After recent system updates, can no longer download files in IE 11 - asp.net

Question

Sometime in the last few weeks, MS pushed out IE 11 updates on Windows 10 which I believe may be interfering with the ability to download Excel and PDF files inside an iframe in my ASP.NET web application.
This issue applies only to certain versions of IE 11 (my machine is on 11.192.16299.0) on Windows 10. It has not been reported to with other Windows versions. Machines operating with certain older versions of IE 11 apparently do not experience the issue.
My application has an iframe, in which my own content can be loaded. After loading a page into the iframe and attempting a file download within that iframe, it appears to the user that nothing happens. Inspecting the console, I notice the following two warnings, which have been present in the application for quite some time:
SEC7131: Security of a sandboxed iframe is potentially compromised by allowing script and same origin access.
DOM7011: The code on this page disabled back and forward caching. For more information, see: http://go.microsoft.com/fwlink/?LinkID=291337
Inspecting network traffic, I can see the file request go out, and a response come back. The response has the following headers:
Cache-Control: no-cache, no-store
Content-Disposition: attachment; filename=doc.pdf
Content-Length: 97542
Content-Type: application/pdf
Date: Thu, 25 Jan 2018 18:27:37 GMT
Expires: -1
Pragma: no-cache
Server: Microsoft-IIS/10.0
X-UA-Compatible: IE=Edge
This appears to be a normal request to me. Cache-Control is properly set to no-cache, as the document contents are dynamically generated.
And finally, here is the code in my codebehind that initiates the file download for a PDF:
Public Sub DownloadPdf() Handles Pdf.Click
Dim pdf As Byte() = GetPdf()
With Response
.Clear()
.ClearHeaders()
With .Cache
.SetCacheability(HttpCacheability.NoCache)
.SetNoStore()
.SetExpires(Date.UtcNow.AddHours(-1))
.SetMaxAge(New TimeSpan(0, 0, 30))
End With
.ContentType = "application/pdf"
.AddHeader("Content-Disposition", "attachment; filename=doc.pdf")
.AddHeader("Content-Length", pdf.Length.ToString)
.BinaryWrite(pdf)
.Flush()
.End()
End With
End Sub
The Excel download code is quite similar. It is fair to assume GetPdf() is working properly, as the PDF generates and downloads in other browsers (Edge, Chrome, Firefox).
I have inspected IE settings for the Internet and Local intranet security zones, and the configuration appears to be normal. File downloads are allowed, plugins are allowed to run (or require prompt in some cases). I even attempted to loosen security settings across the board to determine what may be causing my issue, but I had no luck. In the dev console, I set the debugger to break on all exceptions, but no exception is being thrown during the file download process.
I can view PDFs from other websites and download files, but it seems that most of the test sites I found are serving static content. On the other hand, my files are created on the fly and the download process seems to function a bit differently.
Are there any apparent issues with my setup? Were there any security or settings changes that accompanied recent MS updates that could be interfering with my application? I appreciate any and all help with this. Thanks for looking.

Answer 1

In certain Windows 10 builds (confirmed for 1703 and 1709), IE security settings were changed to prevent sandboxed iframe file downloads. There is no settings change or workaround I discovered that can resolve this other than removing sandboxing from the iframe.