What is the internet Heartbleed Bug?
10 April 2014 Last updated at 06:21 BST
A bug in software used by millions of web servers could have exposed anyone visiting sites they hosted to spying and eavesdropping, say researchers.
The bug is in a software library used in servers, operating systems and email and instant messaging systems.
Called OpenSSL, the software is supposed to protect sensitive data as it travels back and forth.
Victoria Sill explains.