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AWS Security Token Service (STS) is a managed web service that issues temporary, limited-privilege credentials for IAM users, IAM roles, or federated identities. By leveraging STS, you can enforce the principle of least privilege, avoid long-term credentials, and securely grant short-term access to AWS resources.

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Use Case: External Application Access to Amazon S3

Consider an application running on-premises in your corporate data center. To retrieve objects from an S3 bucket without embedding long-term AWS keys, you can integrate STS with your identity provider (IdP) and SAML federation.

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Step 1: Authenticate with Your Identity Provider

  1. The client application prompts the user for corporate credentials.
  2. These credentials are sent to an external LDAP-based IdP for verification.
  3. Upon successful login, the IdP issues a SAML assertion to the client.
SAML federation lets you use existing corporate credentials for AWS access, reducing password sprawl and improving security posture.

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Step 2: Call AssumeRoleWithSAML to Obtain Temporary Credentials

With the SAML assertion in hand, the application calls the STS endpoint: 
aws sts assume-role-with-saml \
  --role-arn arn:aws:iam::123456789012:role/S3AccessRole \
  --principal-arn arn:aws:iam::123456789012:saml-provider/CorpIdP \
  --saml-assertion file://assertion-response.xml
STS validates the SAML assertion, then returns these temporary credentials:
CredentialDescription
Access Key IDUnique identifier for the session
Secret Access KeySecret used to sign AWS API requests
Session TokenToken that authorizes API calls for the session
These credentials inherit the permissions defined in the assumed role’s policy and expire automatically (up to 12 hours).
The image is a flowchart illustrating an STS (Security Token Service) example, showing the interaction between a client app, a portal identity provider (IDP), and AWS as the service provider. It details the authentication process and the exchange of SAML assertions and temporary security credentials.

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Step 3: Use the Temporary Credentials to Access S3

Export the returned credentials into your environment: 
export AWS_ACCESS_KEY_ID=ASIAXXXXXXXXXXXXXXXX
export AWS_SECRET_ACCESS_KEY=XXXXXXXXXXXXXXXXXXXXXXXXXXXX
export AWS_SESSION_TOKEN=XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
Now you can run S3 operations with least-privilege access: 
aws s3 ls s3://your-bucket-name/path/
Temporary credentials automatically expire after the duration specified in the role trust policy (maximum 12 hours). Always handle session renewal and error retries in your application.

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References

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