Users around the world have had trouble accessing some HTTPS websites due to an error at GlobalSign, one of the world's largest certificate authorities.
As part of a planned exercise, GlobalSign revoked one of its cross-certificates that allowed end-user certificates to chain to alternate root certificates. GlobalSign operates multiple roots, which are trusted in browsers and operating systems by default, and links them together through these cross-certificates.
The revocation of such a certificate was interpreted by some browsers and systems also as a revocation of the intermediate certificates that chained back to it. This was not really the case or the company's intention.
