Windows Defender SmartScreen blocks intranet site

Question

Hi Team,

We have a web application hosted on a server, which is accessed internally via the intranet. The application was functioning normally until May 26, 2025. However, starting from May 27, 2025, we began receiving the error message: This site has been reported as unsafe, despite no changes being made to the application.

The initial page of the application is configured to open in a small new window in the Microsoft Edge browser.

Could you please help us understand the possible cause of this sudden issue?

Thanks in advance.

When this setting is off in the EDGE, then it works fine.

Answer 1

Hello Geetha Ganesan,

welcome to Q&A

 

According to your description, an internal web application was suddenly marked as “not secure” by Microsoft Defender SmartScreen, combined with the information in the screenshot (the domain dhhappchne8164.cscidp.net was blocked).

Possible causes of problems

1. Domain Reputation Issues
   • Domain *.cscidp.net is blacklisted by Microsoft The domain belongs to a dynamic DNS service (e.g. No-IP/DynDNS). Hackers often abuse such domains to host malware, causing Microsoft to block entire domain extensions in bulk. Even if your subdomain is harmless, it will be implicated.
     • Trigger point: May 27, 2025 Microsoft updated the blacklist to mark the entire domain of cscidp.net as dangerous.
     1. SSL Certificate Issues
        • Expired certificates, domain name mismatches, or the use of self-signed certificates may trigger security warnings (but SmartScreen is primarily targeted at malicious content and needs to be combined with troubleshooting).
        1. Content False Alarm
           • If the page contains external resources (e.g. JS/CSS libraries) that have been tampered with by hackers, or if there are suspicious redirects, it may be determined by SmartScreen to be malicious.

Method 1: Replace the domain name (with immediate effect)

Discard the dynamic DNS domain name:

Replace dhhappchne8164.cscidp.net with:

Internal private domain name (e.g. appname.internal) and configure the intranet DNS resolution

or apply for a regular corporate domain name (e.g. app.yourcompany.com)

Modify the application configuration : Update the server hostname binding and client access address.

 

Method 2: # Check certificate expiration date (performed at the server)

Get-ChildItem -Path Cert:\LocalMachine\My | Where-Object { $_.Subject -match "dhhappchne8164" } | fl Subject, NotAfter

Make sure that the certificate is valid and the domain name matches (renew if expired).

 

Thank you for your patience and reading.

Best regards.

Kai Ho | Microsoft Community Support Specialist