How can re-access to EC2 instance

I hope it never happens to them

Some time ago I lost/deleted the .pem file (I don’t really know) to access one of my EC2 instances, it was a productive instance and the only person with access was me.

Tried several ways without success. After a while I started to create a new EC2 instance and configure the deploy to that new instance, since the code was safe in a rep, the public resources in an S3 and the DB in an RDS, therefore my plan was “move” everything to that new instance and after a couple of hours everything would be fine, but something in my mind told me that it was not correct and that “something” could be lost if I lowered the instance and raised another… that something It was tricky: there were trusted certificates, configuration of payment method integrations, “hardcoded” integrations that were only inside EC2 and never supported (badly there) anywhere else… so I kept looking and found the following solution (finally).

Error: Authentication failed, permission denied
Solution: Use a userdata script and add the SSH public key to the authorized_keys file to regain access

How?

1. On our computer we obtain the ssh public key and “save / copy” its content for the future.
2. Go to AWS, find the instance and stop it.
3. We go to Actions, Instance Settings, Edit User Data and paste the following content replacing the OS_USER with the username associated with the AMI from which you launched the instance and the PUBLIC_SSH_KEY with our ssh public key from the step

Content-Type: multipart/mixed; boundary="//"
MIME-Version: 1.0
 - //
Content-Type: text/cloud-config; charset="us-ascii"
MIME-Version: 1.0
Content-Transfer-Encoding: 7bit
Content-Disposition: attachment; filename="cloud-config.txt"
#cloud-config
cloud_final_modules:
-[scripts-user, always]
 - //
Content-Type:
 text/x-shellscript; charset="us-ascii"
MIME-Version: 1.0
Content-Transfer-Encoding: 7bit
Content-Disposition: attachment; filename="userdata.txt"
#!/bin/bash
OS_USER=@@@@@@
chown root:root /home
chmod 755 /home
chown $OS_USER:$OS_USER /home/$OS_USER -R
chmod 700 /home/$OS_USER
chmod 700 /home/$OS_USER/.ssh
chmod 600 /home/$OS_USER/.ssh/authorized_keys
echo '$PUBLIC_SSH_KEY' >> /home/$OS_USER/.ssh/authorized_keys
chown $OS_USER:$OS_USER/ home/$OS_USER -R
 - //

Restart the instance and now you can access :)

At last, to delete this “file” you just have to go to where you edited it (step 3) and delete its content :)

I hope this helps you as much as it does me