services.jwt.getToken(ctx);
      
      if ((id || _id) === undefined) throw new Error('Invalid token: Token did not contain required fields');

      ctx.state.user = await strapi.query('user', 'users-permissions').findOne({ _id, id }, ['role']);
    } catch (err) {
      return ctx.unauthorized(err);
    }

    if (!ctx.state.user) {
      return ctx.unauthorized(`User Not Found.`);
    }

    role = ctx.state.user.role;

    if (role.type === 'root') {
      return await next();
    }
  }

  // Retrieve `guest` role.
  if (!role) {
    role = await strapi.query('role', 'users-permissions').findOne({ type: 'guest' }, []);
  }

  const route = ctx.request.route;
  const permission = await strapi.query('permission', 'users-permissions').findOne({
    role: role._id || role.id,
    type: route.plugin || 'application',
    controller: route.controller,
    action: route.action,
    enabled: true
  }, []);

  if (!permission) {
    return ctx.unauthorized();
  }

  // Execute the policies.
  if (permission.policy) {
    return await strapi.plugins['users-permissions'].config.policies[permission.policy](ctx, next);
  }

  // Execute the action.
  await next();
};