Enhancing the Security of Time-Bound Credentials Using AWS Nitro Enclaves with Monzo Bank

Monzo’s engineers built a highly secure infrastructure on Amazon Elastic Compute Cloud (Amazon EC2), which provides secure and resizable compute capacity for virtually any workload. They also used Kubernetes on AWS—which makes it easy to run Kubernetes in the cloud with scalable and highly virtual machine infrastructure—to deploy, scale, and orchestrate their containerized applications. Initially, a small set of trusted engineers with high-level administrative access to the company’s infrastructure managed clusters of compute instances on Amazon EC2. Amazon EC2 is built on AWS Nitro System, which is a combination of dedicated hardware and lightweight hypervisor that facilitates faster innovation and enhanced security. For Monzo the compute capability and built-in security of AWS Nitro System means that no operator on the cloud-provider side can access customer data. However, Monzo wanted to further enhance its security posture as a supplemental measure to protect customers.

Monzo engineers chose to use AWS Nitro Enclaves as a solution. “Ultimately, we are trying to secure our fundamental infrastructure on which everything else runs,” says Craig Ferguson, software engineer at Monzo. The company instituted time limits on privileged credentials and implemented MPA, which requires a second authorized user to approve an action. “I got the feeling that we were doing something really innovative with the technology,” says Dino Fizzotti, senior software engineer at Monzo. “People at AWS were really excited at how we were using the technology and the problems that we were solving.”

In October 2022, Monzo implemented its first use case. While maintaining its well-established authentication methods familiar to engineers, Monzo simultaneously deployed the workload in AWS Nitro Enclaves. Using the highly isolated environment of AWS Nitro Enclaves, Monzo engineers reduced the vulnerable surface area for sensitive data processing applications.

“That first solution gave us all the experience and confidence we needed in the system,” says Fizzotti. “We didn’t have to update our tooling or how we write software to gain the security benefits. It was just a big win for us.” The team quickly deployed a second use case, lifting and shifting its Kubernetes deployments. “The behavior of the system isn’t different from a user perspective,” says Ferguson. “It completely hides the fact that it’s running in an enclave. It’s a seamless process.”

The engineering team built additional resilience into the infrastructure by adhering to the practice of reproducible builds, which further maintains the integrity of source code. “I can trust that the image that we are deploying to production corresponds directly with the code that’s been reviewed and accepted,” says Ferguson. “This is a problem that would have been difficult to solve outside of AWS Nitro Enclaves.”

In January 2023, Monzo engineers further enhanced the security protocols for engineers operating within AWS Nitro Enclaves by rolling out MPA for time-bound credentials. Although engineers must still wait for approval, the process happens quickly. No access to privileged roles remains without MPA. “At first, there was a little bit of additional friction in our MPA solution, which is a natural consequence that we are happy to accommodate, given the additional security benefit,” says Fizzotti. “But now that the engineers are really comfortable with the tooling, the MPA approval happens within seconds.”

To increase visibility into exactly who has access to workloads running within AWS Nitro Enclaves, for how long, and for what reason, Monzo broadcasts authorization requests internally. “The justification is visible to all engineers,” says Fizzotti. “The rest of the organization can see who is making a change according to their request detail and who has approved it.”

By using the cryptographic attestation feature of AWS Nitro Enclaves, Monzo engineers certify that only authorized code is running inside the enclave, adding strong security guarantees to Monzo’s microservices workflow. Additionally, Monzo uses AWS Key Management Service (AWS KMS), which lets engineers create, manage, and control cryptographic keys across applications and AWS services. The use of AWS KMS means that only a specific enclave running authorized code can receive secrets from AWS KMS. In short, Monzo’s solution automatically protects itself from malicious interference that might have tampered with the integrity of the code.

Monzo’s implementation relies on the combination of AWS Nitro Enclaves and the support of AWS KMS for cryptographic attestation so that only trusted workloads running in an enclave can decrypt the persistent secrets necessary to operate MPA. The isolation offered by AWS Nitro Enclaves means that secrets are inaccessible even to sophisticated attackers who might have gained privileged access to the underlying infrastructure.