That error is the system's way of telling you it doesn't trust the files you just downloaded. It happens because Windows has "tagged" the setup files with a security flag because they originated from the internet or a network share, and the .NET Framework refuses to run them with full permissions.

Since you're likely running the setup from a mounted ISO or a folder you just extracted, here is how to kill that error:

1. The "Unblock" Fix (Most Common)

Windows puts a "Mark of the Web" on downloaded files. You need to tell it the file is safe.

1. Go to the folder where you have the Exchange ISO (or the folder you extracted it to).
2. Right-click the ISO file (or the Setup.exe) and select Properties.
3. On the General tab, look at the bottom for a checkbox that says Unblock.
4. Check that box, click Apply, and then OK.
5. If you already extracted the files, you might need to do this for the original ZIP/ISO and then re-extract them, or run this PowerShell command to unblock everything in that folder at once:

Get-ChildItem -Path "C:\Path\To\Your\ExchangeSetupFolder" -Recurse | Unblock-File

2. Move to a Local Drive

If you are running the setup from a Network Drive (mapped drive) or a UNC path (e.g., \\Server\Share), .NET will almost always throw this CAS (Code Access Security) error.

• Fix: Copy the entire setup folder directly onto the C: drive of the GLASS server. Do not run it over the network.

3. Run from an Elevated Prompt (Old School)

Even if you are an admin, the GUI sometimes fails to trigger the correct elevation.

1. Open Command Prompt as Administrator.
2. Navigate to your setup folder (e.g., cd C:\ExchangeCU15).
3. Run the setup with the required license switch:

.\Setup.exe /m:Upgrade /IAcceptExchangeServerLicenseTerms_DiagnosticDataON

Why this is happening now

This is another "security" layer that feels like a hurdle. .NET 4.0 and later disabled the "CAS policy" by default. When Exchange (which is heavily built on .NET) tries to load its own components from a location it deems "remote" or "untrusted," it panics and shuts down to prevent what it thinks might be a malicious code injection.

Once you unblock the file and move it to the C: drive, does the "Initializing Setup" bar finally move past that error?