Internet DRAFT - draft-hofmann-wimse-workload-identity-bcp
draft-hofmann-wimse-workload-identity-bcp
WIMSE B. Hofmann
Internet-Draft H. Tschofenig
Intended status: Standards Track Siemens
Expires: 12 April 2024 10 October 2023
Best Current Practice for Workload Identity
draft-hofmann-wimse-workload-identity-bcp-00
Abstract
The use of the OAuth 2.0 framework for container orchestration
systems poses a challenge as managing secrets, such as client_id and
client_secret, can be complex and error-prone. "Service account
token volume projection", a term introduced by Kubernetes, provides a
way of injecting JSON Web Tokens (JWTs) to workloads.
This document specifies the use of JWTs for client credentials in
container orchestration systems to improve interoperability in
orchestration systems, to reduce complexity for developers, and
motivates authorization server to support RFC 7523.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Architecture and Recommendations . . . . . . . . . . . . . . 5
4. Security Considerations . . . . . . . . . . . . . . . . . . . 6
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
7.1. Normative References . . . . . . . . . . . . . . . . . . 6
7.2. Informative References . . . . . . . . . . . . . . . . . 6
Appendix A. Example . . . . . . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
In workload scenarios dedicated management entities, also called
"control plane" entities, are used to start, monitor and stop
workloads dynamically. These workloads typically run micro services
that interact with each other and with other entities on the
corporate network or on the Internet. When one workload, acting as
an OAuth client, wants to gain access to a protected resource hosted
on another workload or on the Internet (referred here generically as
a resource server) then authorization is typically required.
OAuth has been designed to offer help in scenarios where access to
protected resources needs to be managed dynamically in a distributed
system.
Each workload instance has to be provisioned with unique credentials.
However, these credentials have to be configured prior and are then
attached to the workload. In addition, these credentials do not have
an automated rotation mechanism and are valid for an unspecified
amount of time.
This requires manual configuration effort and the missing automated
rotation mechanism introduce inconvenience and increase the attack
surface.
"Service account token volume projection" is a feature of container
orchestration systems that allows users to create JSON Web Tokens
(JWTs) for their workloads. These JWTs, referred as Service Account
Tokens, can be used as client credentials, as specified in RFC 7523
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[RFC7523]. As these tokens are managed by the "control plane" and
simply mounted to the filesystem, a workload just needs to consume
this file and present it to the authorization server. In addition,
service account token volume projection allows an expiration time on
these JWTs to be set, allowing automated rotation of these
credentials. Finally, the private key for signing these tokens is
managed by the "control plane", hence removing the manual effort of
configuring the client_id and client_secret.
However, there is currently no standardized way to manage these
Service Account Tokens across container orchestrators. This leads to
inconsistencies, and additional effort for developers as they need to
support different client authentication mechanisms. In the worst
case, this approach is ignored in favor of client_id and
client_secret.
This specification specifies the use of Service Account Tokens in
container orchestration systems, which provides a secure and scalable
way to create and manage these tokens, and ensures interoperability
with existing OAuth-based authorization systems.
When OAuth is used as part of the control plane entities, a Service
Account Token is provisioned to the workload via the Agent. This
interaction is shown in Figure 1.
To distinguish the entities, we use the term "Control Plane" to refer
to the OAuth 2.0 Authorization Server that is part of the cluster's
control plane. Since there are also two access tokens in play, we
use the term "Service Account Token" to refer to the token issued by
the Control Plane and thereby distinguish it from the other access
token issued to an OAuth 2.0 client running inside the workload by
the second authorization server.
It is important to note that the workload does not use the Service
Account Token with resource servers directly but instead obtains
access tokens from this second authorization server. To obtain these
access tokens, the OAuth 2.0 client running in the workload uses the
JWT client authentication grant, as defined in [RFC7523], with the
Service Account Token as input. The obtained access token may be a
bearer token, or a proof-of-possession token.
Figure 1 illustrates the interaction in the architecture graphically.
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+---------------+
| |
| Authorization |
| Server |
| |
+---------------+
^ |
| |
+--------------------------|-|--------------+
|Cluster | | OAuth |
| | | Exchange |
| +---------------+ | | to obtain |
| | | | | access token |
| | Control Plane | | | using |
| | | | | Service |
| +---------------+ | | Account |
| ^| | | Token |
| || | v +-----+
| || +----------+ |
| || | |+ | +----------+
| Obtain || | Workload || | | |
| Service|| | ||<-------->| Resource |
| Account|| +----------+| | Access | Server |
| Token || +----------+ | Token | |
| || ^ | +----------+
| || Start Workload: |
| || with Service : |
| || Account Token : |
| || v |
| || +-------+ |
| |+------------->| | |
| +---------------| Agent | |
| | | |
| +-------+ |
| |
+-------------------------------------+
Figure 1: Protocol Interaction.
In Section 3 we provide more details about how the content of the
tokens and the offered security properties.
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2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Architecture and Recommendations
This specification relies on the use of OAuth 2.0 [RFC6749] and
[RFC7523] for client authentication using a JWT.
Service Account Tokens used in container orchestration systems are
vulnerable to different types of threats, as shown in this list:
1. Token theft: Tokens can be stolen by attackers who have already
gained access to a workload. These attackers can then use these
tokens to impersonate the workload and gain access to resources
they should not have access to.
2. Token reuse: Tokens can be reused by attackers to gain access to
the system. However, expiration times limited the token reuse
time.
3. Misconfigured service accounts: Similar to misconfigured access
to secrets, misconfigured service accounts can lead to
applications gaining more privileges then necessary.
4. Theft of token signing key: The token signing key can be stolen
by attackers who have already gained access to the control plane.
However, such attackers also have access to all secrets in the
container orchestration system. Hence, resulting in the same
impact for use of client_id and client_secret compared to using
Service Account Tokens.
The following fields are populated in the Service Account Token:
1. The 'iss' claim MUST contain a string identifying the container
orchestrator.
2. The 'sub' claim MUST contain a string identifying the workload.
This is also the client_id according to [RFC7523].
3. The 'aud' claim MUST identify one or multiple authorization
servers that are intended recipients of the Service Account Token
for client authorization.
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Further processing requirements are specified in [RFC7523].
4. Security Considerations
This entire document is about security.
5. IANA Considerations
This document does not require actions by IANA.
6. Acknowledgements
Add your name here.
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/rfc/rfc2119>.
[RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
RFC 6749, DOI 10.17487/RFC6749, October 2012,
<https://www.rfc-editor.org/rfc/rfc6749>.
[RFC7523] Jones, M., Campbell, B., and C. Mortimore, "JSON Web Token
(JWT) Profile for OAuth 2.0 Client Authentication and
Authorization Grants", RFC 7523, DOI 10.17487/RFC7523, May
2015, <https://www.rfc-editor.org/rfc/rfc7523>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.
7.2. Informative References
[OIDC] Sakimura, N., Bradley, J., Jones, M., de Medeiros, B., and
C. Mortimore, "OpenID Connect Core 1.0 incorporating
errata set 1", November 2014,
<https://openid.net/specs/openid-connect-core-1_0.html>.
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Appendix A. Example
The functionality described in this specification can be verified
using Kubernetes. Modern version of Kubernetes implement service
account token volume projection, which enables the ability to inject
the Service Account Token with a specific issuer and audience into
the workload.
A most important parts of the configuration are (which can be found
at the end of the full configuration):
1. the path, where the application can find the token, as a file
2. the expiration of the token in seconds
3. the audience, which will be in the Service Account Token
serviceAccountToken:
path: token
expirationSeconds: 7200
audience: "https://localhost:5001/connect/token"
The full configuration is shown below:
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apiVersion: apps/v1
kind: Deployment
metadata:
name: simpleapp
labels:
app: simpleapp
spec:
replicas: 1
selector:
matchLabels:
app: simpleapp
template:
metadata:
labels:
app: simpleapp
spec:
containers:
- name: container1
image: curlimages/curl:8.2.1
imagePullPolicy: Always
command:
- sleep
- "3600"
env:
- name: TOKEN_PATH
value: '/var/run/secrets/other_token/token/token'
volumeMounts:
- mountPath: '/var/run/secrets/other_token/token'
name: other-token-path
volumes:
- name: other-token-path
projected:
sources:
- serviceAccountToken:
path: token
expirationSeconds: 7200
audience: "https://localhost:5001/connect/token"
The most important parts of the token, which the workload will
obtain, looks as follows:
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{
"aud": [
"https://localhost:5001/connect/token"
],
"exp": 1691752299,
"iss": "https://kubernetes.default.svc.cluster.local",
"sub": "system:serviceaccount:test:default"
}
A complete token example obtained by the workload is shown below.
{
"aud": [
"https://localhost:5001/connect/token"
],
"exp": 1691752299,
"iat": 1691745099,
"iss": "https://kubernetes.default.svc.cluster.local",
"kubernetes.io": {
"namespace": "test",
"pod": {
"name": "simpleapp-5d7dcf96df-n7csk",
"uid": "9fc443d7-5c7a-48d5-9679-0ee03b17d4c5"
},
"serviceaccount": {
"name": "default",
"uid": "0bea3006-fb60-49a3-bc80-7e6884d378ae"
}
},
"nbf": 1691745099,
"sub": "system:serviceaccount:test:default"
}
To enable the authorization server to use the Service Account Token
for client authentication the following configuration is needed:
1. the client id is set to system:serviceaccount:test:default. In
our case we are using the default service account in the test
namespace.
2. the public key of the token signing key. This can be either
configured manually, or dynamically by referencing the JWK
endpoint Kubernetes exposes, which is
https://kubernetes.default.svc.cluster.local/openid/v1/jwks
Note: Authorization servers that follow the OpenID Connect Core
specification, which profiles RFC 7523, will unfortunately run into
problem. Here is the why.
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For JWT-based client authentication [OIDC] specifies the following:
1. The 'jti' claim is mandated for client authentication.
2. The 'iss' claim must match the 'sub' claim. Since Kubernetes
issues the tokens, and not the workload, the two do not match.
[RFC7523], on the other hand, does not mandate the use of a 'jti'
claim and does not mandate that the 'iss' claim equals the 'sub'
claim.
Authors' Addresses
Benedikt Hofmann
Siemens
Email: hofmann.benedikt@siemens.com
Hannes Tschofenig
Siemens
Email: hannes.tschofenig@siemens.com
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