Protecting a Shared AMI (Linux and UNIX)

Amazon Elastic Compute Cloud: 2009-08-15

Protecting a Shared AMI (Linux and UNIX)

These guidelines are not requirements and you are welcome to follow or ignore them. However, following these guidelines produces a better user experience, helps ensure your users' instances are secure, and can protect you.

To build a shared AMI, follow these guidelines:

Shared AMI Guidelines

1	How to Update the AMI Tools at Boot Time
2	Disable Password-Based Logins for Root
3	Install Public Key Credentials
4	How to Disable sshd DNS Checks (optional)
5	Identify Yourself
6	Protect Yourself
7	Protect Paid AMIs

	Note
	These guidelines are written for Fedora distributions, but the principles apply to any AMI. You might need to modify the provided examples for other distributions. For other distributions, review their documentation or search the AWS forums in case someone else has done it already.

How to Update the AMI Tools at Boot Time

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 will have the latest AMI tools.

To update the AMI tools at startup on Fedora

Add the following to rc.local:

# Update the Amazon EC2 AMI creation tools
echo " + Updating ec2-ami-tools"
wget http://s3.amazonaws.com/ec2-downloads/ec2-ami-tools.noarch.rpm && \
rpm -Uvh ec2-ami-tools.noarch.rpm && \
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.

Note

The preceding procedure applies to Fedora distributions. For other distributions:

On most Red Hat systems, add these steps to your /etc/rc.d/rc.local script.
On Gentoo systems, add them to /etc/conf.d/local.local.
On Ubuntu systems, add them to /etc/rc.local.
On Debian, you might need to create a start up script in /etc/init.d and use update-rc.d <scriptname> defaults 99 (where <scriptname> is the name of the script you created) and add the steps to this script.

Disable Password-Based 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 logins for the root user. Additionally, we recommend you randomize the root password at boot.

To disable password-based logins for root

Open the /etc/ssh/sshd_config file with a text editor and locate the following line:
```
#PermitRootLogin yes
```
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.
To randomize the root password, add the following to your boot process:
```
if [ -f "/root/firstrun" ] ; then
  dd if=/dev/urandom count=50|md5sum|passwd --stdin root
  rm -f /root/firstrun
else
  echo "* Firstrun *" && touch /root/firstrun
```
Note

This step assumes that a /root/firstboot file is bundled with the image. If file was not created, the root password will never be randomized and will be set to the default.

	Note
	If you are using a distribution other than Fedora, you might need to consult the documentation that accompanied the distribution.

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.

The following list shows the SSH files to remove.

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

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) is made available to the instance through an HTTP query against the instance metadata.

To login 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 will be able to launch instances of your AMI with a key pair and log in without requiring a root password.

if [ ! -d /root/.ssh ] ; then
        mkdir -p /root/.ssh
        chmod 700 /root/.ssh
fi
# Fetch public key using HTTP
curl http://169.254.169.254/2009-08-15//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 image 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.

How to Disable sshd DNS Checks (optional)

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

To disable sshd DNS checks

Open the /etc/ssh/sshd_config file with a text editor and locate the following line:
```
#UseDNS yes
```
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 as each AMI is represented by a numeric user ID.

We recommend that you post a description of your AMI, and the AMI ID, in the Amazon EC2 developer forum. This provides a convenient central location for users who are interested in trying new shared AMIs.

Protect Yourself

The previous sections described how to make your shared AMIs safe, secure, and useable 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:

Always delete the shell history before bundling. If you attempt more than one bundle upload in the same image, the shell history contains your secret access key.
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).
Exclude the ssh authorized keys when bundling the image. The Amazon public images store the public key used to launch an instance with its ssh authorized keys file.

	Note
	Unfortunately, it is not possible for this list of guidelines to be exhaustive. Build your shared AMIs carefully and take time to consider where you might expose sensitive data.

Protect Paid AMIs

The simplest way to prevent users from rebundling Paid AMIs that you create is to not provide root access to the AMI and to pay attention to security announcements that involve privilege escalations. Amazon EC2 requires you to have root access any AMI that you rebundle.

If you must provide root access to an AMI, Amazon EC2 tools are designed to protect the product code. Although this is effective, it is not guaranteed and users might create AMIs using other tools.

To ensure users cannot rebundle your paid AMIs, we recommend that you configure your application to check the instance metadata to verify that the product code is intact.

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	Note
	This step assumes that a /root/firstboot file is bundled with the image. If file was not created, the root password will never be randomized and will be set to the default.