GitHub OIDC Sub Claim Format Change Breaks AWS Deployments for New Repos

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A developer discovered that GitHub Actions deployments to AWS S3 were silently failing due to a mismatch in the OIDC token's subject claim format. GitHub introduced a new sub claim format in April 2026, embedding numeric owner and repo IDs into the string, replacing the older name-only format used in IAM trust policies. Repositories created after July 15, 2026 receive this new format by default, causing AWS STS to reject role assumption requests when trust policies still reference the old pattern. The change was designed to prevent trust policy hijacking, where an abandoned repo name could be reclaimed by another user to spoof the original subject claim. The fix requires updating the IAM trust policy condition in Terraform to match the new format that includes the numeric GitHub owner and repository IDs.

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