What Actually Happens During an STS AssumeRole Call

~8 min read

The concrete request/response mechanics behind temporary credential vending, and where it happens invisibly.

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Key points

  • AssumeRole requires the caller to have permission via the target role's trust policy
  • Returns temporary access key ID, secret key, and session token, valid only for a set duration
  • The assumed role's permission policy determines what the temporary credentials can actually do
  • EC2 instance profiles, Lambda execution roles, and cross-account access all use this mechanism invisibly