Yahoo Explains Cookie Forgery Related To Two 2016 Breaches

Yahoo has provided further clarification on the cookie forging activity related to data breaches disclosed in September and December of last year.

Earlier this week, the company began to issue warnings to inform users of forged cookies used to steal their information. A Dark Reading report implied the forged cookies were used in a third incident; in fact, this activity was limited to the two 2016 breaches.

"Yahoo has connected some of the cookie forging activity to the same state-sponsored actor believed to be responsible for the data theft we disclosed on September 22, 2016," a Yahoo spokesperson says.

The alerts sent this week are a continuation of Yahoo's ongoing investigation into the cookie forging disclosed in December, not a new event. Both the September and December data breaches are related to the theft of user data in 2014.

Further, the company states, it believes the separate theft of user data in August 2013, which was disclosed on December 14, 2016, is likely unrelated to the incident announced on September 22.

Yahoo believes the investigation of this breach is in its final stages.

Shuman Ghosemajumder, CTO at Shape Security, says this update doesn't add much to the overall story; Yahoo is simply continuing efforts promised in December.

What's important to remember, he says, is this credential spill drives potential for millions of account takeovers on thousands of major websites. Cybercriminals will have an easier time increasing the number of compromised credentials far beyond the two billion spilled.

"Credential spills are one of the most widespread, yet misunderstood, security breaches," says Ghosemajumder. Most people will focus on users' Yahoo accounts, but the damage affecting those has been done. Yahoo will simply tell users to reset their passwords.

"The real issue now is that these passwords will be used to breach thousands of other websites unrelated to Yahoo, as cybercriminals use advanced automated tools to discover where users have used those same passwords on other sites, through credential stuffing attacks, the most common attacks on web applications and APIs today," he explains.