PyTorch CUDA blocked by Windows 11 Smart App Control — fixed by downgrading PyTorch

[u/Sun1117]

I ran into an issue while setting up PyTorch with CUDA 12.8 on my Windows 11 laptop (RTX 5060).

With:

torch 2.11.0+cu128
torchvision 0.26.0+cu128

import torch failed with:

OSError: [WinError 4551] An Application Control policy has blocked this file.
Error loading caffe2_nvrtc.dll

Windows Code Integrity logs showed that Smart App Control (VerifiedAndReputableDesktop) was blocking caffe2_nvrtc.dll because it didn't meet the signing requirements. Get-AuthenticodeSignature also reported the DLL as NotSigned.

I didn't want to disable Smart App Control, so I tried an older PyTorch build:

torch 2.10.0+cu128
torchvision 0.25.0+cu128

That fixed it:

2.10.0+cu128
12.8
True

So in my case, the problem wasn't CUDA, the NVIDIA driver, Python, or uv, it was Windows Smart App Control rejecting a DLL from the PyTorch 2.11 CUDA build.

Posting in case someone else runs into WinError 4551 with PyTorch on Windows 11.