SharePoint changes files during CU installation

You might think, I’m drunk, but I’m not.

SharePoint, as a document management system, should never ever modify files without any notice. However, it seems that we found a bug, where it does modify files.

What?

We have a lot of CSS files in SharePoint. They are used for some custom apps in SharePoint or were just uploaded to OneDrive for Business. All in a SharePoint 2013 OnPrem environment. The CSS files contain base64 encoded data URIs.

Sample:

.myClass {
    background-image: url(‘data:image/png;base64,MY_IMAGE_AS_A_BASE64_ENCODED_STRING’) !important;
}

Then we installed a Cumulative Update and ran PSCONFIG.

And then?

What happened then is a really strange thing. The CSS files seem to be the same as before. The “Last Modified” date has not changed. However the content of the CSS file changed a little bit:

.myClass {
    background-image: url(‘data:image/png;base64,MY_IMAGE_AS_A_…_HERE_NOW_SOME_OTHER_STRING’) !important;
}

The base64 string before was about 600 characters. After installing the CU it is now 234 characters and only the first about 100 are the same. The rest is some garbage. That leads to broken images and icon-fonts all over the place.
The worst thing here is: What else did the CU installation?
We reproduced the behavior with 2 different CU installations on different sites and it seems, that only the base64 string is modified. Everything else is untouched.

We’ve filed a case at Microsoft and I will update the post when we will have an official statement.

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