The month’s victim comes courtesy of Yahoo, or should I say Yahoo’s HotJobs.com. On October 28th, popular internet research and analysis company Netcraft discovered a vulnerability on the Yahoo site that was being exploited to steal user authentication cookies. These cookies contain user login credentials that can be used to access any of Yahoo’s services, including e-mail. These cookies were being sent remotely to a site in the United States under the control of the attacker.
Yahoo has since corrected the flaw and released the following statement to netcraft:
The team was made aware of this particular Cross-Site Scripting issue yesterday morning (Sunday, Oct. 26) and a fix was deployed within a matter of hours. Yahoo! appreciates Netcraft’s assistance in identifying this issue.
As a safety precaution, we recommend users change their passwords, should they still be concerned. Users should always verify via their Sign-in Seal that they are giving their passwords to Yahoo.com.
How it happened:
This attack was probably executed using the CookieMonster tool that has recently affected netflix.com and bankofamerica. CookieMonster is a cookie stealing toolkit that works with both http and https sites. It siphons authentication cookies from vulnerable sites. These cookies can be used to hijack a users account.
Theregister.co.uk best describes CookieMonster as follows:
The vulnerability stems from website developers’ failure to designate authentication cookies as secure. That means web browsers are free to send them over the insecure http channel, and that’s exactly what CookieMonster causes them to do. It does this by caching all DNS responses and then monitoring hostnames that use port 443 to connect to one of the domain names stored there. CookieMonster then injects images from insecure (non-https) portions of the protected website, and – voila! – the browser sends the authentication cookie.
A CookieMonster blog listed several popular sites that were allegedly vulnerable back in September. Those sites include southwest.com, expedia.com, usairways.com, register.com, newegg.com, ebay.com, any many many more.
What can be done:
In addition to the steps outlined in this XSS tutorial, sites that contain cookies for authentication must not allow cookie values to be translated on the client side. In the early days of cookie based authentication, many sites simply stored authentication information in the cookie, which can be read in any text editor. Today, cookies merely act as a reference point for server side authentication, however if the cookie can be used from any client, it defeats the purpose of even hiding the true value.