Google said late Sunday that an attack mounted against its Gmail service targeted users primarily located in Iran, although the company has taken steps to block further interception attempts.

Google discovered that attackers had acquired a Secure Sockets Layer (SSL) certificate valid for any website in the google.com domain. The SSL certificate is used to vouch for the authenticity of websites and protect against security threats such as "man-in-the-middle" attacks.

Private companies, known as certificate authorities (CAs), make money from issuing digital certificates, although experts have pointed out there are many weaknesses in how certificates are issued that could undermine security.

In this case a Dutch CA, DigiNotar, issued an SSL certificate for the google.com domain on July 10, without Google's knowledge. It has since revoked the certificate.

Using a fake certificate would allow attackers to capture the login details for a person's Gmail account without a warning coming from the browser that something suspicious is happening, allowing them access to the e-mail account.

Google said "the people affected were primarily located in Iran," although the company did not detail further if it believed their accounts were compromised.

To perform the attack, an attacker would need to "poison" a Domain Name System cache. DNS is used to lookup the IP address for where a domain is located, but many organizations run their own DNS servers that caches the information to speed the lookup process up, updating it periodically.

That tampering could allow a random IP address to appear as a "*google.com" site. Combined with the fraudulent certificate that fools the web browser's warning system, a person would not know they've been hacked.

Google uses a different CA, not DigiNotar, to issue certificates for its domains -- and as an additional security measure, it codes information about that issuer into its Chrome browser. This allowed a Chrome user to flag the DigiNotar-issued certificate for google.com as fake, even though it was technically valid. The additional protection won't work for certificates for other companies' domains, where Google can't know in advance who the issuing authority should be.

Google said on Sunday in a blog post that it had now configured its Chrome browser to revoke SSL certificates coming from DigiNotar while the company investigates.

Mozilla, the organization behind the Firefox web browser, said in a blog post that it plans to issue soon new versions of Firefox along with its Thunderbird e-mail application and SeaMonkey application suite to revoke "trust in the DigiNotar root."

Microsoft said on Monday that it would remove DigiNotar from its trusted certificate providers within its Internet Explorer browser. Apple officials could not be immediately reached.

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