What is wrong with 0.0.0.0/0 on SSH in a security group?

What this card says

Short answer

0.0.0.0/0 means every IP on the internet. SSH (port 22) is used to log in to a server. A security group is the AWS firewall. So this rule lets anyone in the world try to log in.

Example

A new EC2 server (a cloud computer on AWS) with SSH open to 0.0.0.0/0 soon shows many failed SSH logins from unknown IPs. This is brute force (bots guessing logins).

Say it in the interview

“Even with strong keys, open SSH means noise and risk from new bugs. I allow port 22 only from known IPs, like my IP /32, or use AWS SSM Session Manager.”

Common mistake / follow-up

Mistake: “I use a key, so it is safe.” Keys help, but the door is still open to everyone.
Follow-up: “How do you find failed SSH logins?”