Key Takeaways
1. The January 2026 Windows 11 KB5074109 security update causes crashes and boot issues on some PCs.
2. The update removes four old modem drivers, making certain modems stop working.
3. Many users have reported that their dial-up and telephony modems fail to operate after installing the update.
4. Microsoft justifies the driver removal as a security measure to address unresolved kernel vulnerabilities.
5. Users relying on affected modems have limited options: uninstall the update or upgrade to new hardware with supported drivers.
Microsoft’s January 2026 Windows 11 KB5074109 security update has been causing issues on some PCs, leading to crashes and boot problems. Additionally, it’s intentionally disabling certain older modems.
Driver Removals
In the official changelog for KB5074109, Microsoft mentions a “Compatibility” change that eliminates four long-standing modem drivers from Windows 11: agrsm64.sys, agrsm.sys, smserl64.sys, and smserial.sys. The documentation clearly states that any modem hardware relying on these drivers will stop functioning after the update.
Users Affected
This situation has been frequently noted by users. After installing KB5074109, many find that their dial-up and telephony modems cease to operate, including some devices marketed as “Windows 11 compatible.” Rolling back the update immediately restores these modems, proving that the removal of the drivers is the cause of the problem.
Microsoft’s Justification
Both Microsoft and external analysts frame this action as a step towards enhancing security, rather than as an error. The drivers that were removed, which are utilized by older Agere/LSI and Motorola soft-modems, have been linked to unresolved vulnerabilities at the kernel level. Therefore, Microsoft opted to remove them from the operating system instead of continuing to distribute code that could be exploited.
For users and small businesses that still depend on these modems for services like faxing, telemetry, or phone-logging, the options are quite limited. They can either uninstall KB5074109 and temporarily pause updates, or swap out the hardware for devices that utilize currently supported drivers. Microsoft’s own guidelines make it clear that there will not be any future support for modems associated with these four outdated drivers.
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