Unsigned .pyd files blocked by Windows WDAC code integrity policies

[NotMyself]

Describe the issue:

NumPy's compiled extension modules (.pyd files) are not Authenticode-signed. On Windows systems with WDAC (Windows Defender Application Control) code integrity policies enforced, this causes numpy to fail to import with:

DLL load failed while importing _mt19937: An Application Control policy has blocked this file.

This is becoming increasingly common as organizations deploy WDAC policies, and it breaks any Python application that depends on numpy — even if the application only needs numpy for optional features.

Reproduce the code example:

# On a Windows system with WDAC code integrity enforced:
import numpy  # Fails with DLL load error

Verify that .pyd files are unsigned:

Get-AuthenticodeSignature (Get-ChildItem -Recurse -Filter "*.pyd" -Path "$env:APPDATA\uv\tools\chunkhound\Lib\site-packages\numpy")
# All show Status: NotSigned

Error message:

ImportError: DLL load failed while importing _mt19937: An Application Control policy has blocked this file.

NumPy/Python version information:

• numpy 2.4.2
• Python 3.13
• Windows 11 Pro 10.0.26100 (x64)
• WDAC CodeIntegrityPolicyEnforcementStatus = 2 (enforced)

Context

This was reported in chunkhound/chunkhound#192 where it crashes the MCP server at startup. The workaround is to lazy-load numpy so it's only imported when actually needed, but that doesn't help applications that require numpy at startup.

Authenticode-signing the .pyd files in the wheel build would allow numpy to work on WDAC-enforced systems. Other major projects with native extensions (e.g., some Microsoft packages) sign their DLLs for this reason.

Related

• WDAC documentation: https://learn.microsoft.com/en-us/windows/security/application-security/application-control/windows-defender-application-control/wdac
• Authenticode signing: https://learn.microsoft.com/en-us/windows/win32/seccrypto/authenticode

[mattip]

The second link is broken. Can you point to the similar signing issue for CPython? I would think they also need this to run.

[rgommers]

I did some searching and cannot find any other report of a WDAC signing issue for numpy, python, pip, or any other relevant package. So it seems likely that this either works as designed (can't install wheels from PyPI on systems with a policy that doesn't allow that), or a hiccup in Windows Defender.

This did remind me of an issue that we had with macOS when they introduced code signing by default, around 2020 or so. The solution was to self-sign packages on install automatically in installers IIRC.

If there is evidence that this is specific to _mt19937 or numpy, it'd be great to see. Absent that, I think there is nothing to do here.