Yahoo has patched a cross-site scripting (XSS) vulnerability capable of running in multiple browsers once Yahoo Mail users open spammed malicious links.
Shahin Ramezany, a hacker and independent researcher, tweeted about the exploit on Sunday, linking to a YouTube video he created that demonstrates the hack. Ramezany also said the exploit puts 400 million users at risk.
It has not been confirmed whether the vulnerability is related to a wave of recent Yahoo Mail accounts that have been compromised, often to deliver spam to a victim's address book.
Yahoo! told SC the XSS vulnerability was patched, but the email hacking incidents could be a separate issue.
“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,” the company said.
“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.
"Separately, we were also recently informed of an online video that demonstrated a potential security vulnerability, which has been fixed."
Ramezany did not respond to a request for comment.
The researcher tweeted Monday that he planned to post proof-of-concept code for the flaw on his site, Abysssec.com, after Yahoo patched the issue.
After the news of Yahoo's fix, Ramezany later tweeted Tuesday that the patch was "not effective enough and users are still [at] risk," since the proof-of-concept code can be easily tweaked to continue attacks.