Microsoft will no longer accept digital certificates that are signed with RSA keys smaller than 1024 bits. This includes SSL certificates, Authenticode code signing certificates, email certificates, and any other certificates validated by the Windows Crypto APIs. The problem is many organizations got stuck in time and have not increased their key strength as flaws have been found and computing power has increased. Perhaps this is a blessing in disguise, as anything you are protecting with weak RSA certificates isnt in fact protected at all.”]
Source: https://nakedsecurity.sophos.com/2012/09/11/microsoft-says-no-to-insecure-certificate-practices/