Microsoft Windows, Antivirus Software at Odds After Latest Update

Microsoft's April security update continues to cause problems with Windows computers running third-party antivirus software, with at least five companies' products suffering slow start-up times or even an inability to boot the system.

McAfee is the latest security company to confirm that the Windows update is causing problems for its customers. Since April 18, the company has issued multiple advisories for its Threat Prevention 10.x, Host Intrusion Prevention 8.0, and VirusScan Enterprise 8.8 software, warning that a patch to Microsoft's Client Server Runtime Subsystem (CSRSS) caused a deadlock with its software.

"We're continuing to investigate the issue's cause and updating our customers on our latest findings," a company spokesperson said. "McAfee's support team is providing customers a workaround until a solution can be identified, developed, and delivered."

The problem first became apparent soon after the regularly scheduled "Patch Tuesday" update on April 9. Six days later, security firm Sophos warned that computers running older versions of Windows — not Windows 10 or Windows Server 2016 — will freeze at boot time if its Sophos Windows endpoint security software is installed.

"Sophos has received reports of computers failing to boot," the company stated in an advisory. "Sophos is actively investigating this issue and will update this article when more information is available."

Meanwhile, security firm Avast has issued a "micro-update" that fixes the issues its software has with the updates to Windows 7 and Windows 8.1, a spokesperson stated. 

"Upon reports of frozen login screens, we released micro-updates via our emergency updater tool to resolve," the company said in a statement sent to Dark Reading. "Avast users need only boot their machine and let stand at the login screen for approximately 15 minutes while the updater runs in the background. Then, reboot the machine and it should now operate normally even with the aforementioned Windows updates installed."

Security software makers Avira and ArcaBit were also impacted by the issue, according to Microsoft's advisory.

This is not the first time that Windows has disagreed with major antivirus programs. In early 2018, Microsoft warned that the January 2018 update to various versions of Windows had caused compatibility issues with certain antivirus software. Antivirus developers often use undocumented functions to get around certain roadblocks put in place by Microsoft. The software giant blamed the antivirus firms for making "unsupported calls into Windows kernel memory."

"We continue to require that AV software be compatible, and in cases where there are known issues of AV driver compatibility, we will block those devices from updates to avoid any issues," Microsoft said in the advisory issued at the time. "We recommend customers check with their AV provider on compatibility of their installed AV software product."

Two factors are likely converging to cause the issues for antivirus companies on Windows systems, says Nick Fritts, a senior software engineer with security firm Endgame. First, antivirus software runs at a privileged level on the system and in the critical path of many system operations. Second, Microsoft is updating the core components of Windows more frequently than in the past.

"Due to the nature of antivirus and security products, they are more likely to run into problems than other software," Fritts says. "I think Microsoft is trying to strike a balance between keeping their operating system updated and not impacting customers. So far they've probably been too aggressive with update cycles, and that may be why they've added the new feature to allow for scheduled or declined version upgrades."

It's unclear how the increasing incidents of incompatibility will impact antivirus makers' products. Microsoft has its own antimalware software, and late last year announced it is running in a sandbox, making it less likely to be the target of a privilege escalation attack and further protecting the system.

Antivirus firms' continued reliance on undocumented function calls to give their software an advantage in scanning the operating system could cause future incidents of incompatibility, says Gabe Landau, principal software developer with Endgame.

"While antivirus companies can test for compatibility against past, current, and upcoming patch versions of Windows, it's always a risk when assumptions are made about undocumented Windows behavior because that behavior can change, breaking assumptions," Landau says. "Microsoft has no formal obligation to leave undocumented behavior of their software unchanged indefinitely, yet security companies cannot protect their customers using only the documented interfaces that Microsoft chooses to expose. It's a hard problem."

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