An SSH certificate authority framework to sign SSH pubkey requests based on SAML assertions
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README.md

sshephalopod (working title)

Providing SSH certificate authority signing of SSH pubkey requests based on authentication from a trusted third party.

What problems does it solve?

  • Distribution of SSH private keys to users who need to use them to authenticate to a remote host, where the corresponding public key is in an authorized_keys file or similar
  • Distribution of user public keys to remote hosts (the inverse of the above problem)
  • SSH key rotation and revocation (which, in practice, rarely happens without implementing some kind of synchronous lookup performed by the sshd on the remote host)
  • Authenticated access to remote hosts without the need for the remote host to be able to contact the authenticating party directly (based on cryptographic trusts previously established)
  • Minimal reconfiguration to either the SSH client or remote host

Deployment Risks and Considerations

Deploying sshephalopod into an AWS account has risks where potential attackers have access to the account as well:

  • because EC2 instances trust the public key retrieved from DNS, an attacker who can change DNS could substitute their own ssh pubkey into the TXT record;
  • because sshephalopod stores keys in S3 to persist certificates across invocations, an attacker could load a copy of keys that they have access to into the S3 bucket, giving them signing access to create their own trusted certificates. They would also have to update the DNS TXT record with their substituted public key.

You can mitigate the S3 bucket risk to some extent by clever use of IAM policies -- preventing users writing to that bucket, for example -- but in general, you get fairly far down the policy-whittling rabbit hole and your overheads in maintaining exceptions and special cases get both heavy and hairy (and nobody likes shaving yaks).

If you decide to deploy sshephalopod into a different account, you pretty much eliminate the above risks, because you control the access at the account boundary; however, you have the tradeoff that the "sshephalopod account" cannot update the TXT and SRV records for a domain that covers a VPC in an alernate account. You would then have to:

  • configure EC2 instances to trust a public key from a domain not controlled by the account that the instance is launched into; and
  • configure client-side wrapper scripts to find the SRV record based on a DNS domain that isn't the one that hosts the EC2 instances.

This makes it a little more clunky, but much more secure. It's up to you to decide what level of assurance you need around your remote-access infrastructure; most people should probably err on the side of caution.

Prerequisites

  • Create a reference to your Service Provider in your Identity Provider (IdP). You need to have your IdP return an Attribute of email as part of the AttributeStatement

Building/Deploying it

Common prep work

Do this whether you're deploying via Docker or via Makefile:

  • Update the top-level Makefile and set:
    • IDP_METADATA to the http(s) endpoint which returns the SAML metadata of your Identity Provider. For Okta, this will look something like https://foo.okta.com/app/ksdulfvlisndklfgjnsk/sso/saml/metadata Note that you do not need to do anything with this URL, just set it in the Makefile -- a Custom Resource will retrieve the data when the sshephalopod CloudFormation stack is created.
    • DOMAIN to the DNS domain that this sshephalopod instance will serve
    • CONFIG_BUCKET to the name of an existing S3 bucket where the Lambda code will be uploaded.
    • KEYPAIR_BUCKET to the name of an existing S3 bucket where the ssh CA keypairs will be persisted, so that subsequent creations of sshephalopod for the same domain will use the same keypair.

Build-time dependencies

  • A SAML Identity Provider (IdP). sshephalopod is known to work with Okta
  • AWS CLI -- a version that can do CloudFormation and S3 commands
  • zip for bundling the Lambda code into S3

Build/deploy

  • from the top level, run make deploy to deploy the CloudFormation stack including Lambda functions, DNS entries, API Gateway configuration

Configuring your instances

For an EC2 instance, include the following code in your UserData:

  "Fn::Base64": {
    "Fn::Join": [
      "",
      [
        "#!/bin/bash\n",
        "CERT=$( eval echo $( dig +short txt sshephalopod-ca-cert.",
                          { "Ref": "DNSDomain" }, " ) )\n",
        "if [ -z \"$CERT\" ]; then\n",
        "    echo ERROR cannot find CA cert\n",
        "    exit 1\n",
        "fi\n",

        "echo $CERT > /etc/ssh/trusted_certs\n",
        "cat >> /etc/ssh/sshd_config <<EoM\n",
        "\n",
        "LogLevel VERBOSE\n",
        "TrustedUserCAKeys /etc/ssh/trusted_certs\n",
        "EoM\n",

        "service sshd restart\n"
      ]
    ]
  }

Automatic CA trusted key rotation

Have a look at the contributed key-rotation script which replaces the one-time example above.

Using it

With a deployed sshephalopod and a configured Okta:

  • Have an SSH public key in $HOME/.ssh/id_rsa.pub

  • Run the wrapper script:

    localhost$ cd ssh-integration && ./wrapper ec2-user domain.where.SRV.record.is
    Using SP https://cnsjdhfbs2.execute-api.us-west-2.amazonaws.com/prod/signing
    Beginning the SAML dance ...
    Please sign in to Okta
    Username: my.username
    Password: ********
    Signing request succeeded: please log in with 'ssh ec2-user@domain.where.SRV.record.is'
    If you are using ssh-agent, please run 'ssh-add' now
    
  • If you're using ssh-agent, run ssh-add to add your private key and the signed certificate to your agent. Note: gnome-keyring-daemon does NOT support RSA-certificate signed SSH keys, you have to use OpenSSH's ssh-agent)

    localhost$ ssh-add -l
    The agent has no identities.
    
    localhost$ ssh-add
    Enter passphrase for /home/user/.ssh/id_rsa: ********
    Identity added: /home/user/.ssh/id_rsa (/home/user/.ssh/id_rsa)
    Certificate added: /home/user/.ssh/id_rsa-cert.pub (my.username@okta-domain)
    
    localhost$ ssh-add -l
    4096 SHA256:nBwKsuL3+RRoyV1mw6N4GZJ8I/o2akuit8QXCm1XUBQ /home/user/.ssh/id_rsa (RSA)
    4096 SHA256:nBwKsuL3+RRoyV1mw6N4GZJ8I/o2akuit8QXCm1XUBQ /home/user/.ssh/id_rsa (RSA-CERT)
    

License

sshephalopod is licensed under the MIT license; please see the file LICENSE for details.