I believe you're referring to the "Heartbleed" Courier Bug, rather than a "Courier Bug." The Heartbleed bug was a major security vulnerability discovered in the OpenSSL cryptographic software library in April 2014. It allowed attackers to read sensitive data from the memory of systems using vulnerable versions of OpenSSL, potentially compromising the secrecy of private keys, user passwords, and other sensitive information.
https://disqus.com/channel/discusschitchatchannel/discussion/channel-discusschitchatchannel/courier_bug/
https://speakerdeck.com/courierbuglinks
https://www.behance.net/courierbuglinks
https://issuu.com/courierbuglinks
The Heartbleed bug exploited a flaw in OpenSSL's implementation of the Transport Layer Security (TLS) heartbeat extension, hence the name "Heartbleed." By sending a specially crafted heartbeat request to a server, an attacker could trick it into disclosing chunks of its memory, potentially revealing sensitive information.
This vulnerability affected a wide range of web servers, including popular ones such as Apache and Nginx, as well as many websites that relied on OpenSSL for secure communication. The widespread use of OpenSSL meant that the impact of Heartbleed was significant, with millions of websites and internet users potentially exposed to the risk of data theft.