Yahoo said it reset passwords for an unspecified number of accounts after detecting an unfolding hack-attack campaign.
"Recently, we identified a coordinated effort to gain unauthorized access to Yahoo Mail accounts. Upon discovery, we took immediate action to protect our users, prompting them to reset passwords on impacted accounts," said Jay Rossiter, who's in charge of Yahoo's platforms and personalization products, in an "important security update for Yahoo Mail users" blog post. Some related notifications, however, have yet to be made.
To help users recover their accounts, Yahoo said it may force users to employ second sign-in verification -- its version of two-factor authentication -- which sends a six-digit code via SMS to a user's registered mobile phone number, provided they have one on file.
According to Litmus email analytics, Yahoo Mail comprises 5% of the world's email clients. In terms of market share -- across desktops, mobile clients, and webmail -- that makes Yahoo Mail the world's eighth most popular email client, trailing iOS Mail clients (38%), Outlook (14%), Android (12%), Apple Mail (8%), Gmail (6%), and Outlook.com (6%), but placing it ahead of Windows Live Mail (3%) and Windows Mail (2%).
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Yahoo said there's no evidence that the recent account-hijacking campaign resulted from attackers stealing credentials from Yahoo itself. Rather, whoever's behind the attack appears to have harvested the usernames and passwords from another site, meaning that Yahoo victims likely reused usernames and passwords across multiple sites. "Based on our current findings, the list of usernames and passwords that were used to execute the attack was likely collected from a third-party database compromise," Rossiter said. "Our ongoing investigation shows that malicious computer software used the list of usernames and passwords to access Yahoo Mail accounts. The information sought in the attack seems to be names and email addresses from the affected accounts' most recent sent emails."
He added: "We are working with federal law enforcement to find and prosecute the perpetrators responsible for this attack."
Who hijacks email accounts? Criminals may hack into accounts to harvest legitimate names and email addresses to target with phishing attacks or to run scams. One well-worn ruse involves an urgent appeal for funds sent under an email account owner's name to everyone in his or her address book. The message claims, for example, that the account owner was mugged in London and a hotel is holding his or her passport ransom until the bill is settled in cash. "Kindly help me send the money via Western Union Money Transfer to my name and hotel address below," the message reads.
Yahoo's account-takeover warning is thus a heads-up, not just for users to beware account takeovers, but for anyone who receives an email sent via Yahoo. "I'd... recommend being wary of any odd email messages from friends with Yahoo accounts that send you links or attachments in the next few days," said Chris Mohan at the Internet Storm Center.
How can Yahoo users better avoid account takeovers altogether? According to Yahoo's tips for safeguarding its accounts, the company recommends that all account users "add an alternate email address and mobile number to your account," which can be used to receive a password reset, in the event that the account has been compromised.
Also never reuse the same password across multiple sites. "If you do make the mistake of reusing passwords, you are running the risk of having your password compromised in one place -- perhaps via a phishing attack or keylogger -- and then hackers using it to unlock your other online accounts," said Graham Cluley, an independent security researcher, in a blog post.
Instead, use a password manager to generate strong passwords -- think long and random -- and then store those passwords. Modern password managers will keep your password database synchronized across PCs, mobile devices, and the cloud.
So many people still reuse passwords, however, that some companies, such as Facebook, have begun testing public dumps of usernames and passwords to see if they unlock any of their users' accounts. If so, the company can lock affected accounts and force users to pick a new password.
Beyond never reusing passwords, for optimum account security Yahoo users should also activate the aforementioned second sign-in verification, which Yahoo first introduced in 2011. Once activated, the two-factor verification system will send a six-digit code via SMS to the mobile phone number on file anytime it sees a login attempt -- with a valid username and password -- from a new system. Without that code, would-be attackers can't access the account.
For access to Yahoo email via any non-Yahoo application, meanwhile, the company also offers one-time codes. "Certain applications -- for instance, iOS Mail, Android Mail, and Outlook -- don't support Yahoo's second sign-in verification," Cluley said. "For those, you will need to generate one-time passwords, separate from the one you use on your Yahoo account."
Having a wealth of data is a good thing -- if you can make sense of it. Most companies are challenged with aggregating and analyzing the plethora of data being generated by their security applications and devices. This Dark Reading report, How Existing Security Data Can Help ID Potential Attacks, recommends how to effectively leverage security data in order to make informed decisions and spot areas of vulnerability. (Free registration required.)