AWS Security Blog

Cyber hygiene and MAS Notice 655

August 31, 2021: AWS KMS is replacing the term customer master key (CMK) with AWS KMS key and KMS key. The concept has not changed. To prevent breaking changes, AWS KMS is keeping some variations of this term. More info.

In this post, I will provide guidance and resources that will help you align to the expectations of the Monetary Authority of Singapore (MAS) Notice 655 – Notice on Cyber Hygiene.

The Monetary Authority of Singapore (MAS) issued Notice 655 – Notice on Cyber Hygiene on 6 Aug 2019. This notice is applicable to all banks in Singapore and takes effect from 6 Aug 2020. The notice sets out the requirements on cyber hygiene for banks across the following six categories: administrative accounts, security patches, security standards, network perimeter defense, malware protection, and multi-factor authentication.

Whilst Notice 655 is specific to all banks in Singapore, the AWS security guidance I provide in this post is based on consistent best practices. As always, it’s important to note that security and compliance is a shared responsibility between AWS and you as our customer. AWS is responsible for the security of the cloud, but you are responsible for your security in the cloud.

To aid in your alignment to Notice 655, AWS has developed a MAS Notice 655 – Cyber Hygiene – Workbook, which is available in AWS Artifact. The workbook covers each of the six categories of cyber hygiene in Notice 655 and maps to the following:

The downloadable workbook contains two embedded formats:

  • Microsoft Excel – coverage includes AWS responsibility control statements, and Well-Architected Framework best practices.
  • Dynamic HTML – same as Microsoft Excel, with the added feature that the Well-Architected Framework best practices are mapped to AWS Config managed rules and Amazon GuardDuty findings, where available or applicable.

Administrative accounts

“4.1. A relevant entity must ensure that every administrative account in respect of any operating system, database, application, security appliance or network device, is secured to prevent any unauthorised access to or use of such account.”

For administrative accounts, it is important to follow best practices for the privileged accounts, keeping in mind both human and programmatic access.

The most privileged user account in an AWS account is the root user. When you first create an AWS account (unless you create it with AWS Organizations), this is the initial user account created. The root user is associated with the provided email address and password used to create the account. The root user account has access to every resource in the account—including the ability to close it. To align to the principle of least privilege, the root user account should not be used for everyday tasks. Instead, AWS Identity and Access Management (IAM) roles should be created and scoped to particular roles and functions within your organization. Furthermore, AWS strongly recommends that you integrate with a centralized identity provider, or a directory service, to authenticate all users in a centralized place. This reduces the requirement for multiple credentials and reduces management complexity.

There are some additional key steps that you should do to further protect your root user account.

Ensure that you have a very long and complex password, and if necessary you should change the root user password to meet this recommendation.

  • Put the root user password in a secure location, and consider a physical or virtual password vault with strong multi-party access protocol.
  • Delete any access keys, and remove any programmatic access keys from the root user account.
  • Enable multi-factor authentication (MFA), and consider a hardware-based token that is stored in a physical vault or safe with a strong multi-party access protocol. Consider using a separate secure vault store for the password and the MFA token, with separate permissions for access.
  • A simple but hugely important step is to ensure your account information is correct, which includes the assigned root email address, so that AWS Support can contact you.

Do keep in mind that there are a few AWS tasks that require root user.

You should use IAM roles for programmatic or system-to-system access to AWS resources that are integrated with IAM. For example, you should use roles for applications that run on Amazon Elastic Compute Cloud (Amazon EC2) instances. Ensure the principle of least privilege is being applied for the IAM policies that are attached to the roles.

For cases where credentials that are not from AWS IAM, such as database credentials, need to be used by your application, you should not hard-code these credentials in the application source code, or stored in an un-encrypted state. It is recommended that you use a secrets management solution. For example, AWS Secrets Manager helps you protect the secrets needed to access your applications, services, and IT resources. Secrets Manager enables you to easily rotate, manage, and retrieve database credentials, API keys, and other secrets throughout their lifecycle. Users and applications can retrieve secrets with a call to Secrets Manager APIs, which eliminates the need to hardcode sensitive information in plain text.

For more information, see 4.1 Administrative Accounts in the MAS Notice 655 workbook on AWS Artifact.

Security Patches

“4.2 (a) A relevant entity must ensure that security patches are applied to address vulnerabilities to every system, and apply such security patches within a timeframe that is commensurate with the risks posed by each vulnerability.”

Consider the various categories of security patches you need to manage, based on the AWS Shared Responsibility Model and the AWS services you are using.

Here are some common examples, but does not represent an exhaustive list:

Operating system

When using services from AWS where you have control over the operating system, it is your responsibility to perform patching on these services. For example, if you use Amazon EC2 with Linux, applying security patches for Linux is your responsibility. To help you with this, AWS publishes security patches for Amazon Linux at the Amazon Linux Security Center.

AWS Inspector allows you to run scheduled vulnerability assessments on your Amazon EC2 instances, and provides a report of findings against rules packages that include operating system configuration benchmarks for common vulnerabilities and exposures (CVEs) and Center for Internet Security (CIS) guidelines. To see if AWS Inspector is available in your AWS Region, see the list of supported Regions for Amazon Inspector.

For managing patching activity at scale, consider AWS Systems Manager Patch Manager to automate the process of patching managed instances with both security-related patches and other types of updates.

Container orchestration and containers

If you are running and managing your own container orchestration capability, it is your responsibility to perform patching for both the primary and secondary nodes. If you are using Amazon Elastic Kubernetes Service (Amazon EKS), then AWS manages the patching of the control plane, and publishes EKS-optimized Amazon Machine Images (AMIs) that include the necessary worker node binaries (Docker and Kubelet). This AMI is updated regularly and includes the most up to date version of these components. You can update your EKS managed nodes to the latest versions of the EKS-optimized AMIs with a single command in the EKS console, API, or CLI. If you are building your own custom AMIs to use for EKS worker nodes, AWS also publishes Packer scripts that document the AWS build steps, to allow you to identify the binaries included in each version of the AMI.

AWS Fargate provides the option of serverless compute for containers, so you can avoid the operational overhead of scaling, patching, securing, and managing servers.

For the container images, you do need to ensure these are scanned for vulnerabilities and patched. The Amazon Elastic Container Registry (Amazon ECR) service offers the ability to scan container images for common vulnerabilities and exposures (CVEs).

Database engines

If you are running and managing your own databases on top of an AWS service such as Amazon EC2, it is your responsibility to perform patching of the database engine.

If you are using Amazon Relational Database Service (Amazon RDS), then AWS will automatically perform the patching of the database engine. This is done within the configurable Amazon RDS maintenance window, and is your opportunity to control when DB instance modifications, database engine version upgrades, and software patching occurs.

In cases where you are using fully-managed AWS database services such as Amazon DynamoDB, AWS takes care of the underlying patching requirements.


For application code and dependencies that you run on AWS services, you own and manage the patching. This applies to applications that your organization has built themselves, or applications from a third-party software vendor. You should make sure that you have a mechanism for ensuring that vulnerabilities in the application code you run are regularly identified & patched.

For more information, see 4.2 Security Patches in the MAS Notice 655 workbook on AWS Artifact.

Security Standards

“4.3. (b)… a relevant entity must ensure that every system conforms to the set of security standards.”

After you have defined your organizational security standards, the next step is to verify your conformance to these standards. In my consultation with customers, I advise that it is a best practice to enforce these security standards as early in the development lifecycle possible. For example, you may have a standard requiring that data of a specific data classification must be encrypted at rest with a AWS Key Management Service (AWS KMS) customer-managed KMS key. The way this is typically achieved is by defining your Infrastructure-as-Code (IaC), for example using AWS CloudFormation. As your projects move through the various stages of development in your pipeline, you can automatically and programmatically check your IaC templates against codified security standards that you have defined. AWS has a number of tools that assist you with defining you rules and evaluating your IaC templates.

In the case of AWS CloudFormation, you may want to consider the tools AWS CloudFormation Guard, cfn-lint or cfn_nag. Enforcing your security standards as early in the development lifecycle as possible has some key benefits. It instills a culture and practice of creating deployments that are aligned to your standards from the outset, and allows developers to move fast by using the tools and workflow that work best for their team, while providing feedback early enough so they have time to resolve any issues & meet security standards during the development process.

It’s important to complement this IaC pipeline approach with additional controls to ensure that security standards remain in place after it is deployed. You should make sure to look at both preventative controls and detective controls.

For preventative controls, the focus is on IAM permissions. You can use these fine-grained permissions to enforce at the level of the IAM principal (such as user or role) to control what actions can or cannot be taken on AWS resources. You can make use of AWS Organizations service control policies (SCPs) to enforce permission guardrails globally across the entire organization, across an organizational unit, or across individual AWS accounts. Some example SCPs that may align to your security standards include the following: Prevent any virtual private cloud (VPC) that doesn’t already have internet access from getting it, Prevent users from disabling Amazon GuardDuty or modifying its configuration. Additionally, you can use the SCPs described in the AWS Control Tower Guardrail Reference, which you can implement with or without using AWS Control Tower.

For detective controls, after your infrastructure is deployed, you should make use of the AWS Security Hub and/or AWS Config rules to help you meet your compliance needs. You should ensure that the findings from these services are integrated with your technology operations processes to take action, or you can use automated remediation.

For more information, see 4.3 Security Standards in the MAS Notice 655 workbook on AWS Artifact.

Network Perimeter Defense

“4.4. A relevant entity must implement controls at its network perimeter to restrict all unauthorised network traffic.”

Having a layered security strategy is a best practice, and this applies equally to your network. AWS provides a number of complimentary network configuration options that you can implement to add network protection to your resources. You should consider using all of the options I describe here for your AWS workload. You can implement multiple strategies together where possible, to provide network defense in depth.

For network layer protection, you can use security groups for your VPC. Security groups act as a virtual firewall for members of the group to control inbound and outbound traffic. For each security group, you add rules that control the inbound traffic to instances, and a separate set of rules that control the outbound traffic. You can attach Security groups to EC2 instances and other AWS services that use elastic network interfaces, including RDS instances, VPC endpoints, AWS Lambda functions, and Amazon SageMaker notebooks.

You can also use network access control lists (ACLs) as an optional layer of security for your VPC that acts as a firewall for controlling traffic in and out of one or more subnets, and supports allow rules and deny rules. Network ACLs are a good option for controlling traffic at the subnet level.

For application-layer protection against common web exploits, you can use AWS WAF. You use AWS WAF on the Application Load Balancer that fronts your web servers or origin servers that are running on Amazon EC2, on Amazon API Gateway for your APIs, or you can use AWS WAF together with Amazon CloudFront. This allows you to strengthen security at the edge, filtering more of the unwanted traffic out before it reaches your critical content, data, code, and infrastructure.

For distributed denial of service (DDoS) protection, you can use AWS Shield, which is a managed service to provide protection against DDoS attacks for applications running on AWS. AWS Shield is available in two tiers: AWS Shield Standard and AWS Shield Advanced. All AWS customers benefit from the automatic protections of AWS Shield Standard, at no additional charge. AWS Shield Standard defends against most common, frequently occurring network and transport layer DDoS attacks that target your web site or applications. When you use AWS Shield Standard with Amazon CloudFront and Amazon Route 53, you receive comprehensive availability protection against all known infrastructure (Layer 3 and 4) attacks. AWS Shield Advanced provides advanced attack mitigation, visibility and attack notification, DDoS cost protection, and specialist support.

AWS Firewall Manager allows you to centrally configure and manage firewall rules across your accounts and applications in AWS Organizations. Also in AWS Firewall Manager, you can centrally manage and deploy security groups, AWS WAF rules, and AWS Shield Advanced protections.

There are many AWS Partner Network (APN) solutions that provide additional capabilities and protection that work alongside the AWS solutions discussed, in categories such as intrusion detection systems (IDS). For more information, find an APN Partner.

For more information, see 4.4 Network Perimeter Defence in the MAS Notice 655 workbook on AWS Artifact for additional information.

Malware protection

“4.5. A relevant entity must ensure that one or more malware protection measures are implemented on every system, to mitigate the risk of malware infection, where such malware protection measures are available and can be implemented.”

Malware protection requires a multi-faceted approach, including all of the following:

  • Training your employees in security awareness
  • Finding and patching vulnerabilities within your AWS workloads
  • Segmenting your networks
  • Limiting access to your critical systems and data
  • Having a comprehensive backup and restore strategy
  • Detection of malware
  • Creating incident response plans

In the previous sections of this post, I covered security patching, network segmentation, and limiting access. Now I’ll review the remaining elements.

Employee security awareness is crucial, because it is generally accepted that the primary vector by which malware is installed within your organization is by phishing (or spear phishing), where an employee is misled into installing malware, or opens an attachment that uses a vulnerability in software to install itself.

For backup and restore, a comprehensive and tested strategy is crucial, especially when the motivation of the malware is deletion, modification, or mass encryption (ransomware). You can review the AWS backup and restore solutions and leverage the various high-durability storage classes provided by Amazon Simple Storage Service (Amazon S3).

For malware protection, as with other security domains, it is important to have complementary detective controls along with the preventative ones, it is important to have systems for early detection of malware, or of the activity indicative of malware presence. Across your AWS account, when you understanding what typical activity looks like, that gives you a baseline of network and user activity that you can continuously monitor for anomalies.

Amazon GuardDuty is a threat detection service that continuously monitors and compares activity within your AWS environment for malicious or unauthorized behavior to help you protect your AWS accounts and workloads. Amazon GuardDuty uses machine learning, anomaly detection, and integrated threat intelligence to identify and prioritize potential threats.

Continuing on the topic of malware detection, you should consider other approaches as well, including endpoint detection and response (EDR) solutions. AWS has a number of partners that specialize in this space. For more information, find an APN Partner.

Finally, you should make sure that you have a security incident response plan to help you respond to an incident, communicate during an incident, and recover from it. AWS recommends that you create these response plans in the form of playbooks. A good way to create a playbook is to start off simple and iterate to improve your plan. Before you need to respond to an actual event, you should consider the tasks that you can do ahead of time to improve your recovery timeframes. Some of the issues to consider include pre-provisioning access to your responders, and pre-deploying the tools that the responders or forensic teams will need. Importantly, do not wait for an actual incident to test your response and recovery plans. You should run game days to practice, improve and iterate.

For more information, see 4.5 Malware protection in the MAS Notice 655 workbook on AWS Artifact.

Multi-factor authentication

“4.6. … a relevant entity must ensure that multi-factor authentication is implemented for the following:
(a)all administrative accounts in respect of any operating system, database, application, security appliance or network device that is a critical system; and
(b)all accounts on any system used by the relevant entity to access customer information through the internet.“

When using multi-factor authentication (MFA), it’s important to for you to think of the various layers that you need to implement.

For access to the AWS API, AWS Management Console, and AWS resources that use AWS Identity and Access Management (IAM), you can configure MFA with a number of different form factors, and apply it to users within your AWS accounts. As I mentioned in the Administrative accounts section, AWS recommends that you apply MFA to the root account user. Where possible, you should not use IAM users, but instead use identity federation with IAM roles. By using identity federation with IAM roles, you can apply and enforce MFA at the level of your identity provider, for example Active Directory Federation Services (AD FS) or AWS Single Sign-On. For highly privileged actions, you may want to configure MFA-protected API access to only allow the action if MFA authentication has been performed.

With regards to third-party applications, which includes software as a service (SaaS), you should consider integration with AWS services or third-party services to provide MFA protection. For example, AWS Single Sign-On (SSO) includes built-in integrations to many business applications, including Salesforce, Office 365, and others.

For your own in-house applications, you may want to consider solutions such as Amazon Cognito. Amazon Cognito goes beyond standard MFA (which use SMS or TOTP), and includes the option of adaptive authentication when using the advanced security features. With this feature enabled, when Amazon Cognito detects unusual sign-in activity, such as attempts from new locations and unknown devices, it can challenge the user with additional verification checks.

For more information, see 4.6 Multi-Factor authentication in the MAS Notice 655 workbook on AWS Artifact.


AWS products and services have security features designed to help you improve the security of your workloads, and meet your compliance requirements. Many AWS services are reviewed by independent third-party auditors, and these audit reports are available on AWS Artifact. You can use AWS services, tools, and guidance to address your side of the shared responsibility model to align with the requirements stated in Notice 655 – Notice on Cyber Hygiene.

Review the MAS Notice 655 – Cyber Hygiene – Workbook on AWS Artifact to understand both the AWS control environment (the AWS side of the shared responsibility model) and the guidance AWS provides to help you with your side of the shared responsibility model. You will find AWS guidance in the AWS Well-Architected Framework best practices, and where available or applicable to detective controls, in AWS Config rules and Amazon GuardDuty findings.

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Boyd author photo

Darran Boyd

Darran is a Principal Security Solutions Architect at AWS, responsible for helping remove security blockers for our customers and accelerating their journey to the AWS Cloud. Darran’s focus and passion is to deliver strategic security initiatives that un-lock and enable our customers at scale across the financial services industry and beyond.