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If you’ve installed Microsoft’s January 2026 security update for Windows 11 and suddenly found your fax machine, dial-up connection, or telephony modem has stopped working, you’re not alone. But here’s the surprising part: it’s not a bug. Microsoft deliberately designed the update to disable certain outdated hardware, and for some users and small businesses, that’s creating real headaches.
The update in question, KB5074109, was released on January 13, 2026, as part of Microsoft’s regular Patch Tuesday cycle. While it brings important security fixes, its release notes also include a small but significant “compatibility” change that’s now making waves. According to the official documentation, the update removes four legacy modem drivers from the Windows 11 operating system:
- agrsm64.sys
- agrsm.sys
- smserl64.sys
- smserial.sys
These drivers, which date back years, were used to support older Agere/LSI and Motorola-based software modems—often found in internal PCI modems, some multifunction printers with fax capabilities, and specialized telephony or alarm system hardware.
What Users Are Reporting
Across tech forums and support threads, users who’ve installed KB5074109 are reporting that their modem-dependent devices have simply stopped functioning. In some cases, this includes hardware marketed as “Windows 11 compatible.”
The common thread? Rolling back the update immediately restores modem functionality, confirming that Microsoft’s removal of the drivers is the direct cause. The official support page for the update, which you can find here, states plainly: “After installing this update, modem hardware that depends on these drivers will no longer work.”
A Security Decision, Not an Accident
So why would Microsoft break functioning hardware? Both the company and independent analysts are framing the move as a necessary security hardening measure.
The drivers in question have long been associated with unresolved, kernel-level vulnerabilities. The kernel is the core of the operating system, and flaws at this level are particularly dangerous, as they can give attackers deep access to a PC. By choosing to completely remove these outdated drivers from the OS image, Microsoft is essentially cutting out what it sees as unpatchable, exploitable code rather than continuing to ship it.
As reported by outlets like Windows Central, this isn’t an oversight—it’s a deliberate end-of-support action. Microsoft’s stance is clear: support for modems tied to these four legacy drivers is not coming back.
What This Means for Affected Users
For the niche but dedicated group still using these modems—whether for fax lines in medical or legal offices, legacy alarm system dialers, or specialized data logging—the options are now limited:
- Uninstall and Pause: The immediate workaround is to uninstall KB5074109 and pause future updates. This is, however, a temporary and security-compromising solution, as it leaves the PC unprotected from other fixes in the update.
- Replace Hardware: The only sustainable path forward is to replace the affected modem or device with newer hardware that uses currently supported and actively maintained drivers.
The situation highlights the ongoing tension between modern security practices and legacy compatibility. While Microsoft’s move may be justified from a security standpoint, it delivers a stark reminder that even “compatible” hardware can have its plug pulled with a single update, leaving users to adapt unexpectedly.
Source: Microsoft Support, Windows Central