Enhance Security: Deploy Custom Cipher Suite Ordering in Windows Server 2016

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This article provides essential guidance for deploying custom cipher suite ordering within Windows Server 2016 environments, specifically for Schannel. Properly configuring cipher suites is a critical aspect of securing network communications and ensuring optimal performance for modern web protocols. This document is designed to equip system administrators and security professionals with the knowledge necessary to effectively manage cipher suites and enhance the security posture of their Windows Server 2016 deployments.

This guidance is directly applicable to systems running Windows Server 2016. The information contained herein is closely related to Knowledge Base article 4032720, providing a practical approach to implementing the recommendations outlined in that resource.

Summary¶

To successfully implement a custom cipher suite ordering strategy for Schannel in Windows Server environments, it is imperative to prioritize cipher suites that exhibit full compatibility with the HTTP/2 protocol. This strategic prioritization involves placing these HTTP/2-friendly cipher suites at the forefront of your configured list. Conversely, cipher suites identified on the HTTP/2 block list, as meticulously detailed in RFC 7540, must be strategically positioned at the bottom of your cipher suite preference order. This meticulous ordering is not merely a best practice, but a fundamental requirement for ensuring seamless communication, robust protocol negotiation, and optimal performance in contemporary web environments.

Cipher suites are essentially sets of cryptographic algorithms that are used to establish secure network connections using protocols like TLS/SSL. They define the algorithms used for key exchange, encryption, and message authentication. The order in which these cipher suites are presented to a connecting client or server dictates the preference during the negotiation process. A misconfigured order can lead to compatibility issues, performance degradation, and even security vulnerabilities.

For instance, Cipher Block Chaining (CBC) mode cipher suites, while historically prevalent, are now often included on block lists due to discovered vulnerabilities and less efficient performance in modern cryptographic standards. Examples of CBC mode cipher suites that might be present in older configurations include:

• TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256
• TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA384
• TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256
• TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA384

These CBC-based suites, while still functional in many scenarios, are generally less preferred compared to more modern and secure alternatives like Galois/Counter Mode (GCM) suites. GCM offers improved performance and inherent resistance to certain types of cryptographic attacks.

Similarly, Non-PFS (Perfect Forward Secrecy) cipher suites represent another category that requires careful consideration in modern security paradigms. Perfect Forward Secrecy is a critical security property that ensures that even if the private key of a server is compromised in the future, past communication sessions remain secure. Non-PFS cipher suites lack this crucial property, making them less desirable for environments where strong security and long-term confidentiality are paramount. Examples of Non-PFS cipher suites include:

• TLS_RSA_WITH_AES_256_GCM_SHA384
• TLS_RSA_WITH_AES_128_GCM_SHA256

It is crucial to understand that while these RSA-based suites offer encryption, the key exchange mechanism does not provide forward secrecy. Modern best practices strongly advocate for the use of cipher suites that incorporate algorithms like Elliptic-curve Diffie-Hellman Ephemeral (ECDHE) for key exchange, as these algorithms inherently provide perfect forward secrecy.

If cipher suites that are on the HTTP/2 block list, such as CBC-mode suites or Non-PFS suites, are inadvertently placed at the top of your custom cipher suite order, it can lead to significant operational problems. Modern HTTP/2 clients and contemporary web browsers might encounter difficulties in negotiating a mutually acceptable HTTP/2-compatible cipher suite. This negotiation failure directly translates into an inability to effectively utilize the HTTP/2 protocol, leading to degraded website performance, increased page load times, and potential compatibility errors that can disrupt user experience.

A common manifestation of such misconfiguration is the appearance of error messages in web browsers. For example, users of the Google Chrome browser might frequently encounter the error message ERR_SPDY_INADEQUATE_TRANSPORT_SECURITY. This specific error is a strong and reliable indicator of underlying cipher suite ordering issues that are actively preventing the establishment of a secure and functional HTTP/2 connection between the client and the server.

It is critically important to recognize and appreciate that the default cipher suite ordering pre-configured in Windows Server 2016 is meticulously designed to be inherently compatible with HTTP/2 cipher suite preferences and modern security standards. Furthermore, this default ordering extends its benefits far beyond mere HTTP/2 compatibility. It proactively prioritizes cipher suites that offer the strongest security characteristics and cryptographic algorithms currently available and widely recommended in the security community. Therefore, adopting and leveraging the default ordering configuration provides a robust and secure foundation, effectively ensuring that HTTP/2 implementations on Windows Server 2016 are virtually free from cipher suite negotiation problems across a wide spectrum of web browsers and client applications. This robust configuration not only enhances security but also contributes to a smoother, more reliable user experience.

Workaround¶

In practical scenarios where protocol negotiation failures unfortunately occur, often as a direct consequence of misconfigured cipher suites, an essential and immediate remedial action is to temporarily disable the HTTP/2 protocol. This temporary disabling of HTTP/2 serves as a crucial interim measure, effectively allowing system administrators to regain control over the server’s configuration and to systematically reorder the cipher suites in an effective and controlled manner, all without causing ongoing service disruption or impacting user accessibility.

To precisely disable and subsequently re-enable the HTTP/2 protocol on Windows Server 2016, a direct modification of the Windows Registry is required. It is paramount to follow these steps with utmost care and precision, as incorrect modifications to the registry can potentially lead to system instability or unexpected behavior. Always back up your registry before making changes. The steps are as follows:

1. Initiate the Registry Editor. The Registry Editor is a powerful system utility that allows direct access and modification of the Windows Registry. To launch it, type regedit in the Windows search bar located in the taskbar, and then press the Enter key. This action will open the Registry Editor application.

2. Navigate to the designated registry subkey. Within the Registry Editor window, carefully use the left-hand navigation pane to locate and access the following specific registry subkey path: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\HTTP\Parameters. This path leads to the section of the registry where HTTP protocol parameters, including HTTP/2 settings, are stored.

3. Locate or create the EnableHttp2Tls DWORD value. Within the Parameters subkey, you need to identify if a DWORD (32-bit) Value named EnableHttp2Tls already exists. If it does not exist, you will need to create it. To create a new DWORD value, right-click in the right-hand pane of the Registry Editor, select New from the context menu, and then choose DWORD (32-bit) Value. Name the new value precisely as EnableHttp2Tls.

4. Modify the EnableHttp2Tls value data to control HTTP/2 status. Once you have located or created the EnableHttp2Tls DWORD value, you need to modify its value data to either disable or enable HTTP/2. To do this, double-click on the EnableHttp2Tls value to open its Edit DWORD (32-bit) Value dialog. In the Value data field, enter the appropriate value according to your desired action:

   • To disable HTTP/2 temporarily, set the Value data to 0 (zero).
   • To enable HTTP/2 (or re-enable it after disabling), set the Value data to 1 (one).
     Ensure you select Decimal or Hexadecimal base appropriately (Hexadecimal is often used for registry values, but Decimal ‘0’ and ‘1’ are equivalent).

5. Restart the computer to apply the changes. This is an absolutely critical step. Simply modifying the registry value is not sufficient for the changes to take effect. The Windows operating system, and specifically the HTTP service, needs to be completely restarted to recognize and load the newly configured registry settings. A full system restart ensures that all services and components are properly initialized with the updated HTTP/2 status.

By meticulously following these detailed steps, system administrators can effectively manage the operational status of the HTTP/2 protocol on Windows Server 2016. This control is essential not only for troubleshooting cipher suite related issues but also for fine-tuning server configurations to achieve an optimal balance between protocol compatibility, robust security, and overall system performance within their Windows Server 2016 environment. Remember to always exercise caution when modifying the registry and ensure you have a proper backup in place before making any changes.

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Do you have any further questions or personal experiences related to custom cipher suite ordering and its deployment in Windows Server 2016? Your insights and comments are highly valuable. Please feel free to share your thoughts and experiences in the comments section below to contribute to a richer understanding of this important security and configuration topic!