Final answer:
The Heartbleed bug is the security flaw in the OpenSSL cryptography library that affected Internet security by allowing attackers to access sensitive data from vulnerable systems.
Step-by-step explanation:
The security bug in the OpenSSL cryptography library, which is a widely used implementation of the Transport Layer Security (TLS) protocol, is the Heartbleed bug. Discovered in 2014, Heartbleed allowed attackers to read sensitive information from a vulnerable server's memory, including private keys, user passwords, and other confidential data.
The bug lay in the heartbeat feature of the OpenSSL library, which when exploited, compromised the secret keys used to identify service providers and to encrypt traffic, user names, passwords, and the actual content. This significantly undermined Internet security and required immediate and widespread action to patch the vulnerability in systems using OpenSSL.