The hacker, known as Comodohacker, said on Monday he had broken into Dutch certificate authority (CA) DigiNotar and that he had access to four other such companies, including GlobalSign, a certificate authority based in Portsmouth, New Hampshire. On Tuesday, GlobalSign said it was investigating the claim and had "decided to temporarily cease issuance of all certificates until the investigation is complete."

"We will post updates as frequently as possible," . "We apologize for any inconvenience."

GlobalSign couldn't immediately be reached for comment, but earlier in the day, Steve Roylance, GlobalSign's business development director, said his company was "taking this very seriously."

Comodohacker, also known as Ich Sun, is the person who earlier this year claimed to have broken into security vendor and certificate issuer Comodo. At the time but he didn't name his other victim.

Little noticed by most Web surfers, digital certificates are an important part of the Internet's foundations. They help browsers know when they are visiting legitimate websites rather than fakes.