Belgian security firm GlobalSign has temporarily stopped issuing authentication certificates for secure websites.
It comes after an anonymous hacker claimed to have gained access to the company's servers.
If confirmed, it would be the second security breach at a European certificate authority in two months.
Hundreds of bogus DigiNotar authentications were issued following an intrusion into its systems.
Certificate authorities (CAs) are companies or public bodies whose job is to confirm that secure websites are genuine.
When computers connect to a site with TLS or SSL authentication, a certificate is issued which verifies the site's identity to the web browser.
Fake certificates could allow someone to spy on a user's activity.
Multiple targetsGlobalSign took action as the result of a posting which appeared on the online notice board Pastebin.
The author, who identified themselves only as "ComodoHacker", claimed to have gained access to four certificate authorities, in addition to DigiNotar.
Only GlobalSign is named, although the message points out that an attack on StartCom was foiled by its boss Eddy Nigg.
ComodoHacker also refers to an attack on US certificate authority Comodo, which was targeted in March.
As a precaution, GlobalSign said it was temporarily ceasing the issuance of all certificates while it investigated the claims.
The hacker also played down suggestions that the attacks were the work of Iranian authorities.
"I'm single person, do not AGAIN try to make an ARMY out of me in Iran. If someone in Iran used certs I have generated, I'm not one who should explain," said the posting.
It had been suggested that, because many of the bogus DigiNotar certificates were issued to users in Iran, that authorities in there may have initiated the CA hack as a tool for spying on dissidents.
A report on the DigiNotar attack said that up to 300,000 Iranians may have had their Gmail accounts monitored as a result of a fake Google certificate being created.
HacktivistWhile the anonymous posting contains no information about the identity of the CA hacker, it does detail a political agenda.
The message states: "Dutch government is paying what they did 16 years ago about Srebrenica, you don't have any more e-Government huh?"
It appears to reference the apparent non-intervention of Dutch peacekeeping forces during the notorious 1995 Srebrenica massacre, where Serbian forces killed more than 8,000 Bosnian Muslims.
DigiNotar certificates are used to authenticate many online services offered by the Dutch government, although the company has said that these use a separate system which was not compromised during the attack.
State prosecutors in the Netherlands are investigating the incident.
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