Yahoo said it has closed a cross-site scripting vulnerability a researcher said left Yahoo Mail users susceptible to having their accounts hijacked – though a security firm is disputing that claim.
The vulnerability, uncovered by security researcher Shahin Ramezany of Abysssec, is believed to have led to millions of Yahoo mail users being at risk. In a YouTube video, Ramezany demonstrated an exploit for the issue, which he described as a document object model-based cross-site scripting bug.
According to Yahoo, this bug is different than the one reported by security blogger Brian Krebs last November.
"At Yahoo! we take security very seriously and invest heavily in measures to protect our users and their data," a spokesperson told SecurityWeek. "We are investigating recent reports of user accounts that may have been compromised to send abusive email and will work diligently to fix any vulnerabilities that are found."
"In general, we recommend using different passwords for online accounts, changing passwords from time to time, and choosing passwords that combine letters, numbers, and symbols," the spokesperson added.
But while Yahoo said the vulnerability demonstrated by Ramezany has been fixed, security certification and pentesting firm Offensive Security say the bug can still be exploited with slight modifications to the original proof-of-concept code. The company posted a video demonstration of its findings online.
"The victim has to be lured to click a link which contains malicious XSS code for the attack to succeed," according to the Offensive Security blog. "This can demonstrated by the video we have created just this morning (10:23 AM EST, Jan 8th, 2013) after Shahin kindly shared proof of concept code with us."
"Yahoo mail users should be on guard against clicking any links for the foreseeable future," according to Offensive Security. "Due to the nature of the vulnerability, XSS filters and similar protections provide little defense against this attack."
Yahoo did not respond to Offensive Security's claims before publication.