Error installing on fresh Windows 8.1 machine

[adang1345]

Bug report

Bug description:

I was testing some custom builds of Python on Windows machines that don't have the Universal C runtime installed, and I discovered a problem with my builds that shows up in the official Windows installer as well. I receive an error when trying to install Python 3.13.7 on a fresh Windows 8.1 VM that does not have the Universal C runtime installed.

The install log contains the following, indicating that installing ucrt.msi failed.

[0DDC:0ED0][2025-09-14T12:49:34]w343: Prompt for source of package: ucrt_AllUsers, payload: ucrt_AllUsers, path: C:\Users\adang\Downloads\ucrt.msi
[0DDC:0ED0][2025-09-14T12:49:34]i338: Acquiring package: ucrt_AllUsers, payload: ucrt_AllUsers, download from: https://www.python.org/ftp/python/3.13.7/amd64/ucrt.msi
[0E80:0E98][2025-09-14T12:49:35]i305: Verified acquired payload: ucrt_AllUsers at path: C:\ProgramData\Package Cache\.unverified\ucrt_AllUsers, moving to: C:\ProgramData\Package Cache\{A5AB937A-A96D-4BF8-BFAB-8702BA813A81}v3.13.7150.0\ucrt.msi.
[0E80:0E84][2025-09-14T12:49:35]i301: Applying execute package: ucrt_AllUsers, action: Install, path: C:\ProgramData\Package Cache\{A5AB937A-A96D-4BF8-BFAB-8702BA813A81}v3.13.7150.0\ucrt.msi, arguments: ' ALLUSERS="1" ARPSYSTEMCOMPONENT="1" MSIFASTINSTALL="7" TARGETDIR="C:\Program Files\Python313"'
[0E80:0E98][2025-09-14T12:49:35]i304: Verified existing payload: core_AllUsers at path: C:\ProgramData\Package Cache\{BE75E968-78F4-411D-92E4-73EC3043F7E4}v3.13.7150.0\core.msi.
[0E80:0E98][2025-09-14T12:49:35]i304: Verified existing payload: exe_AllUsers at path: C:\ProgramData\Package Cache\{E4047598-558F-4468-8B53-9FCEF7F86E0D}v3.13.7150.0\exe.msi.
[0E80:0E98][2025-09-14T12:49:35]i304: Verified existing payload: dev_AllUsers at path: C:\ProgramData\Package Cache\{96A23710-E014-4A5B-96A1-DE64AC37251B}v3.13.7150.0\dev.msi.
[0E80:0E98][2025-09-14T12:49:35]i304: Verified existing payload: lib_AllUsers at path: C:\ProgramData\Package Cache\{A139F43E-8105-465D-AC80-28F349CBE08D}v3.13.7150.0\lib.msi.
[0E80:0E98][2025-09-14T12:49:35]i304: Verified existing payload: test_AllUsers at path: C:\ProgramData\Package Cache\{6BD2B618-9A2B-47D9-B24B-2F05BD2768E4}v3.13.7150.0\test.msi.
[0E80:0E98][2025-09-14T12:49:35]i304: Verified existing payload: doc_AllUsers at path: C:\ProgramData\Package Cache\{CFAAA24B-16EF-4D9D-80A5-F67798771571}v3.13.7150.0\doc.msi.
[0E80:0E98][2025-09-14T12:49:35]i304: Verified existing payload: tcltk_AllUsers at path: C:\ProgramData\Package Cache\{A65B1339-6492-4CA4-AEB5-2B25A83A20B9}v3.13.7150.0\tcltk.msi.
[0E80:0E98][2025-09-14T12:49:35]i304: Verified existing payload: launcher_AllUsers at path: C:\ProgramData\Package Cache\{8B8DEA15-D815-4CB1-AC10-4E7713F3DFA0}v3.13.7150.0\launcher.msi.
[0E80:0E98][2025-09-14T12:49:35]i304: Verified existing payload: pip_AllUsers at path: C:\ProgramData\Package Cache\{CD31E178-F872-466B-A231-9C8AA53A89FD}v3.13.7150.0\pip.msi.
[0E80:0E98][2025-09-14T12:49:35]i304: Verified existing payload: compileall_AllUsers at path: C:\ProgramData\Package Cache\5DB71E6C62D8C3B4121E17DCFA4030A491BFC13E\py.exe.
[0E80:0E98][2025-09-14T12:49:35]i304: Verified existing payload: compileallO_AllUsers at path: C:\ProgramData\Package Cache\5DB71E6C62D8C3B4121E17DCFA4030A491BFC13E\py.exe.
[0E80:0E98][2025-09-14T12:49:35]i304: Verified existing payload: compileallOO_AllUsers at path: C:\ProgramData\Package Cache\5DB71E6C62D8C3B4121E17DCFA4030A491BFC13E\py.exe.
[0E80:0E84][2025-09-14T12:49:35]e000: Error 0x80070643: Failed to install MSI package.
[0E80:0E84][2025-09-14T12:49:35]e000: Error 0x80070643: Failed to execute MSI package.
[0DDC:0DE0][2025-09-14T12:49:35]e000: Error 0x80070643: Failed to configure per-machine MSI package.
[0DDC:0DE0][2025-09-14T12:49:35]i319: Applied execute package: ucrt_AllUsers, result: 0x80070643, restart: None
[0DDC:0DE0][2025-09-14T12:49:35]e000: Error 0x80070643: Failed to execute MSI package.

The file Python 3.13.7 (64-bit)_20250914124833_000_ucrt_AllUsers.log contains the following excerpt.

=== Logging started: 9/14/2025  12:48:43 ===
MSI (s) (78:08) [12:48:43:382]: Note: 1: 2203 2: C:\Windows\Installer\inprogressinstallinfo.ipi 3: -2147287038 
MSI (s) (78:08) [12:48:43:382]: APPCOMPAT: [DetectVersionLaunchCondition] Launch condition already passes.
MSI (s) (78:08) [12:48:43:382]: PROPERTY CHANGE: Adding ACTION property. Its value is 'INSTALL'.
MSI (s) (78:08) [12:48:43:382]: Doing action: INSTALL
MSI (s) (78:08) [12:48:43:382]: Note: 1: 2205 2:  3: ActionText 
Action start 12:48:43: INSTALL.
MSI (s) (78:08) [12:48:43:382]: Running ExecuteSequence
MSI (s) (78:08) [12:48:43:382]: Doing action: FindRelatedProducts
MSI (s) (78:08) [12:48:43:382]: Note: 1: 2205 2:  3: ActionText 
Action start 12:48:43: FindRelatedProducts.
MSI (s) (78:08) [12:48:43:398]: Doing action: AppSearch
MSI (s) (78:08) [12:48:43:398]: Note: 1: 2205 2:  3: ActionText 
Action ended 12:48:43: FindRelatedProducts. Return value 1.
Action start 12:48:43: AppSearch.
MSI (s) (78:08) [12:48:43:398]: Note: 1: 1322 2:  
MSI (s) (78:08) [12:48:43:398]: Note: 1: 1322 2:  
MSI (s) (78:08) [12:48:43:398]: Note: 1: 1324 2: . 3: 1 
MSI (s) (78:08) [12:48:43:398]: Note: 1: 1324 2: . 3: 1 
MSI (s) (78:08) [12:48:43:398]: Note: 1: 1322 2:  
MSI (s) (78:08) [12:48:43:398]: Note: 1: 1322 2:  
MSI (s) (78:08) [12:48:43:398]: Note: 1: 1324 2: . 3: 1 
MSI (s) (78:08) [12:48:43:398]: Note: 1: 1324 2: . 3: 1

Last 4 lines repeated hundreds of times, snipped for brevity

MSI (s) (78:08) [12:48:43:492]: Internal Exception during install operation: 0xc00000fd at 0x00007FFFE243B8DA.
MSI (s) (78:08) [12:48:43:492]: WER report disabled for silent install.
MSI (s) (78:08) [12:48:43:492]: Internal MSI error. Installer terminated prematurely.

After trying to install some older Python versions using installers that I custom-built with Tools\msi\buildrelease.bat, I found that the earliest version with this problem is Python 3.7.13 and that a regression was introduced in #31726 with the addition of the following line:

cpython/Tools/msi/ucrt/ucrt.wxs

Line 7 in 1c984ba

<PropertyRef Id="DetectTargetDir" />

It's not totally clear to me what this line is for, but I noticed that the change in question added it to some .wxs files under Tools/msi and not others. I think that it should not have been added to ucrt.wxs, because when I remove it, my custom-built installers no longer encounter this error. I can create a PR.

CPython versions tested on:

3.13

Operating systems tested on:

Windows

Linked PRs

• gh-138896: Fix error installing C runtime on non-updated Windows machines #138932
• [3.14] gh-138896: Fix error installing C runtime on non-updated Windows machines (GH-138932) #139034
• [3.13] gh-138896: Fix error installing C runtime on non-updated Windows machines (GH-138932) #139035

[ZeroIntensity]

If your fix doesn't break anything, we can probably accept it. But keep in mind that we don't have any guarantees about Windows 8 support, because we follow Microsoft's fixed lifecycle policy.

[zooba]

That line figures out where to install the UCRT, because since a few versions ago we always install into our target directory.

We should move the CRT install step in bundle.wxs to after the exe.wxs entry. That will install the file being searched for before we try to search for it. Removing the line you suggest will just restore security bug that was fixed.

FWIW, sharing your actual log files would help us confirm this. I'm just guessing right now, but it seems like the only thing that could reasonably be changed, so I think it's a good guess (as well as explaining why you're the first to hit this issue, since it has no impact on Win10+).

Though we also don't necessarily have to fix anything here, since 3.13 doesn't support Windows 8.1. I'm not hugely worried either way - reordering the chain like this should be safe.

[adang1345]

Here are the full log files.

Python 3.13.7 (64-bit)_20250914124833.log
Python 3.13.7 (64-bit)_20250914124833_000_ucrt_AllUsers.log

[zooba]

Hah, okay, there's genuinely no more useful information than what you posted! I'd report this as an MSI bug (it should be able to handle this more sensibly), but we've moved away from MSI entirely because they won't fix anything but the most critical bugs these days.

A PR against main would be great if you want to make one, and it can be backported to 3.13.

[adang1345]

Reordering the CRT install step to after the exe.wxs entry indeed fixes the problem. I have made the PR.