By Victor Luckerson
April 10, 2014

It keeps happening. A newly discovered software bug–this one going by the ominous name Heartbleed–allows hackers to bypass the encryption technology used on many websites to access passwords, credit-card numbers and other sensitive data. By one estimate, as much as 66% of the web is affected, including Yahoo and the popular dating site OkCupid.

Companies are scrambling to close off the security flaw in the widely used encryption technology, called OpenSSL, but since the coding error went unnoticed for two years, there’s no telling how much information hackers have stolen. Users are advised to change their passwords–but only after confirming that an affected website is no longer susceptible.

Heartbleed is the latest and farthest-reaching cybersecurity scare in a spate of them affecting everything from cell phones to brick-and-mortar stores like Target. Experts say data will get only more difficult to secure. “There are no secrets on the Internet,” says Ari Takanen, founder of Codenomicon, the security firm that discovered Heartbleed. “Something bad can always happen.”

–VICTOR LUCKERSON

Contact us at editors@time.com.

This appears in the April 21, 2014 issue of TIME.

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