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secrets.md

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Secrets

Objects of type secret are intended to hold sensitive information, such as passwords, OAuth tokens, and ssh keys. Putting this information in a secret is safer and more flexible than putting it verbatim in a pod definition or in a docker image.

Creating and Using Secrets

To make use of secrets requires at least two steps:

  1. create a secret resource with secret data
  2. create a pod that has a volume of type secret and a container which mounts that volume.

This is an example of a simple secret, in json format:

{
  "apiVersion": "v1",
  "kind": "Secret",
  "metadata" : {
    "name": "mysecret",
    "namespace": "myns"
  },  
  "data": {
    "username": "dmFsdWUtMQ0K",
    "password": "dmFsdWUtMg0KDQo="
  }
}

The data field is a map. Its keys must match DNS_SUBDOMAIN. The values are arbitrary data, encoded using base64.

This is an example of a pod that uses a secret, in json format:

{
 "apiVersion": "v1",
 "kind": "Pod",
  "metadata": {
    "name": "mypod",
    "namespace": "myns"
  },
  "spec": {
    "containers": [{
      "name": "mypod",
      "image": "redis",
      "volumeMounts": [{
        "name": "foo",
        "mountPath": "/etc/foo",
        "readOnly": true
      }]
    }],
    "volumes": [{
      "name": "foo",
      "secret": {
        "secretName": "mysecret"
      }
    }]
  }
}

Restrictions

Secret volume sources are validated to ensure that the specified object reference actually points to an object of type Secret. Therefore, a secret needs to be created before any pods that depend on it.

Secret API objects reside in a namespace. They can only be referenced by pods in that same namespace.

Individual secrets are limited to 1MB in size. This is to discourage creation of very large secrets which would exhaust apiserver and kubelet memory. However, creation of many smaller secrets could also exhaust memory. More comprehensive limits on memory usage due to secrets is a planned feature.

Kubelet only supports use of secrets for Pods it gets from the API server. This includes any pods created using kubectl, or indirectly via a replication controller. It does not include pods created via the kubelets --manifest-url flag, its --config flag, or its REST API (these are not common ways to create pods.)

Consuming Secret Values

The program in a container is responsible for reading the secret(s) from the files. Currently, if a program expects a secret to be stored in an environment variable, then the user needs to modify the image to populate the environment variable from the file as an step before running the main program. Future versions of Kubernetes are expected to provide more automation for populating environment variables from files.

Changes to Secrets

Once a pod is created, its secret volumes will not change, even if the secret resource is modified. To change the secret used, the original pod must be deleted, and a new pod (perhaps with an identical PodSpec) must be created. Therefore, updating a secret follows the same workflow as deploying a new container image. The kubectl rolling-update command can be used (man page).

The resourceVersion of the secret is not specified when it is referenced. Therefore, if a secret is updated at about the same time as pods are starting, then it is not defined which version of the secret will be used for the pod. It is not possible currently to check what resource version of a secret object was used when a pod was created. It is planned that pods will report this information, so that a controller could restart ones using a old resourceVersion. In the interim, if this is a concern, it is recommended to not update the data of existing secrets, but to create new ones with distinct names.

Use cases

Use-Case: Pod with ssh keys

To create a pod that uses an ssh key stored as a secret, we first need to create a secret:

{
  "kind": "Secret",
  "apiVersion": "v1",
  "metadata": {
    "name": "ssh-key-secret"
  },
  "data": {
    "id-rsa": "dmFsdWUtMg0KDQo=",
    "id-rsa.pub": "dmFsdWUtMQ0K"
  }
}

Note: The serialized JSON and YAML values of secret data are encoded as base64 strings. Newlines are not valid within these strings and must be omitted.

Now we can create a pod which references the secret with the ssh key and consumes it in a volume:

{
  "kind": "Pod",
  "apiVersion": "v1",
  "metadata": {
    "name": "secret-test-pod",
    "labels": {
      "name": "secret-test"
    }
  },
  "spec": {
    "volumes": [
      {
        "name": "secret-volume",
        "secret": {
          "secretName": "ssh-key-secret"
        }
      }
    ],
    "containers": [
      {
        "name": "ssh-test-container",
        "image": "mySshImage",
        "volumeMounts": [
          {
            "name": "secret-volume",
            "readOnly": true,
            "mountPath": "/etc/secret-volume"
          }
        ]
      }
    ]
  }
}

When the container's command runs, the pieces of the key will be available in:

/etc/secret-volume/id-rsa.pub
/etc/secret-volume/id-rsa

The container is then free to use the secret data to establish an ssh connection.

Use-Case: Pods with prod / test credentials

This example illustrates a pod which consumes a secret containing prod credentials and another pod which consumes a secret with test environment credentials.

The secrets:

{
  "apiVersion": "v1",
  "kind": "List",
  "items":
  [{
    "kind": "Secret",
    "apiVersion": "v1",
    "metadata": {
      "name": "prod-db-secret"
    },
    "data": {
      "password": "dmFsdWUtMg0KDQo=",
      "username": "dmFsdWUtMQ0K"
    }
  },
  {
    "kind": "Secret",
    "apiVersion": "v1",
    "metadata": {
      "name": "test-db-secret"
    },
    "data": {
      "password": "dmFsdWUtMg0KDQo=",
      "username": "dmFsdWUtMQ0K"
    }
  }]
}

The pods:

{
  "apiVersion": "v1",
  "kind": "List",
  "items":
  [{
    "kind": "Pod",
    "apiVersion": "v1",
    "metadata": {
      "name": "prod-db-client-pod",
      "labels": {
        "name": "prod-db-client"
      }
    },
    "spec": {
      "volumes": [
        {
          "name": "secret-volume",
          "secret": {
            "secretName": "prod-db-secret"
          }
        }
      ],
      "containers": [
        {
          "name": "db-client-container",
          "image": "myClientImage",
          "volumeMounts": [
            {
              "name": "secret-volume",
              "readOnly": true,
              "mountPath": "/etc/secret-volume"
            }
          ]
        }
      ]
    }
  },
  {
    "kind": "Pod",
    "apiVersion": "v1",
    "metadata": {
      "name": "test-db-client-pod",
      "labels": {
        "name": "test-db-client"
      }
    },
    "spec": {
      "volumes": [
        {
          "name": "secret-volume",
          "secret": {
            "secretName": "test-db-secret"
          }
        }
      ],
      "containers": [
        {
          "name": "db-client-container",
          "image": "myClientImage",
          "volumeMounts": [
            {
              "name": "secret-volume",
              "readOnly": true,
              "mountPath": "/etc/secret-volume"
            }
          ]
        }
      ]
    }
  }]
}

Both containers will have the following files present on their filesystems:

    /etc/secret-volume/username
    /etc/secret-volume/password

Note how the specs for the two pods differ only in one field; this facilitates creating pods with different capabilities from a common pod config template.

Use-case: Secret visible to one container in a pod

Consider a program that needs to handle HTTP requests, do some complex business logic, and then sign some messages with an HMAC. Because it has complex application logic, there might be an unnoticed remote file reading exploit in the server, which could expose the private key to an attacker.

This could be divided into two processes in two containers: a frontend container which handles user interaction and business logic, but which cannot see the private key; and a signer container that can see the private key, and responds to simple signing requests from the frontend (e.g. over localhost networking).

With this partitioned approach, an attacker now has to trick the application server into doing something rather arbitrary, which may be harder than getting it to read a file.

Security Properties

Protections

Because secret objects can be created independently of the pods that use them, there is less risk of the secret being exposed during the workflow of creating, viewing, and editing pods. The system can also take additional precautions with secret objects, such as avoiding writing them to disk where possible.

A secret is only sent to a node if a pod on that node requires it. It is not written to disk. It is stored in a tmpfs. It is deleted once the pod that depends on it is deleted.

On most Kubernetes-project-maintained distributions, communication between user to the apiserver, and from apiserver to the kubelets, is protected by SSL/TLS. Secrets are protected when transmitted over these channels.

There may be secrets for several pods on the same node. However, only the secrets that a pod requests are potentially visible within its containers. Therefore, one Pod does not have access to the secrets of another pod.

There may be several containers in a pod. However, each container in a pod has to request the secret volume in its volumeMounts for it to be visible within the container. This can be used to construct useful security partitions at the Pod level.

Risks

  • Applications still need to protect the value of secret after reading it from the volume, such as not accidentally logging it or transmitting it to an untrusted party.
  • A user who can create a pod that uses a secret can also see the value of that secret. Even if apiserver policy does not allow that user to read the secret object, the user could run a pod which exposes the secret. If multiple replicas of etcd are run, then the secrets will be shared between them. By default, etcd does not secure peer-to-peer communication with SSL/TLS, though this can be configured.
  • It is not possible currently to control which users of a kubernetes cluster can access a secret. Support for this is planned.
  • Currently, anyone with root on any node can read any secret from the apiserver, by impersonating the kubelet. It is a planned feature to only send secrets to nodes that actually require them, to restrict the impact of a root exploit on a single node.

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