NEW YORK ( TheStreet) -- Hundreds of thousands of usernames and passwords were stolen on Wednesday, as Yahoo! ( YHOO) suffered a massive security breach.

In an email to The Street on Thursday, Yahoo! confirmed that an "older file" from the Yahoo! Contributor Network, which allows writers, photographers, and videographers to share content, was hacked. However, less than 5% of the approximately 400,000 violated accounts had valid passwords. In addition to Yahoo! emails being affected, some Gmail, AOL ( AOL), Hotmail, Comcast ( CMCSA), MSN, SBC Global, Verizon ( VZ), BellSouth and Live.com users were also affected.

"At Yahoo! we take security very seriously and invest heavily in protective measures to ensure the security of our users and their data across all our products," Yahoo! wrote in the email to TheStreet. "We are fixing the vulnerability that led to the disclosure of this data, changing the passwords of the affected Yahoo! users and notifying the companies whose users accounts may have been compromised. We apologize to affected users."

Like Sony ( SNE - Get Report) and LinkedIn ( LNKD), which both dealt with security breaches in the past year, Yahoo! is encouraging subscribers to change their passwords regularly and is directing them to the company's online safety tips. Sucuri Malware Labs, an antivirus research firm, set up a site to enable users to check if their Yahoo! account has been compromised.

Yahoo! is holding its annual shareholder meeting today. It is widely assumed that interim chief Ross Levinsohn will be named its permanent CEO. Levinsohn replaced Scott Thompson in May after controversy over Thompson's resume.

Despite the hacking incident, Yahoo! shares outpaced the NASDAQ on Thursday, trading down .66% to $15.70. The stock is down 2.67% year-to-date.

--Written by Nathalie Pierrepont in New York.

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