SHA-1 is a hashing algorithm that has been used extensively since it was published in 1995, however, it is no longer considered secure. It was deemed vulnerable to attacks from well-funded adversaries back in 2005 and was replaced by SHA-2 and SHA-3 which are considerably more secure hashing functions. Many companies including Google, Mozilla, and Microsoft have already announced that they'll stop accepting SHA-1 TLS certificates by 2017.
Now, Microsoft has detailed how numerous websites, users, and third-party applications will be affected once the company deprecates SHA-1 signed certificates starting February 14, 2017.
Microsoft states that in an effort to further enhance security features on Edge and Internet Explorer 11, the two browsers will prevent sites using SHA-1 signed certificates from loading and will display an "invalid certificate" warning. While it isn't recommended, users will have the option to bypass the warning and access the potentially vulnerable website. The company has clarified that this will only impact websites with SHA-1 signed certificates that link to a Microsoft Trusted Root CA, while manually installed enterprise or self-signed SHA-1 certificates will remain unaffected.
The Redmond giant states that developers who have installed the latest 2016 November Windows updates can test if their websites will be affected by the change. The detailed procedure can be viewed in the company's blog post here.
Microsoft has clarified that third-party Windows applications utilizing the Windows cryptographic API set or older versions of Internet Explorer will not be affected by the changes. Similarly, the update will not prevent clients from using the SHA-1 certificate in client authentication.
Regarding cross-signed certificates, Microsoft has explicitly confirmed that Windows will only check if the thumbprint of the root certificate is in the Microsoft Trusted Root Certified Program. The company has clarified that certificates "cross-signed with a Microsoft Trusted Root that chains to an enterprise/self-signed root" will not be affected by the changes next year.