The social networking giant revealed in a blog post that it had discovered a data compromise potentially exposing approximately 250,000 users' account information to the eyes of a sophisticated attacker.
Twitter did not say exactly how the attackers compromised the data, but it did say that it discovered the exploit when it "detected unusual access patterns" that allowed it to track down the breach. The exploit may have enabled attackers to access usernames, email addresses, session tokens, and encrypted/salted versions of passwords, the company said.
As a precaution, Twitter reset passwords and revoked session tokens for the users' accounts, notifying them by email of the need to change their passwords.
Twitter did not attribute the attack to China, which is suspected in a recent series of media hacks, but an analysis by Internet data tracking company PeerReach indicates that top media figures may have been among the targets.
"This attack was not the work of amateurs, and we do not believe it was an isolated incident," the Twitter blog states. "The attackers were extremely sophisticated, and we believe other companies and organizations have also been recently similarly attacked. For that reason we felt that it was important to publicize this attack while we still gather information."
Several security monitoring companies, including Vigilant, said they detected the attacks. "Although details of entry have not been made public, based on Vigilant's threat intelligence monitoring, we have observed that the motivation was to harvest email address for future spam use," said Lance James, chief scientist at Vigilant, in a statement.
"Multiple emails came through this weekend from victim accounts through bulk mailers, and when contacted, the victims indicated their Twitter account was on the list and that they had also received the email address from their Twitter account," James said. "When passwords were similar, or the same, the attackers used them as a stepping stone to further access email accounts associated with the hack."
Although some of the affected users were frustrated that the attack was not detected sooner, some security experts were encouraged by the fact that Twitter detected it at all. During the past few years, most of the victims studied in the Verizon Data Breach Incident Report found out about their breaches from third parties, rather than discovering it themselves.
"The fact that Twitter detected it quickly and took action is a step in the right direction," says Vinnie Liu, managing partner of security firm Stach & Liu.
But Srinivas Kumar, CTO and co-founder of security startup TaaSERA, says enterprises need to get better at malware detection.
Kumar says there is a need "to provide more visibility at the Web tier for security stakeholders to detect and mitigate risks. Single sign-on, infected access devices, and compromised Web sites together weaken the security posture for Internet-based commerce. We have seen an uptick in this type of activity recently. This is an increasing trend for attacks because it can be used for spam and identity fraud."
Interestingly, Twitter has posted a job advertisement seeking an individual who will help the social networking giant implement two-factor authentication. Experts say they expect the company to follow Google and other leading sites that are now enabling users to protect their identities by authenticating themselves with a step that goes beyond passwords, such as a phone call.
"I think that's something we're going to see from a lot of the major players going forward," says Liu, who has written extensively about passwords and authentication. "When data compromises start to affect their revenue streams, such as we've seen with the gaming sites recently, there's real motivation to increase the sophistication of their access beyond simple passwords."
And if social networks begin to use two-factor authentication as a matter of course, enterprises are likely to follow, Liu says. "Depending on the type of data they are trying to protect, they may not end up using the same second factor, but I think we'll see it becoming more of the norm," he says.
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