Windows 11 KB5074109 kills support for older modems

Microsoft’s January 2026 Windows 11 KB5074109 security update isn’t just causing crashes and boot issues on some PCs... it is also deliberately disabling certain legacy modems.

In the official KB5074109 changelog, Microsoft lists a “Compatibility” change that removes four long-standing in-box modem drivers from Windows 11: agrsm64.sys, agrsm.sys, smserl64.sys and smserial.sys. The notes explicitly state that any modem hardware dependent on these drivers will no longer work after the update.

That behavior has now been widely observed in the wild. Users who install KB5074109 report that dial-up and telephony modems suddenly stop working, including some devices sold as “Windows 11 compatible”. Rolling back the update immediately restores functionality, confirming that the driver removal is the trigger.

Microsoft and third-party analysis describe the move as a security hardening step rather than a bug. The removed drivers, used by older Agere/LSI and Motorola soft-modems, have been associated with unresolved kernel-level vulnerabilities, and Microsoft has chosen to drop them from the OS image instead of continuing to ship exploitable code.

For users and small businesses that still rely on these modems for fax, telemetry or phone-logging systems, the practical options are limited: uninstall KB5074109 and pause updates as a temporary workaround, or replace the hardware with devices that use actively maintained drivers. Microsoft’s own documentation makes clear that support for modems tied to these four legacy drivers is not coming back.