Microsoft to disable vulnerable RC4 cipher in Windows Server by mid-2026

Ars Technica
Ars Technica
19h ago
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Microsoft will disable RC4 cipher in Windows Servers by mid-2026 due to long-known security flaws exploited in major hacks.
Microsoft to disable vulnerable RC4 cipher in Windows Server by mid-2026
A What happened
RC4, an encryption cipher widely used in Windows Server authentication since 2000, has known vulnerabilities that have enabled serious cybersecurity breaches, including the 2024 Ascension hospital system hack. Despite previous efforts to phase out RC4 in favor of AES encryption standards, Windows Servers still responded to RC4 authentication requests, leaving networks exposed to attacks like Kerberoasting. Microsoft will update domain controller defaults to disable RC4 by mid-2026, allowing only AES-SHA1 for Kerberos authentication. Admins must identify and remediate legacy systems still relying on RC4 to secure enterprise networks. Microsoft provides new logging and scripting tools to aid this audit. The decision marks a significant security upgrade in Windows infrastructure after decades of risk.
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Key insights

  • 1

    Legacy dependencies slow security progress: RC4 persisted in Windows Server for over two decades because many legacy systems depended on it. This reflects a broader challenge where outdated technology hampers adoption of secure standards.

  • 2

    Security trade-offs in widespread protocol updates: Microsoft’s slow deprecation of RC4 reveals the complexity of changing security defaults in massively deployed software where backward compatibility risks security and operational disruption.

  • 3

    Importance of strong cryptographic practices beyond cipher strength: Kerberoasting exploits weak password hashing and lack of salting within the authentication protocol using RC4. This shows vulnerabilities can stem from implementation details, not just cipher algorithms.

Takeaways

Disabling RC4 in Windows Servers is a crucial step to strengthen enterprise cybersecurity. Successful implementation depends on administrators auditing and updating legacy authentication systems before mid-2026.

Topics

Technology & Innovation Cybersecurity World & Politics Policy & Regulation