Without knowing what the actual vulnerable code is it is hard to say. However looking at the link it says that System.Security.Cryptography.X509Certificates.dll <= 6.0.125 are impacted. That would lead me to believe that this impacts all versions prior to v6 as well, so yes you are vulnerable.
However System.Security.Cryptography.dll is only for v7 to v8 so .NET Core would be fine. If it weren't fine then you'd expect to see v6 of the binary impacted by the vulnerability as well.