What is going on with RC4 in Kerberos?
Howdy everyone! The cat is out of the bag when it comes to Microsoft’s forward outlook on RC4 usage in Kerberos. In the past few months, we have published the following. Beyond RC4 for Windows authentication Detect and remediate RC4 usage in Kerberos How to manage Kerberos KDC usage of RC4 for service account ticket issuance changes related to CVE-2026-20833 As you can see, the writing is on the wall for RC4. However, we also understand that many folks still have dependencies on RC4 for one reason or another. My goal with this blog post is to help clarify things for those folks. I know that not everyone is interested in the nitty gritty of how everything works and just want quick answers, so I am going to start with those. Frequently Asked Questions (FAQ) If this FAQ does not answer your questions about RC4 dependencies in your environment, please feel free to reach out to stillneedrc4@microsoft.com with your question and information on how you are still leveraging RC4. Is RC4 being removed from Windows? As of January 2026, there are currently no plans to remove RC4 from Windows. However, DES has been removed from Windows Server 2025 and Windows 11 24H2 and newer versions. For additional information please see Removal of DES in Kerberos for Windows Server and Client | Microsoft Community Hub I have a machine running a mission critical service that only supports RC4. What do I do? If you have defined the DefaultDomainSupportedEncTypes registry key on all relevant Windows Key Distribution Centers (KDCs), then you should not expect any breaking changes. Alternatively, if you have defined the msds-SupportedEncryptionTypes attribute for the service account and the bitmask includes RC4 then no action needs to be taken. I have a Windows Server 2003 deployment that we are working to migrate away from, but I need them to work in the meantime. What do I do? As I'm sure you already know Windows Server 2003 left end of life in 2015 . The highest priority should be migrating to a version of Windows that is currently receiving security updates. Thus, this machine is treated the same as any other machine that needs RC4 as Windows Server 2003 does not support AES-SHA1. I am seeing RC4 usage in my environment through Event Id 4768 and Event Id 4769 in the Security Event Logs on my KDC. Will I be broken by these changes? The existence of Event Id 4768 and Event Id 4769 in the Security Event Log is not necessarily an indication that things will break in isolation. While in the initial deployment phase of the 2026 January Cumulative updates (often referred to as audit mode), I recommend looking in the Windows System Event Log for the newly added Event Id 201, 202, 206, and 207. We specifically created these new events to identify interactions that are at risk as part of the RC4 default disablement plan. If you see any of those new events generated in the Windows System Event Log, then you are at risk for breaking changes upon moving to enforcement mode. I have an old service account that doesn't have AES-SHA1 keys. Will I be broken? Maybe. At-risk service accounts will be referenced in Windows System Event Log with Event Id 202, or Event Id 207. As for remediation, there are a few options available to this specific problem. The best recommendation would be to reset the account password on a modern Windows KDC. During a password change, new keys are generated automatically with all the available Kerberos Encryption Types. If that is not possible, then either defining the DefaultDomainSupportedEncTypes or configuring the msds-SupportedEncryptionTypes on the target service account to include RC4 would be your best bet. How does DefaultDomainSupportedEncTypes work? When making a service ticket request, the KDC will search for the target service accounts msds-SupportedEncryptionTypes Active Directory (AD) attribute on the target service account. If that attribute is either 0 or undefined, then the assumed encryption type value will be applied. The assumed encryption type is what the KDC assumes that all accounts running supported Windows versions in an AD domain support. The DefaultDomainSupportedEncTypes registry key controls what the KDC will apply as the assumed encryption types. By default, the assumed encryption types are 0x27 (DES, RC4 and AES-SHA1 session keys). On Windows Server 2025, this default value is 0x24 (RC4, and AES-SHA1 session keys) because DES has been removed from Windows Server 2025. If you would like to define the assumed encryption types, you MUST ensure that all accounts within the domain support the configured value. While reviewing this configuration, it's worth investigating if DES is necessary in your environment. For additional information, please see: DES Detection · microsoft/Kerberos-Crypto Wiki. The configuration of the supported Kerberos encryption types through the DefaultDomainSupportedEncTypes registry keys is almost identical to the msds-SupportedEncryptionTypes bitmask. For example: 0x18 in msds-SupportedEncryptionTypes means AES128-CTS-HMAC-SHA1-96 and AES256-CTS-HMAC-SHA1-96 0x18 in DefaultDomainSupportedEncTypes means AES128-CTS-HMAC-SHA1-96 and AES256-CTS-HMAC-SHA1-96 CAVEAT: The bitmask of 0x20 is ONLY honored when applied to DefaultDomainSupportedEncTypes. More additional details on the Supported Encryption Type bitmask please see [MS-KILE]: Supported Encryption Types Bit Flags | Microsoft Learn What is happening with the January 2026 Cumulative Updates? With the January 2026 Cumulative Updates, we are beginning to change the default behavior of the assumed encryption types on a KDC. We will be removing RC4 as one of the assumed encryption types. We have added new auditing to help detect RC4 usage that is currently permitted through unconfigured assumed encryption types. On installing the April 2026 Windows Cumulative Updates on supported DCs, we will be moving to an enforcement phase where usage of RC4 through the assumed encryption types will now be blocked. Enforcement mode can be rolled back to audit mode. On installing the July 2026 Windows Cumulative Updates, the behavior will be identical to April 2026, however you will no longer roll back to audit mode. For additional reading I recommend reading the How to manage Kerberos KDC usage of RC4 for service account ticket issuance changes related to CVE-2026-20833 - Microsoft Support in its entirety. Closing things out Security hardening is never easy, and we empathize with the IT professionals who are working to harden their environment. Migration and upgrades are not always an easy process, and we at Microsoft want to help make this process as smooth as possible. As mentioned in other blog posts, we have a few Kerberos RC4 identification scripts that are available on GitHub at Microsoft/Kerberos-Crypto. And to help us understand your RC4 dependencies please feel free to drop us a mail at stillneedrc4@microsoft.com.
