A Google software problem inadvertently exposed the names, addresses, email addresses and phone numbers used to register websites after people had chosen to keep the information private.
The privacy breach involves whois, a database that contains contact information for people who've bought domain names. For privacy reasons, people can elect to make information private, often by paying an extra fee.
Craig Williams, senior technical leader for Cisco's Talos research group who discovered the issue, said the data will make it easier for cybercriminals to draft phishing emails that try to trick victims into divulging information or clicking on malicious links.
The cybercriminals are "going to have the right website name, the right name, the right address, the right phone number, the right email," he said.
Cisco said in a blog post that some 282,867 domains were affected. Williams stumbled across the problem last month while doing research on domains associated with malware.
The privacy settings for domain names registered through the company eNom were being turned off right at the time when the domains were up for renewal, starting around mid-2013.
"I immediately knew that was really weird," Williams said. "Nearly everyone these days is very careful about their presence online."
Google partners with registrars including eNom to let people register domain names as part of its Apps product. Williams contacted Google, and in about six days the privacy settings had been restored. In a notice, Google blamed a "software defect." On Thursday, Google said in a statement it is contacting affected Apps customers.
The damage will be long lasting, even if the privacy protections are now back in place. Changes to whois records are immediately recorded by many people and organizations, including security companies.
"A lot of people track this information historically quietly," Williams said.
There may be a small upside to the leak, particularly for computer security researchers.
Although well-intended, whois information is often useless since it's either set to private or is simply fake. Cybercriminals will often buy domains using other people's credit cards and personal information or enter bogus information.
But even the entry of fake information can help track malware campaigns. Williams said fraudsters will often get lazy and reuse the same bogus details, which can still be helpful if it is consistent.
"There are legitimate reasons to track whois information," he said.
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