CNET: The security bug can wipe a server's memory, where sensitive user data is stored like usernames, passwords and credit card numbers.
It's an extremely serious issue, affecting some 500,000 servers, according to Netcraft, an Internet research firm.
Some Web sites that appeared to have been affected included Yahoo and OKCupid, though the companies have said their sites are all or partly fixed. You can check sites on an individual basis here, though caution is still advised even if the site gives you an "all clear" indication. If you're given a red flag, avoid the site for now.
Read more here.