Yahoo is investigating the claims of a hacker who is selling an exploit that apparently hijacks Yahoo mail accounts.
"The exploit, being sold for $700 by an Egyptian hacker on an exclusive cybercrime forum, targets a cross-site scripting (XSS) weakness in yahoo.com that lets attackers steal cookies from Yahoo! Webmail users," wrote Brian Krebs, a noted security blogger who reported the illegal offer to Yahoo. "Such a flaw would let attackers send or read email from the victim’s account. In a typical XSS attack, an attacker sends a malicious link to an unsuspecting user; if the user clicks the link, the script is executed, and can access cookies, session tokens or other sensitive information retained by the browser and used with that site. These scripts can even rewrite the content of the HTML page."
Demonstrating an apparent flair for marketing, the hacker, under the alias "TheHell" also posted a video on YouTube, providing a demo for potential customers. He claims it works with all browsers and does not require a bypass of XSS filters in either Chrome or Internet Explorer. He also says the exploit will be sold only to trusted individuals who are not likely to turn it over to Yahoo, which would undoubtedly develop a patch that will foil the attack.
"TheHell" claims that his exploit attacks a "stored" XSS flaw. Krebs explains that this type of attack injects a code that is permanently stored on targeted servers until it is found and deleted. The malicious code is then passed to the victim’s machine when that particular server is accessed for legitimate download.
A standard phishing attempt is used to access the user's cookies, from which the attacker can access the person's email, or take full control of the account.
As of Tuesday morning, Yahoo was in the process of trying to identify the infected URL. Once the identification is successful, the malicious portion of code will be deleted.
"As powerful as XSS attacks can be, they are unfortunately also extremely common," Krebs writes. "Xssed.com, the definitive archive of reported XSS vulnerabilities, lets users index them by Google Pagerank, making it quite easy to find several examples of other unfixed and recently-patched XSS flaws in yahoo.com and other properties. Also, the Open Source Vulnerability Database (OSVDB) lists new and fixed vulnerabilities by vendor."
PUBLISHED NOV. 27, 2012