Windows application developer guidance

Created:  09 May 2018
Windows Applications
Recommendations for the secure development, procurement and deployment of Windows applications.

This guidance contains recommendations for the secure developmentprocurement and deployment of Windows applications. Please familiarise yourself with the generic application development guidance before continuing.


1. Secure Windows application development
2. Questions for application developers during the procurement process
3. Secure deployment of Windows applications


1. Secure Windows application development

1.1 Datastore hardening

Universal Windows Platform (UWP) applications run in a container, meaning data storage is achieved in a sandboxed environment with its own file system and registry. Additionally, contained applications have restricted access to files on the host system or to data stored by other applications. Application developers do not need to implement anything to take advantage of this secure storage capability.

In cases where it is necessary to share data, UWP provides secure functionality for the following:

  • reading specific files on the host operating system as selected by the user
  • data sharing between applications
  • data storage for the same application across multiple Windows users

Developers should utilise the platform’s native features for those purposes.

Note that while UWP implements the principle of least privilege and ensures its applications cannot access external resources directly, applications with administrator privileges will still be able to read and write data to a UWP application.

1.2 Network protection

The diagram below, taken from the Windows 10 security guidance, illustrates the recommended network configuration for UWP devices handling sensitive data. In summary, a VPN is used to bring device traffic back to the enterprise. Access to internal services is brokered through a reverse proxy server which protects the internal network from attack.

Windows 1709 - figure 1

1.3 Authentication

If sensitive data is handled by the application, two-factor authentication should be required when the user logs in. You should integrate Windows Hello into UWP applications to achieve this. Windows Hello provides a biometric system built in to the operating system, and utilises the device’s Trusted Platform Module (TPM) chip for private key generation and storage (if available). This is a recommended option for key management as the TPM protects against several known attacks. Windows Hello can also require a PIN, which is backed by a TPM, if the organisation does not choose to use biometrics.

The Windows Hello two-factor authentication mechanism provides an alternative to smartcards. However, if Windows Hello is unavailable, smartcards can still be used to provide an additional layer of security.

If an application requires user authentication on launch, you should also implement additional checks for when the application has been backgrounded, or its use has been suspended for a length of time. This is necessary to ensure that the current user is still the authenticated user that launched the application. Where two-factor authentication has in place for the session, it would not be user-friendly to require additional two-factor authentication each time the user returns. 

For authentication provided by an online identity provider, Single Sign-On (SSO) authentication should be enabled with the use of the Web Authentication Broker APIs native to UWP.

1.4 Secure data storage

Data-at-rest should be protected with use of the encryption and hashing APIs provided by UWP:

  • The SymmetricKeyAlgorithmProvider and AsymmetricKeyAlgorithmProvider classes should be used to implement encryption.
  • The CryptographicEngine class provides encryption, decryption, digital signing and signature verification capabilities.
  • The Security.Cryptography.DataProtection.DataProtectionProvider class should be used to encrypt and decrypt stored local data.

UWP provides a range of functionality with built-in support for use of a device’s Trusted Platform Module (TPM), which protects against a range of attacks. Data protected by a TPM is very difficult for an attacker to access. The following TPM based functionality can be used to store data securely:

  • Platform Crypto Provider gives access to robust cryptography schemes, including those that are backed by the TPM. It can be used to securely store data on the device.
  • Windows Hello has integrated TPM support and can be used to authenticate and validate users for access to sensitive datastores.

Cryptographic keys and sensitive data should not be stored on the device unless they are stored using a TPM via a UWP feature such as those listed above.

When storing credentials, the Credential Locker feature should be used as it prevents other UWP applications from accessing them. Note that non-UWP applications and elevated users are able to access credentials within the Credential Locker, so for increased protection they could be encrypted before being stored. Credential Locker documentation details the following best practices for its use:

  • only use the credential locker for passwords and not for data blobs
  • never store credentials in plain-text using app data or roaming settings
  • only save passwords in the credential locker if the user has:
    a) successfully signed in
    b) opted to save passwords


1.5 Server-side controls

Applications storing credentials should have robust server-side control procedures in place to revoke credentials or data stored on the device if it's compromised. If credentials are stored using Credential Locker, they can be deleted from all connected devices by using the PasswordVault.Remove functionality.


1.6 Secure data transmission

In order to transmit sensitive data securely, Windows applications should conform to the following:

  • For TLS connections, certificate pinning to known organisation services should be enforced.

  • Encrypt any Websocket connections using the wss: URI scheme.
  • All HTTP or HTTPS connections should use the Web.Http API.
  • Note that because UWP applications run in a contained environment, issuer certificates to be used for validation are stored in an isolated cache within the container; as a result, you do not need to do anything to store issuer certificates securely.


1.7 Application security

In order to prevent potential memory corruption vulnerabilities and protect against reverse engineering, Windows applications should conform to the following:

  • The application should be compiled using the latest supported security flags.
  • The application should be compiled in release mode with all debug information stripped from the binaries.
  • When applications are updated, the new version should target the latest SDK version.


1.8 General security recommendations

The following additional behaviours can increase the overall security of an application:

  • Applications should minimise the amount of sensitive data stored on the device, retrieving data from the server when needed over a secure connection, and erase it when it is no longer required.
  • Applications that require authentication on application launch should also request this authentication credential when returning back into the foreground after previously being backgrounded by a user allowing for a small grace period.


2. Questions for application developers during the procurement process

For anyone procuring an application built by a third party, you can ask developers the example questions below. Their answers will help you gain more (or less) confidence about the security of their products.

The most thorough way to assess an application before deploying it would be to conduct a full source code review to ensure it meets the security recommendations and contains no malicious or unwanted functionality. Unfortunately, for the majority of third party applications, this will be infeasible or impossible. However, the responses from the third party should help provide confidence that the application is well written and likely to protect information properly.

2.1 Secure data storage

The following questions will help you gain confidence that a UWP application stores sensitive data in a secure manner:


What to look for in answers

What is the flow of data through the application - source(s), storage, processing, and transmission?

Answers should cover all forms of input including data entered by the user, network requests, and inter-process communication.

How is sensitive data stored on the device?

Data should be stored in a location that cannot be accessed by other applications on the device.

Data should not be accessible to other applications on the device through inter-process communication provided by the application, with the following exceptions:

  • App-to-app data sharing has been enabled with the implementation of share contracts.
  • The applications in question were published by the same vendor and Publisher Cache is enabled to share user data between them.

Data should be encrypted when stored on the device.

Encryption of sensitive data should be performed with a key stored either using TPM or on a remote server, as documented in the Section 1.4 above.

What device or user credentials are being stored?
Are these stored in the Credential Locker?

User credentials should be encrypted and stored using Credential Locker.

Are cloud services used by the app?
What data is stored there?
How is it protected in transit?

Data in transit to cloud services should be protected using appropriate encryption.


2.2 Secure data transmission

The following question will help you gain confidence that a UWP application transmits sensitive data securely:


What to look for in answers

Is transmitted and received data protected by cryptographic means, using well-defined protocols?
If not, why not?

Mutually authenticated TLS, Secure Chorus, or other mutually authenticated secure transport should be used to protect information as it travels between the device and other resources. This should be answered specifically for each service the application communicates with to send and receive sensitive information.


2.3 IPC mechanisms

The following question will help you gain confidence that a UWP application shares sensitive data securely:


What to look for in answers

Are any UWP Share contracts, Universal Links, or other inter-process data sharing procedures declared or handled by the application?

What actions can these invoke?

Answers should cover any instances of use and suitably explain how and why they are used, rather than a 'yes' or 'no' reply.  Any inability to explain rationale should be taken as concern about the application.

Can other applications cause the application to perform a malicious action or request on its behalf?

Interactions with other applications should be limited to only those essential for the application to function.


2.4 Binary protection

The following questions will help you gain confidence that a UWP application protects its data within a binary:


What to look for in answers

Is the application compiled in release mode and has all debug information been removed from the binary?

All debugging information should have been removed from the application.

If there is potential for loss of sensitive data as a result of an attacker fully understanding the workings of the application, does the application make use of obfuscated code or other protections against reverse engineering?

Code obfuscation should be used to help hinder reverse engineering of applications.

Is the application digitally signed by either the store or a trusted certificate?

Are signature verifications in place for tamper detection?

These protections improve the security of the application, if they are not used reasons should be given to suitably explain why.


2.5 Server side controls

The following questions will help you gain confidence that a UWP application protects its data on the server side:


What to look for in answers

If the application connects to remote services to access sensitive data, how can that access be revoked?

How long does that revocation take?

Assessors should be aware of the length of window of opportunity for data theft.


3. Secure deployment of Windows applications

This section recommends how to securely deploy the application, should it be from third party organisation or via an in-house application.

3.1 Third party Windows Store applications

Windows supports a number of methods to install new third party applications. The following section divides such applications into two types: untrusted and trusted.

Untrusted third party applications

Untrusted applications are those that have been produced by developers with whom the organisation does not have an existing relationship. This includes applications hosted by both Windows Store and on third party application stores. In these instances you should assume that the third party application may have unwanted functionality, either due to weaknesses in its design, or deliberately malicious code. The Windows Store does check apps for known malware during the certification process but this should not be relied upon by itself. Untrusted third party applications should therefore never be granted access to sensitive data, and should be assessed before inclusion in your organisation’s private enterprise application catalogue.

The Windows Store indicates which app permissions are required by the application. These should be reviewed carefully to ensure they are appropriate and do not provide the application with unnecessary or unmanaged access to sensitive data.

Network architecture components such as the reverse proxy can be used to help restrict third party applications from accessing corporate infrastructure. Although such features should be regarded as techniques to help mitigate the potential threat posed by the installation of third party applications, they cannot guarantee complete protection.

The ideal method of mitigation is to not allow any third party applications to be installed on the device, though in reality this must be taken on a 'per application' basis. Where possible, developers of the application should be consulted in order to understand better the limitations and restrictions of the application. To help your evaluation, you can use the questions given above (feel free to ask more, these represent the minimum you should find out).

Trusted third party applications

You are encouraged to learn as much as possible about the security posture of an application, so the risks of deploying it can be understood and managed wherever possible. Your organisation should ideally establish a relationship with the developers of these products and work with them to understand how their product does, or does not, meet the security posture expected.

Trusted applications have been assessed as posing an acceptable amount of risk to the organisation. This should, however, not result in complete trust of the application and steps should be taken to mitigate relevant risks and impacts. A third party application should never have access to sensitive data unless designed and accredited to do so. For those applications deemed to be trusted, their access to data should be restricted and the principle of least privilege should be applied. If devices are managed, restrictions should be applied with use of native operating system features such as Windows Information Protection (WIP), which defines the third party applications that can access enterprise protected files, Virtual Private Networks (VPN) and enterprise data on the clipboard or through a share contract.

Where a third party application is being considered to handle sensitive data, organisations may also wish to consider commissioning independent assurance. This is particularly recommended if the application implements its own protection technologies (such as a VPN or data-at-rest encryption) and does not use the native protections provided by UWP.

Many enterprise applications feature server side components, which should be considered as part of the wider risk assessment.

Security considerations

When deploying third party applications, the primary concern for an organisation is whether applications could affect the security of the enterprise network, or access data held in a sensitive datastore.

UWP applications must pass a certification process before being published to the Windows Store, which includes checks for known viruses and malware. However, this should not be taken as an indication that the application poses no threat to the organisation and additional threat mitigation activities should be undertaken. Specifically, this involves taking the recommended steps to protect both data-at-rest and data-in-transit while third party applications are permitted on the device.

You should also consider security features of the devices that will host their applications. A number of manufacturers offer custom security features to protect corporate data from other applications. If the application will only be used on these devices, permitting third party applications on the same devices may be deemed acceptable.

Security requirements

 Best practice when using third party applications is as follows:

  • Server side components such as a reverse proxy should be used to restrict enterprise network access to trusted applications.
  • The developers should be contacted in order to better understand the security posture of the application. Use the Questions for Application Developers section as your starting point.
  • Data should be protected from third party applications by restricting the application’s access to sensitive data and functionality.


3.2 In-house Windows store applications

In-house applications are those applications which are designed and commissioned by an organisation to fulfil a particular business requirement. The organisation can stipulate the functionality and security requirements of the application, and can enforce these contractually if the development work is subcontracted. For the purposes of this document, these applications are assumed to access, store, and process sensitive data. The intention when securing these applications is to minimise the opportunity for data leakage, and to harden them against physical and network-level attacks.

The following best practice guidelines should be followed when developing applications for use internally:


3.3 General security advice

Microsoft provide a selection of technologies that can be used to assist the secure deployment of applications for Windows. UWP applications can be deployed to any Windows device through the Windows Store. Non-UWP applications can be packaged with Desktop Bridge for Windows Store deployment on systems running Windows 10 Anniversary edition or later. Desktop Bridge brings the advantages of publishing applications using a store, such as automatically deploying updates, but does not bring the security or containerisation advantages offered by UWP. 

When deploying UWP applications from third parties, review whether it is a native UWP application or one provided by Desktop Bridge. In most cases, applications with the 'Uses all system resources' permission are Desktop Bridge applications.

Managed deployment

Where possible, applications should be provided to users as part of a managed Windows domain or MDM service, in an environment protected by other security controls such as restricted user accounts and AppLocker.

As an alternative to the Windows Store, application deployment can be controlled on managed devices through enterprise software management solutions such as SCCM. These solutions can be used together to control access to the software and ensure that users are always running the latest version of the UWP application.

Unmanaged deployment

Deployment to unmanaged devices introduces some risk, as it reduces the complexity of attack needed to compromise the software. In this scenario, the following guidelines should be considered:

  • Minimise storage of sensitive data on the device.
  • In a client-server model, input from the software should not be trusted by the server unless further authentication (such as described in the Secure Windows Application Development – Authentication section) is supplied to verify and authenticate an actual user.
  • Obfuscation and similar technologies could be used to increase the effort required to reverse-engineer the software. However, obfuscation should be considered only for this purpose, and should not be relied upon to provide complete protection.


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