Following the news that thousands of Hotmail passwords were made available online, Google, AOL and Yahoo have admitted their webmail services were also subject to a similar attack.
Google confirmed that passwords for its free Gmail online email service had been harvested by hackers, but downplayed the phishing attack as involving just a "small number" of accounts.
"We recently became aware of a phishing scheme through which hackers gained user credentials for Web-based mail accounts including a small number of Gmail accounts," a Google spokesman said.
"As soon as we learned of the attack, we forced password resets on the affected accounts. We will continue to force password resets on additional accounts if we become aware of them."
Google told Gmail users to change their passwords if they suspected that their accounts had been compromised. "If you can no longer sign into your account, you can regain access by answering security questions," the company added, referring to Gmail's single-question automated password reset function .
Microsoft has already blocked all hijacked Hotmail accounts, and offered tools to help users who had lost control of their email.
"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.
Neither Google or Microsoft, however, has directly alerted users to the possible danger by sending messages to Gmail or Hotmail accounts, respectively, or by posting a warning on those services.
Like Microsoft, Google today denied that Gmail had been hacked, and Gmail usernames and passwords stolen because of a lapse on its end.
"This was not a Gmail security issue, but rather a phishing scheme," said the Google spokesman.
Last year, a Tennessee college student was accused of breaking into former Alaska governor Sarah Palin's Yahoo Mail account by abusing Yahoo's similar reset tool.
Shortly after Palin's account was hijacked, PC Advisor's sister magazine Computerworld confirmed that the reset mechanisms used by Hotmail, Yahoo Mail and Google's Gmail could be exploited by anyone who knew an account's username and could answer a single security question.
Phishing attacks are on the rise, according to the Anti-Phishing Working Group (APWG), an industry association dedicated to stamping out online identity theft.
The APWG's most recent data reported that the number of unique phishing-oriented websites had surged to nearly 50,000 in June, the largest number since April 2007 and the second-highest total since it 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.
See also: 15,000 caught by 'spear-phishing' attack