[BUG] Stack overflow crash in atcuf64.dll on folder compare startup

[cybertek-jeff]

Description

WinMerge crashes with a stack overflow (0xC00000FD) when attempting a folder compare on Win11 Pro x64. The faulting module is atcuf64.dll at offset 0x000A0CCA (a third-party shell extension, not part of WinMerge from my research). Looks like the extension is recursing into WinMerge's process and exhausting the stack. Crash dump available on request.

Steps to reproduce

1. Launch WinMerge
2. Attempt a folder compare
3. Crash occurs

WinMerge version

Version 2.16.56.0 (2026.04) X64

Platform

Windows x64

Additional information

Crashes with what is below:
Exception Code: 0xC00000FD (Stack Overflow)
Module Name: atcuf64.dll
Offset: 0x000A0CCA

[sdottaka]

You could try adding WinMergeU.exe to the BitDefender AV exclusion list and see if that avoids the crash. This may be related to BitDefender’s atcuf64.dll shell/AV integration:

Directory Opus forum discussion about atcuf64.dll crashes