AcroCEF.exe crash loop (libcef.dll, 0x80000003) on Windows Server 2025 RDS after October 2026 CU KB5126646 – same pattern as KB5034129 in 2024

Hi all,

since installing the October 2026 cumulative update KB5126646 (OS build 26100.33650) on our Windows Server 2025 Remote Desktop Session Hosts, Adobe Acrobat shows a permanently spinning busy cursor as long as it is open. This affects all users on the servers.

What happens:
AcroCEF.exe keeps spawning child processes that crash immediately and are restarted in an endless loop. Process Explorer shows new AcroCEF child processes starting and terminating every second. The busy cursor comes from these constant process starts.

Event log (Application, Event ID 1000), repeated many times:

Faulting application name: AcroCEF.exe, version: 26.2.22301.0
Faulting module name: libcef.dll, version: 112.7.0.0, time stamp: 0x631bae2d
Exception code: 0x80000003
Fault offset: 0x00000000034b4e33
Faulting application path: C:\Program Files\Adobe\Acrobat DC\Acrobat\AcroCEF\AcroCEF.exe
Faulting module path: C:\Program Files\Adobe\Acrobat DC\Acrobat\AcroCEF\libcef.dll

Environment:

• Windows Server 2025, RDS session hosts
• Adobe Acrobat 64-bit, version 26.002.22301
• KB5126646 installed on 21.10.2026

Confirmed cause:
After removing KB5126646 on one test server (via DISM, back to build 26100.33451), the problem is gone immediately. Reinstalling the update brings it back.

This looks like the same issue as in January 2024, when the Windows Server 2022 update KB5034129 caused exactly the same crash: same module, same libcef.dll timestamp 0x631bae2d, same exception code 0x80000003, almost identical fault offset:

• https://community.adobe.com/t5/acrobat-discussions/acrocef-exe-rdrcef-exe-errors-with-acrobat-reader-32-bit-on-latest-update/m-p/14450915
• https://community.adobe.com/t5/acrobat-reader-discussions/application-crash-event-id-1000-in-version-23-8-20470-0-with-acrocef-exe-on-windows-server-2022/m-p/14361274

The workaround from back then does not apply here: there are no Image File Execution Options keys for AcroCEF.exe or RdrCEF.exe on our servers.

Already ruled out:

• No IFEO keys for AcroCEF.exe/RdrCEF.exe
• Exploit Protection: no system-wide mitigations, none for AcroCEF.exe (only ForceRelocateImages on Acrobat.exe)
• No third-party DLLs injected into AcroCEF.exe (only Microsoft, Adobe and Chromium modules)
• No GPU acceleration available or in use
• Disabling Protected Mode at startup: no change
• Not user-profile related (all users affected)

Rolling back a security update with actively exploited vulnerabilities on production RDS servers is not an option for us. Is Adobe aware of this issue, and is there a fix or a supported workaround planned? I can provide ProcMon logs and WER crash dumps if needed.

Thanks!