Microsoft Will Finally Kill Obsolete Cipher That Has Wrecked Decades of Havoc

An anonymous reader quotes a report from Ars Technica: Microsoft is killing off an obsolete and vulnerable encryption cipher that Windows has supported by default for 26 years following more than a decade of devastating hacks that exploited it and recently faced blistering criticism from a prominent US senator. When the software maker rolled out Active Directory in 2000, it made RC4 a sole means of securing the Windows component, which administrators use to configure and provision fellow administrator and user accounts inside large organizations. RC4, short for Rivist Cipher 4, is a nod to mathematician and cryptographer Ron Rivest of RSA Security, who developed the stream cipher in 1987. Within days of the trade-secret-protected algorithm being leaked in 1994, a researcher demonstrated a cryptographic attack that significantly weakened the security it had been believed to provide. Despite the known susceptibility, RC4 remained a staple in encryption protocols, including SSL and its successor TLS, until about a decade ago.

Last week, Microsoft said it was finally deprecating RC4 and cited its susceptibility to Kerberoasting, the form of attack, known since 2014, that was the root cause of the initial intrusion into Ascension's network. 'By mid-2026, we will be updating domain controller defaults for the Kerberos Key Distribution Center (KDC) on Windows Server 2008 and later to only allow AES-SHA1 encryption,' Matthew Palko, a Microsoft principal program manager, wrote. 'RC4 will be disabled by default and only used if a domain administrator explicitly configures an account or the KDC to use it.' Following next year's change, RC4 authentication will no longer function unless administrators perform the extra work to allow it. In the meantime, Palko said, it's crucial that admins identify any systems inside their networks that rely on the cipher. Despite the known vulnerabilities, RC4 remains the sole means of some third-party legacy systems for authenticating to Windows networks. These systems can often go overlooked in networks even though they are required for crucial functions.

To streamline the identification of such systems, Microsoft is making several tools available. One is an update to KDC logs that will track both requests and responses that systems make using RC4 when performing requests through Kerberos. Kerberos is an industry-wide authentication protocol for verifying the identities of users and services over a non-secure network. It's the sole means for mutual authentication to Active Directory, which hackers attacking Windows networks widely consider a Holy Grail because of the control they gain once it has been compromised. Microsoft is also introducing new PowerShell scripts to sift through security event logs to more easily pinpoint problematic RC4 usage. Microsoft said it has steadily worked over the past decade to deprecate RC4, but that the task wasn't easy. 'The problem though is that it's hard to kill off a cryptographic algorithm that is present in every OS that's shipped for the last 25 years and was the default algorithm for so long, Steve Syfuhs, who runs Microsoft's Windows Authentication team, wrote on Bluesky. 'See,' he continued, 'the problem is not that the algorithm exists. The problem is how the algorithm is chosen, and the rules governing that spanned 20 years of code changes.'

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