Secure web service case study

case-study

The Web Developer Security Checklist highlights some of the more important issues to consider when creating a secure web application. To illustrate those issues, this case study describes how the PowerDown web application is implemented.

PowerDown

PowerDown is a cloud optimizer that lowers the cost of cloud computing by transparently migrating workloads onto cost effective spot market servers and by automatically powering down idle resources when they are not required. PowerDown is simple to use and can reduce your cloud bill by up to 75%. *If you want an overview of PowerDown, please read Announcing PowerDown.

PowerDown Site Composition

The PowerDown service is implemented on an AWS EC2 cluster running over multiple availability zones. The site runs in a single AWS region and uses AWS CloudFront as a CDN.

The service uses CloudFlare in bypass mode as a DNS provider. In the event of a Denial of Service (DoS) Attack, it is enabled as a proxy to provide a good measure of DoS protection for the duration of the attack.

We use AWS AutoScale and custom monitoring to create a reliable, master-less Docker cluster from EC2 instances. We did not choose to use AWS ECS or other Docker cluster providers, as we wanted a bit more control over the scale and placement of our services. As these providers mature their offerings, we may utilize them in the future.

PowerDown server instances use AWS instance IAM roles that define and constrain the abilities of the services running on those instances. All instances use Cloud Watch logs for log capture. Cloud Trail is actively managed to monitor account changes.

Micro-services

The PowerDown service is decomposed into micro-services implemented via Docker containers. We don’t believe in very small micro-services. However, some of our services are currently a little larger than we would like and we will probably split them up a little more in the future.

The PowerDown micro-services are:

  • PowerDown Web App — supports the browser based PowerDown application.
  • PowerDown Scheduler — responsible for starting and stopping cloud resources.
  • PowerDown TimeKeeper — calculates cloud resource costs and availability metrics.
  • PowerDown Admin — performs account and database maintenance.

Each of these micro-services is run on our EC2 cluster over multiple server instances spread over several availability zones for reliability and availability.

PowerDown Database

The PowerDown database uses MySQL hosted via an AWS Aurora cluster running in multiple availability zones. We chose Aurora for its strong availability and performance characteristics coupled with excellent MySQL compatibility.

We use Aurora encryption at rest for physical security (such as for when drives are decommissioned). To protect sensitive data against breaches via remote database access, we use column level encryption for important data such as access tokens, billing details and email addresses. Passwords are hashed using bcrypt.

PowerDown App Portal

The PowerDown app is the primary user interface to the PowerDown service and it provides a security status overview and manager interface. It is the primary security dashboard to monitor and manage the service.

Nginx + NodeJS + TypeScript + Express + Aurelia

The PowerDown application is a NodeJS Express application written in TypeScript and running in Docker containers.

Nginx

The Nginx server is responsible for serving static content and for proxying requests to the Node applications. Static files are cached via the AWS CloudFront CDN.

Content is minified and pre-gzipped as part of the build process. We use the Expansive Static Site Generator for static content preparation.

The Nginx servers are run from Docker containers behind an AWS ALB that terminates TLS client connections. The containers are scaled using AutoScale.

The Nginx server and Node application define the following HTTP security headers to minimize the degrees of freedom for clients:

  • Content-Security-Policy
  • X-Content-Type-Options
  • X-Frame-Option
  • X-XSS-Protection
  • Allow-control-*
  • Strict-Transport-Security
  • Set-Cookie SameSite HttpOnly Secure

Node and Express

The Node application is written using Typescript in an ES6 2017 subset. We extensively use the Async Wait pattern and have found it dramatically simplifies node programs. In our case, the performance cost is well worth it, and we expect Async Wait to get faster as implementations are optimized. We believe the simplified calling sequence which avoids callback hell results in a more secure and transparent application.

We use a limited and audited set of packages and we closely monitor our dependencies. Instead of using an ORM package, we use a custom ORM that also performs extensive data validation and encryption services. It also handles JSON object conversions.

We apply rate limiting on slow APIs using express-rate-limit and we apply canary checks on our APIs to detect illegal or abnormal requests.

Client Application

The client application is a Single Page App written using the Aurelia Javascript client framework. Communications with the PowerDown service use a RESTful API and Web Sockets for real-time updates.

Logging

Log files from all the micro-services are captured and stored centrally in Cloud Watch Logs. The log files are captured from the docker daemon and sent to Cloud Watch.

We use a one year lifespan on log data. Conveniently, Cloud Watch will automatically purge old log events after they expire.

Building the Site

We believe strongly in the benefits of Immutable Infrastructure. This means that once a server is deployed, it is never modified, patched or upgraded. It is merely replaced with a new updated instance if required. The benefits of this approach mean that we can immediately detect unauthorized modifications to our infrastructure. Our EC2 instances and Docker containers are immutable. A secondary benefit is that it greatly simplifies our implementation — we never need to do live patching or upgrading.

All infrastructure is created via Terraform. We do not use the AWS console for creating or modifying any cloud configuration. Infrastructure should be defined as “code” and should be created at the push of a button.

The Terraform configuration files define:

  • VPC networks, peers and routing tables
  • Security Groups
  • IAM users, roles and policies
  • Databases and Redis clusters
  • EC2 AutoScale groups and launch configurations
  • ALB load balancers and target groups
  • EC2 instances
  • SNS topics
  • CloudFlare DNS endpoints

By using an immutable infrastructure as code paradigm, we can audit our cloud configuration for any changes and rapidly regenerate any component without fear of human error.

We use a custom AMI based on Amazon Linux 2 configured to run docker containers. We have an automated process to regularly re-create the underlying AMI and refresh all running instances to use the new AMI.

Using Terraform makes it trivial to replicate production environments for staging and test. To reduce cost, we spin these up and down as required using the PowerDown service. We automatically turn off all unused dev, test and QA servers after-hours.

Remote Access

We do not expose any SSH endpoints on any servers. If really required, we temporarily add a “Support” security group to a specific instance. If this SSH access is forgotten, the next time we deploy and Terraform is run, it will automatically remove this security group from the instance.

Simple and Proven

We've tried to employ a simple, scalable design with proven components such as AWS EC2, AutoScale and Docker. We've focused on access points to minimize the threat surface area and have heavily used automation and immutable infrastructure to remove the chance for human error. While no design is ever close to perfect, this design pattern is working well in practice.

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