Create short-lived credentials for multiple service accounts

This page explains how to create short-lived credentials by impersonating multiple service accounts. These credentials let you use all of the roles and permissions that are available to each of the service accounts.

If you can access or create a single service account with all of the required roles, you should create short-lived credentials for that service account directly, which is a simpler approach to service account impersonation.

About creating short-lived credentials

Service accounts can use short-lived credentials to authenticate calls to Google Cloud APIs, other Google APIs, and non-Google APIs. Short-lived credentials have a limited lifetime, with durations of just a few hours or shorter. Short-lived service account credentials are useful for scenarios where you need to grant limited access to resources for trusted service accounts. They also create less risk than long-lived credentials, such as service account keys.

Short-lived credentials can be represented as OAuth 2.0 access tokens, OpenID Connect ID tokens, self-signed JSON Web Tokens (JWTs), and self-signed binary objects (blobs). The most commonly used credential types are OAuth 2.0 access tokens and OpenID Connect (OIDC) ID tokens. You might use each type of token in the following scenarios:

  • OAuth 2.0 access token: An OAuth 2.0 access token is useful for authenticating access from a service account to Google Cloud APIs. Consider the following example use case: to get elevated permissions on a project, an administrator can create an OAuth 2.0 access token that belongs to a service account, then use that token to impersonate the service account when calling Google Cloud APIs. The token has a short lifetime so that the elevated permissions are temporary. Using short-lived tokens helps you implement the principle of least privilege across your identities and resources. It can also be useful when there is an emergency in a production environment, and an administrator needs a short-term elevated authorization for debugging.
  • OIDC ID token: An OIDC ID token is useful for authenticating the identity of a service account to services that accept OpenID Connect. Consider the following example use case: by creating an OIDC ID token belonging to a service account, a workload running on Google Cloud can authenticate itself to another workload deployed on a third-party cloud provider, such as a data pipeline job. If the target service is configured with OIDC, then authentication will succeed.

Delegated request flow

The delegated request flow is similar to a direct request flow, except that for a delegated request, the request is delegated to one or more service accounts in a delegation chain. In this flow, multiple service accounts act as intermediaries between the original caller and the service account for whom the credential is created. Each service account in the delegation chain must have the required permissions on the next service account in the chain, so that it can pass along the request.

This flow is useful for scenarios where a project contains tiers of limited- privilege service accounts, each of which has been configured to perform a specific or limited function on certain resources. For example, one service account only has permissions for Cloud Storage resources, another only has permissions for Compute Engine resources, and so on.

If one service account provides all of the permissions you need, you should use the simpler flow described in Create short-lived credentials from a service account.

Before you begin

  • Enable the IAM and Service Account Credentials APIs.

    Enable the APIs

  • Understand IAM service accounts

  • If you haven't already, enable billing and the IAM API by following the steps in the quickstart.

  • Identify the service accounts you will use in your delegation chain.

    You can create a new service account and include it in the delegation chain if needed.

Providing required permissions

A delegated request involves more than two identities: the caller, one or more service accounts in a delegation chain, and finally the service account. In this flow, consider the following identities:

  • Service Account 1 (SA_1), the caller who issues a request for the short-lived credentials.
  • Service Account 2 (SA_2), an intermediary service account that will delegate the initial request to SA_3.
  • Service Account 3 (SA_3), the limited-privilege account for whom the credential is created.

To allow delegation, each account must grant the Service Account Token Creator role (roles/iam.serviceAccountTokenCreator) to the previous account in the chain.

In this particular example, SA_1 must be granted the Service Account Token Creator role (roles/iam.serviceAccountTokenCreator) on SA_2. This is an example of the SA_2 service account being treated as a resource: when you grant the role on SA_2, you update its allow policy the same way that you would update any other resource.

In this example flow, there is only one intermediary service account. To delegate access through more than one service account, you must also assign this role to any other service account in the chain.

Next, SA_2 must also be granted the Service Account Token Creator role (roles/iam.serviceAccountTokenCreator) on SA_3. This allows SA_2 to create short-lived credentials for SA_3.

The following steps use the REST API to grant the roles. However, you can also use the console or the gcloud CLI.

API

First, get the allow policy for SA_2 (the intermediary service account):

The serviceAccounts.getIamPolicy method gets a service account's allow policy.

Before using any of the request data, make the following replacements:

  • PROJECT_ID: Your Google Cloud project ID. Project IDs are alphanumeric strings, like my-project.
  • SA_2: The name of Service Account 2.
  • POLICY_VERSION: The policy version to be returned. Requests should specify the most recent policy version, which is policy version 3. See Specifying a policy version when getting a policy for details.

HTTP method and URL: 

POST https://iam.googleapis.com/v1/projects/PROJECT_ID/serviceAccounts/SA_2@PROJECT_ID.iam.gserviceaccount.com:getIamPolicy

Request JSON body: 

{
  "options": {
    "requestedPolicyVersion": POLICY_VERSION
  }
}

To send your request, expand one of these options:

You should receive a JSON response similar to the following:

{
  "version": 1,
  "etag": "BwWKmjvelug=",
  "bindings": [
    {
      "role": "roles/serviceAccountAdmin",
      "members": [
        "user:admin@example.com"
      ]
    }
  ]
}

If you have not granted a role to the service account, the response contains only an etag value. Include that etag value in the next step.

Next, modify the allow policy to grant SA_1 the Service Account Token Creator role (roles/iam.serviceAccountTokenCreator).

For example, to modify the sample response from the previous step, add the following:

{
  "version": 1,
  "etag": "BwWKmjvelug=",
  "bindings": [
    {
      "role": "roles/serviceAccountAdmin",
      "members": [
        "user:admin@example.com"
      ]
    },
    {
      "role": "roles/iam.serviceAccountTokenCreator",
      "members": [
        "serviceAccount:SA_1@PROJECT_ID.iam.gserviceaccount.com"
      ]
    }
  ]
}

Then, write the updated allow policy for SA_2:

The serviceAccounts.setIamPolicy method sets an updated allow policy for the service account.

Before using any of the request data, make the following replacements:

  • PROJECT_ID: Your Google Cloud project ID. Project IDs are alphanumeric strings, like my-project.
  • SA_2: The name of Service Account 2.

  • POLICY: A JSON representation of the policy that you want to set. For more information about the format of a policy, see the Policy reference.

    For example, to set the allow policy shown in the previous step, replace POLICY with the following:

    {
  "version": 1,
  "etag": "BwWKmjvelug=",
  "bindings": [
    {
      "role": "roles/serviceAccountAdmin",
      "members": [
        "user:admin@example.com"
      ]
    },
    {
      "role": "roles/iam.serviceAccountTokenCreator",
      "members": [
        "serviceAccount:SA_1@PROJECT_ID.iam.gserviceaccount.com"
      ]
    }
  ]
}

HTTP method and URL: 

POST https://iam.googleapis.com/v1/projects/PROJECT_ID/serviceAccounts/SA_2@PROJECT_ID.iam.gserviceaccount.com:setIamPolicy

Request JSON body: 

{
  "policy": POLICY
}

To send your request, expand one of these options:

The response contains the updated allow policy.

Now, get the allow policy for SA_3 (the service account for whom the credential is created):

The serviceAccounts.getIamPolicy method gets a service account's allow policy.

Before using any of the request data, make the following replacements:

  • PROJECT_ID: Your Google Cloud project ID. Project IDs are alphanumeric strings, like my-project.
  • SA_3: The name of Service Account 3.
  • POLICY_VERSION: The policy version to be returned. Requests should specify the most recent policy version, which is policy version 3. See Specifying a policy version when getting a policy for details.

HTTP method and URL: 

POST https://iam.googleapis.com/v1/projects/PROJECT_ID/serviceAccounts/SA_3@PROJECT_ID.iam.gserviceaccount.com:getIamPolicy

Request JSON body: 

{
  "options": {
    "requestedPolicyVersion": POLICY_VERSION
  }
}

To send your request, expand one of these options:

You should receive a JSON response similar to the following:

{
  "version": 1,
  "etag": "BwWKmjvelug=",
  "bindings": [
    {
      "role": "roles/serviceAccountAdmin",
      "members": [
        "user:admin@example.com"
      ]
    }
  ]
}

If you have not assigned a role to the service account, the response contains only an etag value. Include that etag value in the next step.

Next, modify the allow policy to grant SA_2 the Service Account Token Creator role (roles/iam.serviceAccountTokenCreator).

For example, to modify the sample response from the previous step, add the following:

{
  "version": 1,
  "etag": "BwWKmjvelug=",
  "bindings": [
    {
      "role": "roles/serviceAccountAdmin",
      "members": [
        "user:admin@example.com"
      ]
    },
    {
      "role": "roles/iam.serviceAccountTokenCreator",
      "members": [
        "serviceAccount:SA_2@PROJECT_ID.iam.gserviceaccount.com"
      ]
    }
  ]
}

Finally, write the updated allow policy:

The serviceAccounts.setIamPolicy method sets an updated allow policy for the service account.

Before using any of the request data, make the following replacements:

  • PROJECT_ID: Your Google Cloud project ID. Project IDs are alphanumeric strings, like my-project.
  • SA_3: The name of Service Account 3.

  • POLICY: A JSON representation of the policy that you want to set. For more information about the format of a policy, see the Policy reference.

    For example, to set the allow policy shown in the previous step, replace POLICY with the following:

    {
  "version": 1,
  "etag": "BwWKmjvelug=",
  "bindings": [
    {
      "role": "roles/serviceAccountAdmin",
      "members": [
        "user:admin@example.com"
      ]
    },
    {
      "role": "roles/iam.serviceAccountTokenCreator",
      "members": [
        "serviceAccount:SA_2@PROJECT_ID.iam.gserviceaccount.com"
      ]
    }
  ]
}

HTTP method and URL: 

POST https://iam.googleapis.com/v1/projects/PROJECT_ID/serviceAccounts/SA_3@PROJECT_ID.iam.gserviceaccount.com:setIamPolicy

Request JSON body: 

{
  "policy": POLICY
}

To send your request, expand one of these options:

The response contains the updated allow policy.

Requesting short-lived credentials

After you have granted the appropriate roles to each identity, you can request short-lived credentials for the desired service account. The following credential types are supported:

To understand how to specify a delegation chain for these requests, see the Specifying a delegation chain section on this page.

Generating an OAuth 2.0 access token

By default, OAuth 2.0 access tokens are valid for a maximum of 1 hour (3,600 seconds). However, you can extend the maximum lifetime for these tokens to 12 hours (43,200 seconds). To do so, identify the service accounts that need an extended lifetime for tokens, then add these service accounts to an organization policy that includes the constraints/iam.allowServiceAccountCredentialLifetimeExtension list constraint. You can then specify a lifetime up to 43,200 seconds when you create a token for these service accounts.

To generate an OAuth 2.0 access token for a service account, do the following:

API

The Service Account Credentials API's serviceAccounts.generateAccessToken method generates an OAuth 2.0 access token for a service account.

Before using any of the request data, make the following replacements:

  • SA_NAME: The name of the service account that you want to create a token for.
  • PROJECT_ID: Your Google Cloud project ID. Project IDs are alphanumeric strings, like my-project.
  • DELEGATES: If you are using a delegated request flow, see Specifying a delegation chain on this page. If you are using a direct request flow with no delegation, omit the delegates field in the request body.

  • LIFETIME: The amount of time until the access token expires, in seconds. For example, 300s.

    By default, the maximum token lifetime is 1 hour (3,600 seconds). To extend the maximum lifetime for these tokens to 12 hours (43,200 seconds), add the service account to an organization policy that includes the constraints/iam.allowServiceAccountCredentialLifetimeExtension list constraint.

HTTP method and URL: 

POST https://iamcredentials.googleapis.com/v1/projects/-/serviceAccounts/SA_NAME@PROJECT_ID.iam.gserviceaccount.com:generateAccessToken

Request JSON body: 

{
  "delegates": [
    DELEGATES
  ],
  "scope": [
    "https://www.googleapis.com/auth/cloud-platform"
  ],
  "lifetime": "LIFETIME"
}

To send your request, expand one of these options:

If the generateAccessToken request was successful, the response body contains an OAuth 2.0 access token and an expiration time. The accessToken can then be used to authenticate a request on behalf of the service account until the expireTime has been reached: 

{
  "accessToken": "eyJ0eXAi...NiJ9",
  "expireTime": "2020-04-07T15:01:23.045123456Z"
}

Generating OpenID Connect ID tokens

OpenID Connect ID tokens are valid for 1 hour (3,600 seconds). To generate an ID token for a service account, do the following:

API

The Service Account Credentials API's serviceAccounts.generateIdToken method generates an OIDC ID token for a service account.

Before using any of the request data, make the following replacements:

  • PRIV_SA: The email address of the privilege-bearing service account for which the short-lived token is created.
  • AUDIENCE_NAME: The audience for the token, usually the URL of the application or service that the token will be used to access.

HTTP method and URL: 

POST https://iamcredentials.googleapis.com/v1/projects/-/serviceAccounts/PRIV_SA:generateIdToken

Request JSON body: 

{
  "audience": "AUDIENCE_NAME",
  "includeEmail": "true"
}

To send your request, expand one of these options:

If the generateId request was successful, the response body contains an ID token that is valid for 1 hour. The token can then be used to authenticate a request on behalf of the service account: 

{
  "token": "eyJ0eXAi...NiJ9"
}

Creating a self-signed JSON Web Token (JWT)

Self-signed JSON Web Tokens (JWTs) are useful in a variety of scenarios, such as:

  • Authenticating a call to a Google API as described in Google's Authentication Guide.
  • Securely communicating between Google Cloud or non-Google services, such as App Engine applications. In this scenario, one application can sign a token that can be verified by another application for authentication purposes.
  • Treating a service account as an identity provider by signing a JWT that contains arbitrary claims about a user, account, or device.

To generate a self-signed JWT for a service account, do the following:

API

The Service Account Credentials API's serviceAccounts.signJwt method signs a JWT using a service account's system-managed private key.

Before using any of the request data, make the following replacements:

  • SA_NAME: The name of the service account that you want to create a token for.
  • PROJECT_ID: Your Google Cloud project ID. Project IDs are alphanumeric strings, like my-project.
  • DELEGATES: If you are using a delegated request flow, see Specifying a delegation chain on this page. If you are using a direct request flow with no delegation, omit the delegates field in the request body.

  • JWT_PAYLOAD: The JWT payload to sign, which is a JSON object that contains a JWT Claims Set. Include the claims that are necessary for your desired use case and to meet the validation requirements for the service you are calling. If you are calling a Google API, see Google's Authentication Guide for claim requirements.

    The exp (expiration time) claim must be no more than 12 hours in the future. If you are calling a Google API, the exp claim must be set no more than 1 hour in the future.

    The following example payload contains claims to call a Google API, where EXP is an integer timestamp representing the expiration time: 

    { \"iss\": \"SA_NAME@PROJECT_ID.iam.gserviceaccount.com\", \"sub\": \"SA_NAME@PROJECT_ID.iam.gserviceaccount.com\", \"aud\": \"https://firestore.googleapis.com/\", \"iat\": 1529350000, \"exp\": EXP }

HTTP method and URL: 

POST https://iamcredentials.googleapis.com/v1/projects/-/serviceAccounts/SA_NAME@PROJECT_ID.iam.gserviceaccount.com:signJwt

Request JSON body: 

{
  "delegates": [
    DELEGATES
  ],
  "payload": "JWT_PAYLOAD"
}

To send your request, expand one of these options:

If the signJwt request was successful, the response body contains a signed JWT and the signing key ID that was used to sign the JWT. You can use the signedJwt value as a bearer token to directly authenticate a request on behalf of the service account. The token is valid up to the expiration time specified in the request: 

{
  "keyId": "42ba1e...fc0a",
  "signedJwt": "eyJ0eXAi...NiJ9"
}

Creating a self-signed blob

Self-signed blobs are useful in scenarios when you need to securely transmit arbitrary binary data, usually for authentication purposes. For example, if you want to use a custom protocol/token type (not JWT), you can include that data in a signed blob for use by a downstream service.

To generate a self-signed blob for a service account, do the following:

API

The Service Account Credentials API's serviceAccounts.signBlob method signs a blob using a service account's system-managed private key.

Before using any of the request data, make the following replacements:

  • SA_NAME: The name of the service account that you want to create a token for.
  • PROJECT_ID: Your Google Cloud project ID. Project IDs are alphanumeric strings, like my-project.
  • DELEGATES: If you are using a delegated request flow, see Specifying a delegation chain on this page. If you are using a direct request flow with no delegation, omit the delegates field in the request body.
  • BLOB_PAYLOAD: A base64-encoded string of bytes. For example, VGhlIHF1aWNrIGJyb3duIGZveCBqdW1wZWQgb3ZlciB0aGUgbGF6eSBkb2cu.

HTTP method and URL: 

POST https://iamcredentials.googleapis.com/v1/projects/-/serviceAccounts/SA_NAME@PROJECT_ID.iam.gserviceaccount.com:signBlob

Request JSON body: 

{
  "delegates": [
    DELEGATES
  ],
  "payload": "BLOB_PAYLOAD"
}

To send your request, expand one of these options:

If the signBlob request was successful, the response body contains a signed blob and the signing key ID that was used to sign the blob. You can use the signedBlob value as a bearer token to directly authenticate a request on behalf of the service account. The token is valid until the service account's system-managed private key expires. This key's ID is the value of the keyId field in the response. 

{
  "keyId": "42ba1e...fc0a",
  "signedBlob": "eyJ0eXAi...NiJ9"
}

Specifying a delegation chain

When using a delegated request flow to create short-lived service account credentials, the request body for each API must specify the service account delegation chain in the correct order and in the following format:

projects/-/serviceAccounts/SA_ID

Replace SA_ID with either the service account's unique numeric ID or the service account's email address.

For example, in a delegation chain that flows from SA_1 (caller) to SA_2 (delegated) to SA_3 (delegated) to SA_4, the delegates[] field would contain SA_2 and SA_3 in the following order:

{
  "delegates": [
    "projects/-/serviceAccounts/SA_2@PROJECT_ID.iam.gserviceaccount.com",
    "projects/-/serviceAccounts/SA_3@PROJECT_ID.iam.gserviceaccount.com"
  ]
}

The caller and the service account for whom the credential is created are not included in the delegation chain.