Google's Gmail and Yahoo's Mail were also targeted by a large-scale phishing attack, perhaps the same one that harvested at least 10,000 passwords from Microsoft's Windows Live Hotmail, according to a report by the BBC.
Microsoft, for its part, said late yesterday that it had blocked all hijacked Hotmail accounts, and offered tools to help users who had lost control of their e-mail.
Gmail was the target of what Google called a large-scale phishing campaign, the company told the BBC. "We recently became aware of an industry-wide phishing scheme through which hackers gained user credentials for Web-based mail accounts including Gmail accounts," a Google spokesperson told the news network.
The BBC also said it has seen a list of some 20,000 hijacked e-mail accounts; the list included accounts from Gmail, Yahoo Mail, AOL, Comcast and EarthLink. The latter two are major US Internet service providers.
"As soon as we learned of the attack, we forced password resets on the affected accounts," the Google spokesperson also told the BBC. "We will continue to force password resets on additional accounts when we become aware of them."
Neither Google's or Yahoo's US representatives responded to e-mails from Computerworld seeking confirmation that their Gmail and Yahoo Mail services were targeted by phishers, or answers to questions about how many accounts had been compromised and what the firms are doing to help users.
Neowin.net, the site that first reported the Hotmail account hijacking early Monday, today added that it had seen the same list of compromised accounts as the BBC.
"Neowin can today reveal that more lists are circulating with genuine account information and that over 20,000 accounts have now been compromised," said the Windows enthusiast site . "[The] new list contains e-mail accounts for Gmail, Yahoo, Comcast, EarthLink and other third-party popular Web mail services."
Microsoft has acknowledged that log-on credentials for "several thousand" Hotmail accounts had been obtained by criminals, probably through a phishing attack that had duped users into divulging their usernames and passwords.
Late Monday, Microsoft said it was blocking access to all the accounts whose details had been posted on the Web last week. "We are taking measures to block access to all of the accounts that were exposed and have resources in place to help those users reclaim their accounts," the company said on its Windows Live blog .
Microsoft posted an online form where users who have been locked out of their accounts can verify their identity and reclaim control, and also pointed users to a support page from October 2008 that spells out steps users can take if they think their accounts have been hijacked.
After a slump earlier this year, phishing attacks are on the upswing, according to the Anti-Phishing Working Group (APWG). Its most recent data -- for the first half of 2009 ( download PDF ) -- noted that the number of unique phishing-oriented Web sites had surged to nearly 50,000 in June, the largest number since April 2007 and the second-highest total since the industry association started keeping records.
Yesterday, Dave Jevans, the chairman of APWG, called the Hotmail phishing attack one of the largest ever, but cautioned that the usernames and passwords may have been harvested over several months, and not by a single, defined attack.