ContainerImage.Pinniped/site/content/docs/howto/configure-auth-for-webapps.md

title	description	cascade	menu
Using the Pinniped Supervisor to provide authentication for web applications	Allow your Kubernetes cluster users to authenticate into web apps using the same identities.	layout docs	docs name weight parent Web Application Authentication 800 howtos

The Pinniped Supervisor is an OpenID Connect (OIDC) issuer that can be used to bring your user identities from an external identity provider into your Kubernetes clusters for all your kubectl users. It can also be used to bring those same identities to web applications that are intended for use by the same users. For example, a Kubernetes dashboard web application for cluster developers could use the Supervisor as its OIDC identity provider.

This guide explains how to use the Supervisor to provide authentication services for a web application.

Note that this feature is not part of how Pinniped provides authentication for kubectl users. By default, the Pinniped Supervisor will contain an OIDC client called pinniped-cli which requires no configuration and is used to provide authentication for kubectl (and other kubeconfig-based Kubernetes API clients). If you are only setting up authentication for kubectl users of your Kubernetes clusters, then you do not need to read this guide. If you want to use the Pinniped Supervisor to provide authentication services for a web application, then this guide is for you.

Prerequisites

This guide assumes that you have installed and configured the Pinniped Supervisor, and configured it with an external identity provider, as described in the other guides.

This guide also assumes that you have a web application which supports configuring an OIDC provider for user authentication, or that you are developing such a web application. From the point of view of the Supervisor, your webapp is called a "client" (as defined in the OAuth 2.0 spec).

Typically, the web application should use the OIDC client support from its web application development framework (e.g. Spring, Rails, Django, etc.) to implement authentication. The Supervisor requires that:

• Clients must use the OIDC authorization code flow. Clients must use code as the response_type at the authorization endpoint.
• Clients must use PKCE during the authorization code flow.
• Clients must be confidential clients, meaning that they have a client ID and client secret. Clients must use client secret basic auth for authentication at the token endpoint.
• Clients must use query as the response_mode at the authorization endpoint, or not specify the response_mode param, which defaults to query.

Most web application frameworks offer all these capabilities in their OAuth2/OIDC libraries.

Performance implications of using OIDCClients in the Supervisor

The Pinniped Supervisor is an efficient application which typically does not use a lot of CPU and memory resources. Using the OIDCClient CR, as described below, will cause the Supervisor to perform bcrypt operations to validate the client's secret during authorization and refresh flows. While each of these bcrypt operations takes only about a quarter second of CPU time, in aggregate, when lots of users are perform authorization and refresh flows, these bcrypts will constitute the majority of the CPU usage of the Supervisor.

The administrator of the Supervisor may need to adjust the Supervisor Deployment once they are familiar with usage patterns of their Supervisor. Very heavy usage by clients might result in the Supervisor pods reaching their cpu limit and being throttled, resulting in poor performance. This can be alleviated by adjusting the number of Pod replicas, and the CPU requests and limits on each Pod.

Create an OIDCClient

For each web application, the administrator of the Pinniped Supervisor will create an OIDCClient describing what that web application is allowed to do:

apiVersion: config.supervisor.pinniped.dev/v1alpha1
kind: OIDCClient
metadata:
  # name must have client.oauth.pinniped.dev- prefix
  name: client.oauth.pinniped.dev-my-webapp-client 
  namespace: supervisor # must be in the same namespace as the Supervisor
spec:
  allowedRedirectURIs:
    - https://my-webapp.example.com/callback
  allowedGrantTypes:
    - authorization_code
    - refresh_token
    - urn:ietf:params:oauth:grant-type:token-exchange
  allowedScopes:
    - openid
    - offline_access
    - pinniped:request-audience
    - username
    - groups

If you've saved this into a file my-oidc-client.yaml, then install it into your cluster using:

kubectl apply -f my-oidc-client.yaml

Do not share OIDCClients between multiple web applications. Each web application should have its own OIDCClient.

The name of the OIDCClient will be the client ID used by the web application in the OIDC flows.

The allowedGrantTypes and allowedScopes decides what the web application is allowed to do with respect to authentication. There are several typical combinations of these settings:

1. A web application which is allowed to use the Supervisor for authentication, and furthermore is allowed to authenticate into Kubernetes clusters and perform actions on behalf of the users (using the user's identity):

     allowedGrantTypes:
       - authorization_code
       - refresh_token
       - urn:ietf:params:oauth:grant-type:token-exchange
     allowedScopes:
       - openid
       - offline_access
       - pinniped:request-audience
       - username
       - groups
   

2. A web application which is allowed to use the Supervisor for authentication, but cannot perform actions on Kubernetes clusters.

     allowedGrantTypes:
       - authorization_code
       - refresh_token
     allowedScopes:
       - openid
       - offline_access
       - username
       # "groups" can be excluded from this list when the webapp does
       # not need to see the group memberships of the users.
       - groups 
   

3. A web application which is allowed to use the Supervisor for authentication, but cannot see the username or group memberships of the authenticated users, and cannot perform actions on Kubernetes clusters.

     allowedGrantTypes:
       - authorization_code
       - refresh_token
     allowedScopes:
       - openid
       - offline_access
   

Create a client secret for the OIDCClient

For each OIDCClient created by the Supervisor administrator, the administrator will also need to generate a client secret for the client. The client secrets are random strings auto-generated by the Supervisor upon request. The plaintext secret will only be returned once upon creation.

cat <<EOF | kubectl create -o yaml -f -
apiVersion: clientsecret.supervisor.pinniped.dev/v1alpha1
kind: OIDCClientSecretRequest
metadata:
  name: client.oauth.pinniped.dev-my-webapp-client # the name of the OIDCClient
  namespace: supervisor # the namespace of the OIDCClient
spec:
  generateNewSecret: true
EOF

The server will respond with the newly generated client secret, e.g.:

apiVersion: clientsecret.supervisor.pinniped.dev/v1alpha1
kind: OIDCClientSecretRequest
metadata:
  creationTimestamp: "2022-09-22T19:04:46Z"
  name: client.oauth.pinniped.dev-my-webapp-client
  namespace: supervisor
spec:
  generateNewSecret: true
  revokeOldSecrets: false
status:
  generatedSecret: e593049b02d0b647af4ac99bd5963c3612f9ea9c414a9b8f6acd23bc43cbf084
  totalClientSecrets: 1

Take care to make a note of the status.generatedSecret. It can never be retrieved again. After it has been returned once in the response of the create API, there is no other way to retrieve it in the future. The secret is not stored in plaintext on the server, which only stores a bcrypt-hashed version of the secret.

The status.totalClientSecrets reports the total number of client secrets associated with this OIDCClient at the end of the request. This can also be observed on the status of the OIDCClient CR itself.

This is the client secret that should be used, along with the client ID, by the web application when interacting with the Supervisor's OIDC token endpoint.

The OIDCClientSecretRequest is a special API which only supports the create verb. After creating a client secret, you cannot use kubectl get, kubectl delete, kubectl apply, or any other API verbs to access those client secret resources.

Rotating the client secret for an OIDCClient

To facilitate rotating client secrets, an OIDCClient may have several active secrets. This enables the following process for the Supervisor administrator to change a client secret without causing web application downtime:

1. Add a new, second secret to the OIDCClient by calling the create OIDCClientSecretRequest API again, as shown above. Make note of the plaintext secret returned by the API. Now you have an old secret and a new secret, both of which will work.

2. Reconfigure the web application to use the new client secret.

3. Once the web application has been redeployed and is using the new client secret, call the client secret API again to remove the old client secret:

   cat <<EOF | kubectl create -o yaml -f -
   apiVersion: oauth.virtual.supervisor.pinniped.dev/v1alpha1
   kind: OIDCClientSecretRequest
   metadata:
     # the name of the OIDCClient
     name: client.oauth.pinniped.dev-my-webapp-client
     namespace: supervisor # the namespace of the OIDCClient
   spec:
     revokeOldSecrets: true
   EOF
   

Note that when there are multiple active client secrets for an OIDCClient, clients who use an older client secret will pay a small performance penalty during authorization and refresh flows. The client secret provided by client during authorization and refresh flows is compared against the stored bcrypt hashes of each active client secret, in order from the most recently generated secret to the least recently generated secret. Each comparison operation is a somewhat expensive bcrypt. As a best practice, an OIDCClient should usually have one active secret, except during a window of rotation, when it will have two active secrets.

The server will only allow an OIDCClient to have five active secrets. Asking the server to generate a sixth secret will fail, unless you also ask the server to revoke all the old secrets in the same (or in a previous) request.

Deleting an OIDCClient

An OIDCClient can be deleted in the usual way that Kubernetes CRs are deleted. User sessions using that client will fail at their next refresh request. The corresponding client secrets will also be deleted. Even if the client is created again with the same name, a new client secret will need to be generated for that client. Since the client secret is new, webapps that were using the old client secret will not be able to perform refresh requests (unless they are updated to use the new secret).

What the web application will receive from the authorization code flow

When the web application completes the authorization code flow with the Supervisor, it will receive three tokens:

• An ID token. The ID token is a JWT which is readable by the web application. It will contain the user's identity, to the extent that the client is allowed to learn the details of the user's identity.
• An opaque access token. This token may be used to perform an RFC 8693 token exchange to get a cluster-scoped ID token to gain access to the Kubernetes API of a cluster with the identity of the user who authenticated. This workflow is described further in another section below.
• An opaque refresh token. The ID token and access tokens are short-lived, and are intended to be refreshed often by using the OIDC refresh flow. The refresh flow will return new access, ID, and refresh tokens to the web application. Each refresh must use the latest refresh token.

The ID token returned at the end of the authorization code flow will contain the following standard claims, as defined by the OIDC spec:

• iss: the issuer URL of the Supervisor's FederationDomain
• sub: the subject, a unique identifier of the user (usually not the same as the username)
• exp: expiration time of the ID token
• rat: the timestamp of when authorization was requested
• auth_time: the timestamp of the user authentication
• iat: the timestamp of when this ID token was issued
• aud: the client ID that requested this ID token
• azp: the client ID that requested this ID token, again
• jti: the JWT ID
• nonce: a string value used to associate a Client session with an ID Token, and to mitigate replay attacks

Refreshed ID tokens will contain the same claims, except that a refreshed ID token will also contain an at_hash claim, and will not contain a nonce claim. (The original ID token should also contain an at_hash claim, but it is excluded due to a bug in one of Pinniped's dependencies. The Pinniped maintainers have submitted a PR to that library to fix the bug and are waiting for the next release of that library to incorporate the fix into Pinniped.)

Additionally, the following custom claims may be included in the ID tokens, if the client requested the username and/or groups scopes in the original authorization request, and if the client is allowed to request those scopes:

• username: the user's username for Kubernetes clusters
• groups: an array of strings containing the names of the groups to which the user belongs, for defining their group memberships in Kubernetes clusters

Note that if the groups list is empty for a user, the claim will be excluded rather than appear as an empty list in the ID token. This can happen when the user does not belong to any groups in the external identity provider, or when the Supervisor administrator did not configure Pinniped to extract group memberships from the external identity provider.

Refreshing the user's identity

The ID and access tokens issued at the end of the authorization code flow are only valid for a short period of time. Clients should not assume that the user's identity as described by the results of the initial authorization code grant is still valid beyond the lifetime of those initial ID and access tokens.

The short lifetime of these tokens ensures that the user's session with the external identity provider is validated by the Supervisor often, during each refresh request. For example, if the user's group membership in the external identity provider has changed since the initial authorization, the group membership will be updated during a refresh. Or if the user's account in the external identity provider was suspended, the refresh will fail.

The client may use the refresh token to request new tokens by making a standard OIDC refresh request.

Refresh tokens are typically valid for a number of hours. Once a refresh token has expired, a web application should ask the user the log in again by starting the authorization code flow from the beginning.

How a web application can perform actions as the authenticated user on Kubernetes clusters

If allowed, a web application may perform actions on Kubernetes clusters on behalf of the signed-in user. The actions will use the identity of the signed-in user, so the RBAC policies on the workload cluster related to that user will take effect.

Exactly how this works depends on your configuration and how you choose to use the various components of Pinniped to aid in your Kubernetes authentication setup. If you are using the typical Pinniped setup as described in the [Learn to use Pinniped for federated authentication to Kubernetes clusters]({{< ref "../tutorials/concierge-and-supervisor-demo" >}}) tutorial, then the next sections will apply.

Cluster-scoped ID tokens

The ID token issued at the end of the authorization code flow contains the user's Kubernetes identity. However, this ID token is typically not used directly to provide authentication to the Kubernetes clusters' API servers.

In a typical configuration, the Pinniped Concierge is installed on each workload cluster and is configured with a JWTAuthenticator resource to validate ID tokens issued by the Pinniped Supervisor. However, typically each workload cluster's JWTAuthenticator is configured to validate a unique audience value (aud claim) of the ID tokens. This ensures that an ID token which is used to access one workload cluster cannot also be used to access other workload clusters, to limit the impact of a leaked token.

In this typical configuration, the client must make an extra API call to the Supervisor after the authorization code flow before it can access a particular workload cluster, in order to get a cluster-scoped ID token for a specific workload cluster (technically, for the audience value of that workload cluster).This request is made to the token endpoint, using parameters described in RFC 8693. This request requires that the access token was granted the username and pinniped:request-audience scopes in the authorization code flow, and preferably was also granted the groups scope. It also requires that the client's OIDCClient configuration allows it to use the urn:ietf:params:oauth:grant-type:token-exchange grant type.

The client has already called the Supervisor FederationDomain's /.well-known/openid-configuration discovery endpoint at the beginning of the authorization code flow, so the client is already aware of the location of the FederationDomain's token endpoint. The client makes an HTTPS request to the token endpoint to request a cluster-scoped ID token. The client sends its client ID and client secret as a basic auth header. It sends the Supervisor-issued access token as the subject_token param to identify the user's active session, along with the other required parameters.

POST /federation-domain-path/oauth2/token HTTP/1.1
Host: my-supervisor.example.com
Content-Type: application/x-www-form-urlencoded
Authorization: Basic czZCaGRSa3F0MzpnWDFmQmF0M2JW

grant_type=urn:ietf:params:oauth:grant-type:token-exchange
  &subject_token=<supervisor-issued-access-token-value>
  &subject_token_type=urn:ietf:params:oauth:token-type:access_token
  &requested_token_type=urn:ietf:params:oauth:token-type:jwt
  &audience=<workload-cluster-audience-name>

A successful request will result in a 200 OK response with a JSON body. One of the top-level keys in the returned JSON object will be id_token, and the value at that key will be the cluster-scoped ID token.

This exchange is typically repeated for each workload cluster, right before the client needs to access the Kubernetes API of that workload cluster.

mTLS client certificates

Once the client has a cluster-scoped ID token for a particular workload cluster, the next step towards accessing the Kubernetes API of that workload cluster, in a typical configuration, is to request an mTLS client certificate from that workload cluster. The client certificate will act as the credential for the Kubernetes API server.

This is done by making a request to the /apis/login.concierge.pinniped.dev/v1alpha1/tokencredentialrequests API of the Kubernetes API of that cluster. This API is an aggregated API hosted on the Kubernetes API server, but behind the scenes is actually served by the Pinniped Concierge. It can be accessed just like any other Kubernetes API. It does not require any authentication on the request.

The details of the request and response formats are documented in the [API docs](https://github.com/vmware-tanzu/pinniped/blob/main/generated/{{< latestcodegenversion >}}/README.adoc#tokencredentialrequest).

Here is a sample YAML representation of a request:

apiVersion: login.concierge.pinniped.dev/v1alpha1
kind: TokenCredentialRequest
spec:
  token: <cluster-scoped ID token value>
  authenticator:
    apiGroup: authentication.concierge.pinniped.dev/v1alpha1
    kind: JWTAuthenticator
    name: <the metadata.name of the JWTAuthenticator to be used>

And here is a sample YAML representation of a successful response:

apiVersion: login.concierge.pinniped.dev/v1alpha1
kind: TokenCredentialRequest
status:
  credential:
    expirationTimestamp: <timestamp>
    clientCertificateData: <PEM-encoded client TLS certificates>
    clientKeyData: <PEM-encoded private key for the above certificate>

The returned mTLS client certificate will contain the user's identity (username and groups) copied from the cluster-scoped ID token. It may be used to make calls to the Kubernetes API as that user, until it expires.

These mTLS client certificates are short-lived, typically good for about 5-15 minutes. After it expires, a client which wishes to make more Kubernetes API calls will need to perform an OIDC refresh request to the Supervisor to get a new access token, and then repeat the steps described above to get new cluster-scoped ID tokens and mTLS client certificates. Requiring these steps to be repeated often ensures that the user's session with the external identity provider is validated often, to ensure any changes to the user's level of access will quickly be reflected in the Kubernetes clusters.