Microsoft says the August 2026 security update may trigger 0xc0000409 errors on Windows Server 2016 systems where the Compatibility Appraiser diagnostic service is enabled.
On affected systems, users will see the CompatTelRunner.exe (Compatibility Appraiser Telemetry Runner) process crashing.
Microsoft Compatibility Appraiser, which controls CompatTelRunner and is a Windows Compatibility Telemetry component, is a background system task that checks if the device meets the hardware and software requirements for the next major Windows cumulative update or feature upgrade.
According to Microsoft, this known issue impacts both physical devices and virtual machines, including VMware and Azure environments.
“After installing the August 2026 Windows security update (the Originating KBs listed above), some Windows Server 2016 devices might generate recurring Application Error events (Event ID 1000, with exception code 0xc0000409) associated with CompatTelRunner.exe,” Microsoft explained in a service alert spotted by Microsoft MVP Susan Bradley.
“Although recurring CompatTelRunner.exe failures might generate Application event log entries, they do not affect device functionality. The associated event log warnings can be safely dismissed temporarily until we release a resolution in an upcoming update.”
Microsoft is working on a fix that will ship with a future Windows update and has not yet shared a timeline for a permanent solution.
In June, Microsoft fixed another Windows Server 2016 known issue that caused June 2026 security updates to fail on systems that weren’t up to date.
Last week, it also warned customers that they may experience application crashes on some Windows Server 2025 because of recent memory management changes.
This issue only affects apps that use Address Windowing Extensions (AWE), a set of extensions that lets them use more than 4GB of physical memory within a 32-bit virtual address space.
On impacted systems, users are seeing memory corruption errors, access violation exceptions (with 0xC0000005 error codes), SQL Server crash dumps, and SQL Server services stopping or restarting unexpectedly.
Overall prevention scores can hide what happens after initial access. Once attackers are using valid credentials, prevention drops sharply.
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