Threat/Impact Level: HIGH | Downtime Risk: HIGH | Time to Fix: 5–15 mins

TL;DR

• What broke: Your EKS pod's service account cannot assume its IAM role because AWS STS rejected the OIDC token — the OIDC provider in IAM is either missing its TLS thumbprint, has a stale one after an AWS-side certificate rotation, or the issuer URL has a trailing slash mismatch.
• How to fix it: Re-derive the OIDC provider thumbprint from the live endpoint, update the IAM OIDC provider resource, and verify the trust policy StringEquals condition matches the exact service account ARN format.
• Fast path: Use our Client-Side Sandbox above to auto-refactor your trust policy and OIDC provider Terraform block without leaking your ARNs.

The Incident (What does the error mean?)

Your pod logs or kubectl describe pod surfaces this:

An error occurred (AccessDenied) when calling the AssumeRoleWithWebIdentity operation:
Not authorized to perform sts:AssumeRoleWithWebIdentity

Or from the AWS SDK inside the container:

botocore.exceptions.ClientError: An error occurred (AccessDenied) when calling the 
AssumeRoleWithWebIdentity operation: Not authorized to perform sts:AssumeRoleWithWebIdentity

Immediate consequence: Every pod using this service account loses all AWS API access. If this is your logging, secrets, or storage workload — it is down. STS is not retrying. The token is being presented, but the OIDC provider record in IAM cannot be validated because the thumbprint chain is broken or absent.

The Attack Vector / Blast Radius

This is not just an ops inconvenience. The OIDC thumbprint is the cryptographic anchor that tells AWS IAM: "trust JWTs signed by this OIDC issuer." When it's wrong or missing:

1. All pods bound to any service account using this OIDC provider lose IAM access simultaneously — not just one pod, the entire provider is untrusted.
2. A stale thumbprint (AWS rotated the EKS OIDC endpoint certificate — this has happened in production) causes a silent, total IRSA failure across your cluster with no pre-warning.
3. If you "fix" this by broadening the trust policy to remove the StringEquals condition on sub (the service account), you've created a privilege escalation vector: any pod in the cluster that can obtain an OIDC token can now assume the role.
4. Misconfigured thumbprints are frequently the result of copy-pasting an OIDC provider ARN from a different cluster/region — the issuer URL looks identical but the thumbprint is environment-specific.

How to Fix It (The Solution)

Step 1: Re-derive the live thumbprint

Do not guess or copy from docs. Derive it from the live endpoint:

# Get your cluster's OIDC issuer URL
aws eks describe-cluster --name <cluster-name> --query "cluster.identity.oidc.issuer" --output text
# Example output: https://oidc.eks.us-east-1.amazonaws.com/id/EXAMPLED539D4633E53DE1B71EXAMPLE

# Extract the thumbprint (last cert in chain, SHA-1 fingerprint)
OIDC_URL=$(aws eks describe-cluster --name <cluster-name> \
  --query "cluster.identity.oidc.issuer" --output text | sed 's|https://||')

openssl s_client -servername $OIDC_URL -showcerts \
  -connect ${OIDC_URL}:443 </dev/null 2>/dev/null \
  | awk '/-----BEGIN CERTIFICATE-----/{c++} c==2{print}' \
  | openssl x509 -fingerprint -noout -sha1 \
  | sed 's/://g' | awk -F= '{print tolower($2)}'

⚠️ You must target the second-to-last (root CA) certificate in the chain, not the leaf. The c==2 awk filter handles this. AWS docs confirm SHA-1 of the root CA is what IAM validates.

Basic Fix: Update the OIDC Provider thumbprint via CLI

aws iam update-open-id-connect-provider-thumbprint \
  --open-id-connect-provider-arn arn:aws:iam::<account-id>:oidc-provider/oidc.eks.<region>.amazonaws.com/id/<oidc-id> \
  --thumbprint-list <new-thumbprint-hex>

Enterprise Best Practice: Terraform with locked thumbprint + least-privilege trust policy

- resource "aws_iam_openid_connect_provider" "eks" {
-   url = data.aws_eks_cluster.main.identity[0].oidc[0].issuer
-   client_id_list  = ["sts.amazonaws.com"]
-   # thumbprint_list omitted — THIS IS THE BUG
- }

+ resource "aws_iam_openid_connect_provider" "eks" {
+   url             = data.aws_eks_cluster.main.identity[0].oidc[0].issuer
+   client_id_list  = ["sts.amazonaws.com"]
+   thumbprint_list = [data.tls_certificate.eks_oidc.certificates[0].sha1_fingerprint]
+ }
+
+ data "tls_certificate" "eks_oidc" {
+   url = data.aws_eks_cluster.main.identity[0].oidc[0].issuer
+ }

And the trust policy — never omit the sub condition:

  {
    "Effect": "Allow",
    "Principal": {
      "Federated": "arn:aws:iam::<account>:oidc-provider/oidc.eks.<region>.amazonaws.com/id/<id>"
    },
    "Action": "sts:AssumeRoleWithWebIdentity",
    "Condition": {
      "StringEquals": {
-       "oidc.eks.<region>.amazonaws.com/id/<id>:aud": "sts.amazonaws.com"
-       // Missing sub condition — ANY pod can assume this role
+       "oidc.eks.<region>.amazonaws.com/id/<id>:aud": "sts.amazonaws.com",
+       "oidc.eks.<region>.amazonaws.com/id/<id>:sub": "system:serviceaccount:<namespace>:<serviceaccount-name>"
      }
    }
  }

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Prevention in CI/CD

1. Checkov — catch missing thumbprint at plan time

Checkov rule CKV_AWS_79 flags OIDC providers with empty thumbprint_list. Add to your pipeline:

checkov -d . --check CKV_AWS_79 --framework terraform

2. OPA/Conftest policy — enforce `sub` condition in trust policies

# policy/irsa_trust.rego
package irsa

deny[msg] {
  doc := input.resource_changes[_]
  doc.type == "aws_iam_role"
  assume := doc.change.after.assume_role_policy
  policy := json.unmarshal(assume)
  stmt := policy.Statement[_]
  stmt.Action == "sts:AssumeRoleWithWebIdentity"
  not _has_sub_condition(stmt)
  msg := sprintf("Role %v is missing sub condition on IRSA trust policy", [doc.address])
}

_has_sub_condition(stmt) {
  keys := object.keys(stmt.Condition.StringEquals)
  endswith(keys[_], ":sub")
}

3. Renovate / scheduled Terraform drift detection

The EKS OIDC endpoint certificate can rotate without notice. Run terraform plan on a schedule (daily) in your CI. A thumbprint drift will surface as a planned change before it causes an outage:

# .github/workflows/drift-detect.yml
schedule:
  - cron: '0 6 * * *'
jobs:
  drift:
    steps:
      - run: terraform plan -detailed-exitcode

Exitcode 2 = drift detected. Alert on it.