ucrtbase.dll is missing or crashes a program: what the file is and how to fix it

Each file below is Microsoft's own ucrtbase.dll for one Windows release, fetched from Microsoft's own server and checked against Winbindex, the index of the files in Windows images and updates. Pick your Windows version (Settings, System, About shows it) and the build: 64-bit for System32, 32-bit for SysWOW64. The repair steps above are still the better fix, because Windows protects these folders and replaces the file with every cumulative update. Use a file here to compare it with a copy you have, or to see which build your release should have.

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The wording tells you which problem you have. These are the texts Windows shows, as written, with the file name filled in.

ucrtbase.dll is the Universal C Runtime, often shortened to UCRT: the standard C library of Windows, with functions for files, memory, strings, time, locale and maths that programs built with Visual C++ 2015, 2017, 2019 and 2022 call constantly. Before 2015 every Visual C++ version brought its own runtime, such as msvcr120.dll; since then the C part is one shared Windows component, and the Visual C++ 2015-2022 Redistributable adds only the compiler-specific part on top of it. Microsoft made the UCRT part of Windows 10, Windows 11 and Windows Server 2016 and later, and delivered it to Windows Vista, 7, 8 and 8.1 as update KB2999226. The api-ms-win-crt-*.dll files are small forwarders: a program asks for them by name and Windows passes the calls on to ucrtbase.dll. What Windows reports for the genuine file:

On 64-bit Windows, System32 holds the 64-bit file and SysWOW64 the 32-bit one; the folder names are historical. Winbindex also lists copies in the downlevel subfolders of both; they belong to Windows too and are repaired the same way. The genuine file describes itself as Microsoft® C Runtime Library (Properties, Details, File description), carries a Windows version such as 10.0.26100 and is about 0.9 to 1.4 MB: in the Windows 11 24H2 and 25H2 builds the 64-bit file is 1,377,512 bytes, in Windows 10 22H2 it is 1,045,976 bytes. ucrtbased.dll, with a d, is the debug build that comes with Visual Studio and is not part of Windows.

Every Windows release has its own build of ucrtbase.dll, updated together with Windows by the monthly cumulative updates. Winbindex lists 289 distinct Microsoft builds; the table shows the newest one indexed for each release and the update that delivered it.

Public DLL catalogues have collected 34 copies of ucrtbase.dll uploaded from PCs, from version 10.0.10137.0 to 10.0.18362.815. Many come from older Windows releases or from the Windows 7 and 8.1 update. The table lists each with its checksums, so you can tell which one you have.

None of these replaces the file your Windows ships: the newest build indexed for Windows is 10.0.28000.1450, and each release keeps its own. If the check on this page finds your file in this list, it is most likely an older Microsoft build that was copied somewhere by hand. Confirm the Microsoft signature, remove the stray copy, and let Windows use its own.

Copies uploaded by users to a public DLL catalogue, read on 14 October 2026. We list their checksums so you can identify a file; we do not offer them for download and we did not verify each one against Microsoft's signature.