Workload Identity in a Multi System Environment (WIMSE) Architecture

3.1.1. Trust Domain

A trust domain is a logical grouping of systems that share a common set of security controls and policies. Workload certificates and tokens are issued under the authority of a trust domain. Trust domains SHOULD be identified by a fully qualified domain name associated with the organization defining the trust domain. THe FQDN format of trust domain helps to ensure uniqueness of the trust domain identifier. A trust domain maps to one or more trust anchors for validating X.509 certificates and a mechanism to securely obtain a JWK Set [RFC7517] for validating WIMSE WIT tokens. This mapping MUST be obtained through a secure mechanism that ensures the authenticity and integrity of the mapping is fresh and not compromised. This secure mechanism is out of scope for this document.¶

A single organization may define multiple trust domains for different purpose such as different departments or environments. Each trust domain must have a unique identifier. Workload identifiers are scoped within a trust domain. If two identifiers differ only by trust domain they still refer to two different entities.¶

3.1.2. Workload Identifier

The WIMSE architecture defines a workload identifier as a URI [RFC3986]. This URI is used in the subject fields in the certificates and tokens defined later in this document. The URI MUST meet the criteria for the URI type of Subject Alternative Name defined in Section 4.2.1.6 of [RFC5280].¶

• The name MUST NOT be a relative URI, and it MUST follow the URI syntax and encoding rules specified in [RFC3986]. The name MUST include both a scheme and a scheme-specific-part.¶

In addition the URI MUST include an authority that identifies the trust domain within which the identifier is scoped. The trust domain SHOULD be a fully qualified domain name belonging to the organization defining the trust domain to help provide uniqueness for the trust domain identifier. The scheme and scheme specific part are not defined by this specification. An example of an identifier format that conforms to this definition is SPIFFE ID.¶

While the URI encoding rules allow host names to be specified as IP addresses, IP addresses MUT NOT be used to represent trust domains except in the case where they are needed for compatibility with existing naming schemes.¶

A workload identity only has meaning within the scope of a specific issuer. Two identities of the same value issued by different issuers may or may not refer to the same workload. In order to avoid collisions identity URIs SHOULD specify, in the URI's "authority" field, the trust domain associated with an issuer that is selected from a global name space such as host domains. However, the validator of an identity credential MUST make sure that they are using the correct issuer credential to verify the identity credential and that the issuer is trusted to issue tokens for the defined trust domain.¶

3.1.3. Workload Identity Credentials

The Agent provisions the identity credentials to the workload. These credentials are represented in form of JWT tokens and/or X.509 certificates.¶

JWT bearer tokens are presented to another party as a proof of identity. They are signed to prevent forgery, however since these credentials are often not bound to other information it is possible that they could be stolen and reused elsewhere. To mitigate these risks and make the token more generally useful the WIMSE architecture defines a workload identity token that binds a JWT to a cryptographic key.¶

Both X.509 certificate and workload identity token credentials consist of two parts:¶

• a certificate or WIT is a signed data structure that contains a public key and identity information¶

• a corresponding private key¶

The certificate or WIT is presented during authentication, however the private key is kept secret and only used in cryptographic computation to prove that the presenter has access to the private key corresponding to the public key.¶

3.2.1. Basic Workload Identity Scenario

The above diagram presents a basic workload application system. The large box represents a trust domain within which the workload application is hosted. Within this example there are three workloads, a gateway, that accepts external clients and a CA/credential service that issues workload identity credentials for the trust domain. External to the workload application system there is an application client that calls APIs on workloads.¶

Here is a brief summary of each component¶

• Trust Domain¶

The large box represents a trust domain of the application that is composed of several workloads. A trust domain may have a more complex internal structure with more workloads, multiple gateways, internal infrastructure, and other services.¶

• Workload¶

Three workloads are shown. Each workload is an instance of running software executing for a specific purpose. Workloads obtain their identity credentials from a Credentials Service (1) and use them to authenticate to other workloads and systems in the process of sending and receiving requests to and from external systems or other internal workloads.¶

• Gateway Service¶

A gateway service typically acts as an intermediary between the internal application trust domain and external systems. The gateway is responsible for ensuring appropriate isolation between external and internal domains. It also routes incoming requests to the correct workload. The gateway may also handle authentication, token exchange, and token transformation.¶

• CA/Credential Service¶

In this diagram the token/Credential service is a service responsible for issuing workload identities to workloads in the same trust domain. The credentials are often X.509 based or JWT based.¶

High level flows within the diagram¶

• (1) Workload Identity Credential Distribution¶

Workloads typically retrieve their workload identity credentials early in their lifecycle from a credentials service associated with their trust domain. The protocol interaction for obtaining credentials varies with deployment and is not detailed here.¶

• (2) Application client Requests¶

Clients send API requests to the application. In the example above, the gateway routes the request to the correct workload. In addition, the gateway may assist in authenticating the incoming request and provide information resulting from the authentication to the target workload. The authentication exchange is not covered in detail in this example. The client request is typically made over HTTPS, but other protocols may be used in some systems. The gateway usually terminates the TLS session so it has visibility into the request in order to route it correctly.¶

• (3) API request to workload 1¶

The gateway is configured to forward requests to the correct workload. The gateway often modifies the request to include specific authentication information about the application client and to remove any information that should not be forwarded internally. The gateway authenticates the workload before forwarding the request. This authentication usually uses TLS. The target workload may authenticate the gateway using TLS or some other means. As part of servicing the request the workload must make a request to another workload in the system. In this scenario the workload is making a request to workload 3 over HTTPS. Workload 1 may be able to authenticate the identity of workload 3 through the TLS protocol to ensure it is making a request of the right party. Workload 3 will authenticate workload 1 using its workload identity credentials. If the workload identity credentials are X.509 certificates then this can happen through TLS client authentication (mutual TLS). Alternatively, the workloads can use a JWT based authentication mechanism to authenticate on another. Workload three can use the authenticated identity of workload 1 to determine which APIs workload 1 is authorized 2 and to associated the authenticated identity with logs and other audit information.¶

• (4) API request to workload 2¶

Similarly to the previous flow, the gateway may determine that for another API call, the application client's request needs to be handled by workload 2. The case behaves the same as the previous flow except that the gateway may need to authenticate workload 2 before forwarding traffic to it. Workload 3 will then authorize and audit the request based on the authenticated identity of workload 2. Workload 2 and workload 1 may be authorized to use different APIs on workload 3. If workload 1 or 2 makes an API request that it is not authorized for, then workload 3 will reject the request.¶

3.2.2. Context and workload Identity

In many cases the application system uses other security context information about the request during authorization and auditing. The following is a basic scenario that illustrates the propagation of security context in the workload system. Some of the components and interactions have been removed from the previous scenario for simplicity.¶

• Context Service This scenario adds a context service component which is responsible for creating security context based on authentication and other calculations. Context can be represented in many ways; it can be a plaintext data structure, a signed data structure such as a jwt or a pointer used to lookup the context as a data structure stored somewhere else. In one common example, creating the context may involve a token exchange converting an OAuth 2.0 access token into a different token format, such as a transaction token, that is understood by internal services.¶

• (1) Initial Authentication In the initial authentication the gateway service obtains credentials it can use with the gateway service. This authentication may involve several steps and may be performed by an external entity such as an identity provider. The authentication process will result in a credential that the gateway service can evaluate. For example, the credential could be an OAuth Access token. If the client already has an access token that it can use to authenticate to the gateway, such as an X.509 certificate, then it may skip this step.¶

• (2) Application Client Request The application client makes a request to the gateway over HTTPS. The client may be authenticated to the gateway through TLS client authentication (mutual TLS) or through a credential such as an access token obtained in step 1.¶

• (3) Establishing the request context The gateway service requests a security context token (c) from a token service. This process may entail sending an access token (a) along with other information to a token exchange endpoint to obtain the context token, which contains information about the entity accessing the system. This context is typically only relevant to the internal system and is not returned to the client. The gateway may use alternative mechanisms to get the internal security context information (c).¶

• (4) Forwarding Request to Workload The gateway forwards the request along with the context information (c) to the appropriate workload. A bearer token, such as an access token (a), is not usually forwarded as it is only meant for external access. The workload uses information in the context token in applying authorization policy to the application client's request. If the workload does not receive a context token, then it will deny requests that rely on information from the token.¶

• (5) Making Additional Workload Originated Requests The workload may need to make requests of other workloads. When making these requests, the workload includes the context information so Workload 2 can authorize and audit the request. Workload 2 may have a policy requiring Workload 1 to authenticate its service identity and provide valid context information (c) to access certain APIs.¶

3.2.3. Cross-Domain Communication

In many applications workloads must make requests of infrastructure that operates as a different trust domain or external services that are in a separate trust domain. Figure Figure 3 shows this scenario. The scenario shows some new components described below. Components and interactions from previous scenarios are still relevant to this example, but are omitted for simplicity.¶

• Token Service - the token service is responsible for exchanging information that is internal to the system such as service identity and/or security context information for a token that can be presented to an external service to gain access to infrastructure or an external service. Note that in some cases the token service may reside in a different trust domain or there may be token services in multiple trust domains. In some cases the workload will need to contact token services in various domains in order to gain access to infrastructure or external service.¶

• Infrastructure Service - this service is often part of the application, but it is managed by an infrastructure provider and may require different information to access it.¶

• External Service - this service is distinct from the application and hosted in a separate trust domain. This trust domain often has different access requirements that workloads in the internal trust domain.¶

Some example interactions in this scenario:¶

• (1) The application client is making requests with authentication information as in the other scenarios¶

• (2) The gateway forwards the request to the appropriate workload with the security context information¶

• (3) The workload needs to access an infrastructure service and, because it is managed by the same organization, it authenticates to the service directly using its workload credentials.¶

• (4) Workload 1 contacts Workload 2 to perform an operation. This request is accompanied by a security context as in the other scenarios.¶

• (5) Workload 2 determines it needs to communicate with an external service. In order to gain access to this service it must first obtain a token/credential (t) that it can use externally. It authenticates to the token service using its workload identity and provides security context information. The token service determines what type of externally usable token to issue to the workload.¶

• (6) Workload 2 uses this new token/credential (t) to access the external service.¶

• Note that in (3) and (6) the workload may need to authenticate to an external token service to obtain an access token to access the system in the foreign trust domain.¶

3.3.1. Bootstrapping Workload Identity

[TODO: this section will need to be updated to discuss workload identifier as a concept as well]¶

A workload needs to obtain its identity early in its lifecycle. This identity is the base identity upon which further identity and security context are built.¶

Identity bootstrapping often utilizes identity information provisioned through mechanisms specific to hosting platforms and orchestration services. This initial bootstrapping information is used to obtain specific identity credentials for a workload. This process may use attestation to ensure the workload receives the correct identity credentials. An example of a bootstrapping process follows.¶

Figure 4 provides an example of software layering at a host running workloads. During startup, workloads bootstrap their identity with the help of an agent. The agent may be associated with one or more workloads to help ensure that workloads are provisioned with the correct identity. The agent provides attestation evidence and other relevant information to a server. The server validates this information and provides the agent with identity credentials for the workloads it is associated with. The server can use a variety of internal and external means to validate the request against policy. After obtaining identity credentials from the Server, the agent passes them to the workload.¶

How the workload obtains its identity credentials and interacts with the agent is subject to different implementations. Some common mechanisms for obtaining this initial identity include:¶

• File System - in this mechanism the identity credential is provisioned to the workload via the filesystem.¶

• Local API - the identity credential is provided through an API, such as a local domain socket (for example SPIFFE or QEMU guest agent) or network API (for example Cloud Provider Metadata Server).¶

• Environment Variables - identity credential may also be injected into workloads using operating system environment variables.¶

3.3.2. Service Authentication

One of the most basic use cases for workload identity is authentication of one workload to another, such as in the case where one service is making a request to another service as part of a larger, more complex application. Following authentication, the request to the service offered by the workload needs to be authorized. Even in this simple case the identity of a workload is often composed of many attributes such as:¶

• Trigger Information¶

• Service Name¶

• Instance Name¶

• Role¶

• Environment¶

• Trust Domain¶

• Software Attestation¶

• Hardware Attestation¶

These attributes are used for various purposes such as:¶

• ensuring the request is made to the correct service or service instance¶

• authorizing access to APIs and resources¶

• providing an audit trail of requests within a system¶

• providing context for other decisions made within a service¶

There are several methods defined to perform this authentication. Some of the most common include:¶

• TLS authentication of the server using X.509 certificates and client bearer token, encoded as JWTs.¶

• Mutual TLS authentication using X.509 certificate for both client and server.¶

• TLS authentication of the server and HTTP request signing using a secret key.¶

Figure 5 illustrates the communication between different workloads. Two aspects are important to highlight: First, there is a need to consider the interaction with workloads that are external to the trust domain (sometimes called cross-domain). Second, the interaction does not only occur between workloads that directly interact with each other but instead may also take place across intermediate workloads (in an end-to-end style).¶

3.3.3. Security Context Establishment and Propagation

In a typical system of workloads additional information is needed in order for the workload to perform its function. For example, it is common for a workload to require information about a user or other entity that originated the request. Other types of information may include information about the hardware or software that the workload is running or information about what processing and validation has already been done to the request. This type of information is part of the security context that the workload uses during authorization, accounting and auditing. This context is propagated and possibly augmented from workload to workload using tokens. Workload identity comes into play to ensure that the information in the context can only be used by an authorized workload and that the context information originated from an authorized workload.¶

3.3.4. Service Authorization

After authentication of the peer, a workload can perform authorization by verifying that the authenticated identity has the appropriate permissions to access the requested resources and perform required actions. This process involves evaluating the security context described previously. The workload validates the security context, and checks the validity of permissions against its security policies to ensure that only authorized actions are allowed.¶

3.3.5. Delegation and Impersonation

When source workloads send authenticated requests to destination workloads, those destination workloads may rely on upstream dependencies to fulfill such requests. Such access patterns are increasingly common in a microservices architecture. While X.509 certificates can be used for point-to-point authentication, such services relying on upstream microservices for answers, may use delegation and/or impersonation semantics as described in RFC 8693 OAuth 2.0 Access Token Exchange.¶

WIMSE credentials constrain the subjects and actors identified in delegation and impersonation tokens to be bound by TrustDomains, and to follow their issuing authorities' trust configurations. Upstream workloads should consider the security context of delegation and/or impersonation tokens within and across Trust Domains, when arriving at authorization decisions.¶

3.3.6. Asynchronous and Batch Requests

Source workloads may send authenticated asynchronous and batch requests to destination workloads. A destination workload may need to fulfill such requests with requests to authorized upstream protected resources and workloads, after the source workload credentials have expired. Credentials identifying the original source workload as subject may need to be obtained from the credential issuing authority with appropriately-downscoped context needed access to upstream workloads. These credentials should identify the workload as the actor in the actor chain, but may also identify other principals that the action is taken on behalf. To mitigate risks associated with long-duration credentials, these credentials should be bound to the workload identity credential such as an X.509 certificate or Workload Identity Token (WIT) of the acting service performing asynchronous computation on the source workload's behalf.¶

3.3.7. Cross-boundary Workload Identity

As workloads often need to communicate across trust boundaries, extra care needs to be taken when it comes to identity communication to ensure scalability and privacy. (TODO: align with OAuth cross domain identity and authorization)¶