On the internet, to maintain security, you have to have something called a certificate that is used as part of establishing a secure connection using TLS (the secure part of https). The way it works is using a public/private key pair -- one half is public, and is used to verify that the certificate is signed by the correct authority, and the other is secret, where no-one else can figure it out. Each key is just a really big number that goes into a cryptographic math problem. One certificate authority, eMudhra, used 8 different private keys that were extremely small numbers, including one that is just the number 4 (represented as 0000000...4, but literally just the number 4). The math problem is much easier to solve with small numbers, and a common way to break the cypher is to check every possible option starting at 0, which gets to 4 pretty fast. It's remarkable in terms of a security breach because no-one actually had to leak anything for it to be insecure, it's just that the "password" was so easy to guess that we had to "rotate the passwords" for basically the entire internet, because now anyone can sign a certificate using the private key 4 and it will look like a real certificate for google.com or wellsfargo.com or whatever.