The Egyptian hacker ViruS_HimA has claimed to have discovered a number of critical vulnerabilities in the Yahoo server that allowed him to access Full files backup for one of Yahoo domains and Full access to "12" of Yahoo Databases.
He also claimed to have doscovered a cross site scripting vulnerability. To demonstrate his findings, he has provided several screenshots in pastebin(http://pastebin.com/Pxnszw7b).
The hacker highlighted the fact that he is not malicious hacker - He notified about the vulnerability to the company but they failed to respond. So he just decided to publicly disclose his findings.
"Hey Yahoo! you have to think well about making Hall of fame for security researchers because this will get you much reports for your vulnerabilities." The hacker The hacker advises Yahoo.
Finally, Yahoo now considering the hacker claim. According to The Next Web, they are currently investigating ViruS_HimA's claims and they’re working on addressing any vulnerabilities that are discovered in the process.