[AWS] EC2 SSH Error — Fixing UNPROTECTED PRIVATE KEY FILE (.pem chmod 400)

1. The Error

When trying to SSH into an AWS EC2 instance, you sometimes get the following warning and are denied access.

@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@         WARNING: UNPROTECTED PRIVATE KEY FILE!          @
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
Permissions 0755 for 'myKey.pem' are too open.
It is required that your private key files are NOT accessible by others.
This private key will be ignored.
Load key "myKey.pem": bad permissions
ubuntu@{my-ip}: Permission denied (publickey).

The key part is Permissions for 'myKey.pem' are too open. In other words, the .pem key file's permissions are too open, so the system deemed it a security risk and blocked the connection.

Figure 1. The 'UNPROTECTED PRIVATE KEY FILE' warning when connecting via SSH

2. The Cause

For security, SSH clients on Linux/macOS are designed to use only keys that no one but the owner can read.

• The problematic permission — 0755 or 0644 means other users (group, others) can also read this key.
• The security rule — if a private key could be exposed to others, SSH doesn't trust it and refuses the connection.

3. The Fix: chmod 400

In the terminal, move to the directory with the key file (.pem) and change its permission to owner read-only (400) to fix it instantly.

# Change the pem key file permission to 400
chmod 400 myKey.pem

Note: if 400 doesn't work, you can try 600, but the AWS guide usually recommends 400.

💡 What permission 400 means on Linux

4. Verify

After changing the permission, try connecting again.

ssh -i "myKey.pem" ubuntu@<Your-EC2-IP>

You can now confirm the instance connects normally without the warning.

📦 Migrated from my own Korean blog (my own writing). Original: taehyuklee.tistory.com/31