The Heartbleed bug is a real threat. Here's what you should do.

Starting Monday, the internet-connected world was introduced to a new bug, colorfully named Heartbleed, that has exposed about two-thirds of web servers — and probably about a quarter of all sites — to potential pilfering of sensitive, supposedly encrypted information: passwords, credit card numbers, etc. Google engineers discovered the bug last week in the OpenSSL encryption software, then quietly notified OpenSSL, which started secretly helping companies patch the bug before going public amid fears that hackers had discovered the hole, too.

How big of a deal is Heartbleed? "It's easily the worst vulnerability since mass-adoption of the internet," Matthew Prince, CEO of cybersecurity firm CloudFlare Inc., tells The Wall Street Journal. "It's going to be really bad."

How bad? "We don't know to what extent this flaw has been targeted by hackers, we are in the dark here about the extent of how it is been used," David Emm, senior security researcher at Kaspersky Lab, tells CNBC. "We can't quantify the scale of the damage."

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So, what can you do about it? Unless you're an IT person at a bank or social media service or other websites that relies on OpenSSL encryption, not a whole lot. Those companies have to update their encryption — a process that involves more than just affixing the OpenSSL patch.

Once a vulnerable site is secure again, you should change your password. Seriously, change it. If a site hasn't fixed the encryption problem, changing your password is useless, or worse.

How can you tell? CNET has a list of popular sites and their Heartbleed status. And a company called LastPass has a useful tool where you can enter any website and it will tell you its vulnerability and advise you what to do. For more information about Heartbleed, here's a brief report from CNBC. Good luck. --Peter Weber