The Dutch certificate authority (CA) KPN Corporate Market has stopped issuing SSL certificates after it discovered a security breach that allowed hackers to store tools for denial-of-service attacks on its servers.

According to The Register, the compromise may have taken place four years ago, but there is no evidence that the compromise affects KPN servers. According to a translated statement by KPN last week, it stopped issuing new security certificates "temporarily as a precaution pending external, independent research".

It said: “Existing certificates [that have been] already issued remain valid. During such an investigation in the server of the website where companies can go for information about certificates, spores were detected that could indicate abuse.

“Although there is no evidence that the production of the certificate is compromised, [it] cannot be completely excluded that this did happen. Therefore, KPN Corporate Market (formerly Getronics) decided the application and issuance of new certificates [should be] temporarily discontinued, pending further investigation.”

KPN also said it has replaced these web servers and will conduct an additional, independent investigation with government departments "closely involved in the process". It confirmed that an additional study will be available "in the coming days".

KPN previously told Reuters that it had sold hundreds of new certificates in the days immediately following the DigiNotar hack earlier this year.

Meanwhile, Microsoft has said it will be revoking trust in Malaysian intermediate CA 'DigiCert Sdn. Bhd'. A notification by Entrust revealed that the CA had issued 22 certificates with weak 512-bit keys, and issued certificates without the appropriate usage extensions or revocation information.

Jerry Bryant, group manager of response communications at Microsoft Trustworthy Computing, said: “There is no indication that any certificates were issued fraudulently, however these weak keys have allowed some of the certificates to be compromised.

“These compromised certificates could allow an attacker to impersonate the legitimate owner and make a user believe they are trusting a website or signed software that was created for malicious use.

“The subordinate CA has clearly demonstrated poor CA security practices and Microsoft intends to revoke trust in the intermediate certificates.”