Menu
Amazon Elastic Compute Cloud
User Guide for Linux Instances

Guidelines for Shared Linux AMIs

Use the following guidelines to reduce the attack surface and improve the reliability of the AMIs you create.

Note

No list of security guidelines can be exhaustive. Build your shared AMIs carefully and take time to consider where you might expose sensitive data.

If you are building AMIs for AWS Marketplace, see Building AMIs for AWS Marketplace for guidelines, policies and best practices.

For additional information about sharing AMIs safely, see the following articles:

Update the AMI Tools at Boot Time

For AMIs backed by instance store, we recommend that your AMIs download and upgrade the Amazon EC2 AMI creation tools during startup. This ensures that new AMIs based on your shared AMIs have the latest AMI tools.

For Amazon Linux, add the following to /etc/rc.local:

# Update the Amazon EC2 AMI tools
echo " + Updating EC2 AMI tools"
yum update -y aws-amitools-ec2
echo " + Updated EC2 AMI tools"

Use this method to automatically update other software on your image.

Note

When deciding which software to automatically update, consider the amount of WAN traffic that the update will generate (your users will be charged for it) and the risk of the update breaking other software on the AMI.

For other distributions, make sure you have the latest AMI tools.

Disable Password-Based Remote Logins for Root

Using a fixed root password for a public AMI is a security risk that can quickly become known. Even relying on users to change the password after the first login opens a small window of opportunity for potential abuse.

To solve this problem, disable password-based remote logins for the root user.

To disable password-based remote logins for root

  1. Open the /etc/ssh/sshd_config file with a text editor and locate the following line:

    #PermitRootLogin yes
  2. Change the line to:

    PermitRootLogin without-password

    The location of this configuration file might differ for your distribution, or if you are not running OpenSSH. If this is the case, consult the relevant documentation.

Disable Local Root Access

When you work with shared AMIs, a best practice is to disable direct root logins. To do this, log into your running instance and issue the following command:

[ec2-user ~]$ sudo passwd -l root

Note

This command does not impact the use of sudo.

Remove SSH Host Key Pairs

If you plan to share an AMI derived from a public AMI, remove the existing SSH host key pairs located in /etc/ssh. This forces SSH to generate new unique SSH key pairs when someone launches an instance using your AMI, improving security and reducing the likelihood of "man-in-the-middle" attacks.

Remove all of the following key files that are present on your system.

  • ssh_host_dsa_key

  • ssh_host_dsa_key.pub

  • ssh_host_key

  • ssh_host_key.pub

  • ssh_host_rsa_key

  • ssh_host_rsa_key.pub

  • ssh_host_ecdsa_key

  • ssh_host_ecdsa_key.pub

  • ssh_host_ed25519_key

  • ssh_host_ed25519_key.pub

You can securely remove all of these files with the following command.

[ec2-user ~]$ sudo shred -u /etc/ssh/*_key /etc/ssh/*_key.pub

Warning

Secure deletion utilities such as shred may not remove all copies of a file from your storage media. Hidden copies of files may be created by journalling file systems (including Amazon Linux default ext4), snapshots, backups, RAID, and temporary caching. For more information see the shred documentation.

Important

If you forget to remove the existing SSH host key pairs from your public AMI, our routine auditing process notifies you and all customers running instances of your AMI of the potential security risk. After a short grace period, we mark the AMI private.

Install Public Key Credentials

After configuring the AMI to prevent logging in using a password, you must make sure users can log in using another mechanism.

Amazon EC2 allows users to specify a public-private key pair name when launching an instance. When a valid key pair name is provided to the RunInstances API call (or through the command line API tools), the public key (the portion of the key pair that Amazon EC2 retains on the server after a call to CreateKeyPair or ImportKeyPair) is made available to the instance through an HTTP query against the instance metadata.

To log in through SSH, your AMI must retrieve the key value at boot and append it to /root/.ssh/authorized_keys (or the equivalent for any other user account on the AMI). Users can launch instances of your AMI with a key pair and log in without requiring a root password.

Many distributions, including Amazon Linux and Ubuntu, use the cloud-init package to inject public key credentials for a configured user. If your distribution does not support cloud-init, you can add the following code to a system start-up script (such as /etc/rc.local) to pull in the public key you specified at launch for the root user.

if [ ! -d /root/.ssh ] ; then
        mkdir -p /root/.ssh
        chmod 700 /root/.ssh
fi
# Fetch public key using HTTP
curl http://169.254.169.254/latest/meta-data/public-keys/0/openssh-key > /tmp/my-key
if [ $? -eq 0 ] ; then
        cat /tmp/my-key >> /root/.ssh/authorized_keys
        chmod 700 /root/.ssh/authorized_keys
        rm /tmp/my-key
fi

This can be applied to any user account; you do not need to restrict it to root.

Note

Rebundling an instance based on this AMI includes the key with which it was launched. To prevent the key's inclusion, you must clear out (or delete) the authorized_keys file or exclude this file from rebundling.

Disabling sshd DNS Checks (Optional)

Disabling sshd DNS checks slightly weakens your sshd security. However, if DNS resolution fails, SSH logins still work. If you do not disable sshd checks, DNS resolution failures prevent all logins.

To disable sshd DNS checks

  1. Open the /etc/ssh/sshd_config file with a text editor and locate the following line:

    #UseDNS yes
  2. Change the line to:

    UseDNS no

Note

The location of this configuration file can differ for your distribution or if you are not running OpenSSH. If this is the case, consult the relevant documentation.

Identify Yourself

Currently, there is no easy way to know who provided a shared AMI, because each AMI is represented by an account ID.

We recommend that you post a description of your AMI, and the AMI ID, in the Amazon EC2 forum. This provides a convenient central location for users who are interested in trying new shared AMIs. You can also post the AMI to the Amazon Machine Images (AMIs) page.

Protect Yourself

The previous sections described how to make your shared AMIs safe, secure, and usable for the users who launch them. This section describes guidelines to protect yourself from the users of your AMI.

We recommend against storing sensitive data or software on any AMI that you share. Users who launch a shared AMI might be able to rebundle it and register it as their own. Follow these guidelines to help you to avoid some easily overlooked security risks:

  • We recommend using the --exclude directory option on ec2-bundle-vol to skip any directories and subdirectories that contain secret information that you would not like to include in your bundle. In particular, exclude all user-owned SSH public/private key pairs and SSH authorized_keys files when bundling the image. The Amazon public AMIs store these in /root/.ssh for the root account, and /home/user_name/.ssh/ for regular user accounts. For more information, see ec2-bundle-vol in the Amazon EC2 Command Line Reference.

  • Always delete the shell history before bundling. If you attempt more than one bundle upload in the same AMI, the shell history contains your secret access key. The following example should be the last command executed before bundling from within the instance.

    [ec2-user ~]$ shred -u ~/.*history

    Warning

    The limitations of shred described in the warning above apply here as well.

    Be aware that bash writes the history of the current session to the disk on exit. If you log out of your instance after deleting ~/.bash_history, and then log back in, you will find that ~/.bash_history has been re-created and contains all of the commands executed during your previous session.

    Other programs besides bash also write histories to disk, Use caution and remove or exclude unnecessary dot-files and dot-directories.

  • Bundling a running instance requires your private key and X.509 certificate. Put these and other credentials in a location that is not bundled (such as the instance store).