What is ServiceAccount? | DevOps Dictionary

ServiceAccount — Giving Pods Their Own Identity

Why Pods Need an Identity

Your application pods often need to talk to the Kubernetes API — to discover other services, watch ConfigMaps for config reloads, or scale other workloads. The cluster needs to know: who is this pod, and what is it allowed to do?

ServiceAccounts are the answer. Think of them as IAM roles for pods.

◈ DIAGRAM

+------------------------------------------+
| Pod (running your app)                   | <- "I am serviceaccount:
|                                          |    payment-processor"
+------------------------------------------+
                    |
                    v
+------------------------------------------+
| Kubernetes API Server                    | <- Receives request with
|                                          |    Bearer token from pod
+------------------------------------------+
                    |
                    v
+------------------------------------------+
| RBAC Authorization Check                 | <- Does payment-processor
|                                          |    have GET on secrets?
+------------------------------------------+
            |               |
            v               v
    +------------+    +------------+
    | ALLOWED    |    | DENIED     |
    | 200 OK     |    | 403 Error  |
    +------------+    +------------+

The Default ServiceAccount Problem

Every namespace gets a default ServiceAccount automatically. If you don't set serviceAccountName in your pod spec, your pods use this default SA.

◈ DIAGRAM

+------------------------------------------+
| Namespace: payments-prod                 |
|                                          |
|  default ServiceAccount (auto-created)   | <- ALL pods use this unless
|                                          |    you specify otherwise
|  payments-api pod  ──> default SA        |
|  fraud-checker pod ──> default SA        | <- Both pods share the same
|  batch-job pod     ──> default SA        |    identity and permissions
+------------------------------------------+

COMMON MISTAKE / WARNING
**Security:** In older clusters, the `default` SA often has broad permissions inherited from cluster-admin bindings added during setup. In production, always create dedicated ServiceAccounts with the minimum permissions each workload actually needs — never rely on the default SA.

Creating a Scoped ServiceAccount — Real Example

This example sets up a ServiceAccount for a Prometheus pod that needs to scrape metrics endpoints across the cluster:

YAML

# 1. Create the ServiceAccount
apiVersion: v1
kind: ServiceAccount
metadata:
  name: prometheus-scraper
  namespace: monitoring
---
# 2. Create a ClusterRole with exactly the permissions needed
apiVersion: rbac.authorization.k8s.io/v1
kind: ClusterRole
metadata:
  name: prometheus-metrics-reader
rules:
  - apiGroups: [""]
    resources: ["nodes", "pods", "services", "endpoints"]
    verbs: ["get", "list", "watch"]   # Read-only — cannot create or delete
  - apiGroups: [""]
    resources: ["nodes/metrics"]
    verbs: ["get"]
---
# 3. Bind the ClusterRole to the ServiceAccount
apiVersion: rbac.authorization.k8s.io/v1
kind: ClusterRoleBinding
metadata:
  name: prometheus-scraper-binding
subjects:
  - kind: ServiceAccount
    name: prometheus-scraper
    namespace: monitoring
roleRef:
  kind: ClusterRole
  name: prometheus-metrics-reader
  apiGroup: rbac.authorization.k8s.io
---
# 4. Assign the ServiceAccount to the pod
apiVersion: apps/v1
kind: Deployment
metadata:
  name: prometheus
  namespace: monitoring
spec:
  template:
    spec:
      serviceAccountName: prometheus-scraper  # <- This is what links pod to SA
      containers:
        - name: prometheus
          image: prom/prometheus:v2.48.0

How the Token Gets Into the Pod

Kubernetes automatically mounts the ServiceAccount token into every pod as a projected volume:

◈ DIAGRAM

+------------------------------------------+
| Pod filesystem                           |
|                                          |
| /var/run/secrets/                        |
|   kubernetes.io/                         |
|     serviceaccount/                      |
|       token        <- JWT Bearer token   |
|       ca.crt       <- API server CA cert |
|       namespace    <- Current namespace  |
+------------------------------------------+

Bash

# Read the token from inside a running pod
kubectl exec -it api-server-7d9f8b -n production -- \
  cat /var/run/secrets/kubernetes.io/serviceaccount/token
 
# Use the token to call the Kubernetes API from inside the pod
TOKEN=$(cat /var/run/secrets/kubernetes.io/serviceaccount/token)
curl -k -H "Authorization: Bearer $TOKEN" \
  https://kubernetes.default.svc/api/v1/namespaces/production/pods
 
# Inspect the token's contents (decoded JWT)
kubectl create token prometheus-scraper -n monitoring --duration=1h | \
  cut -d. -f2 | base64 -d 2>/dev/null | jq

Disabling Auto-Mount for Non-API Pods

Most application pods don't need Kubernetes API access at all. Disabling the token mount reduces the attack surface — a compromised pod cannot use the token to probe the API:

YAML

spec:
  serviceAccountName: payments-api-sa
  automountServiceAccountToken: false   # Don't mount the token — app doesn't need it
  containers:
    - name: payments-api
      image: registry.razorpay.in/payments-api:v3.1.2

Troubleshooting Common ServiceAccount Problems

Problem	Symptom	Fix
Pod gets 403 calling the API	`Error: Forbidden` in app logs	SA lacks the required verb — check with `kubectl auth can-i` as the SA
Pod stuck in `Pending`	`ServiceAccount not found` event	SA not created before the pod — create SA first, then deploy
Prometheus scraping fails	`403 Forbidden` on `/metrics`	ClusterRoleBinding is in wrong namespace or references wrong SA name
Token expired in long-running pods	API calls start failing after 24h	Use projected service account tokens with `expirationSeconds: 86400` and enable token rotation
CI bot has too much access	Blast radius concern	Replace cluster-admin SA with a minimal `ci-deployer` Role bound only to `deployments/patch`

PLACEMENT PRO TIP
**Tip:** At Swiggy's scale, every microservice has its own dedicated ServiceAccount with the least-privilege permissions it actually needs. This means a compromised payment pod cannot read secrets from the delivery namespace — the blast radius is contained to one workload.

REMEMBER THIS
**Remember:** `kubectl auth can-i get pods --as=system:serviceaccount:monitoring:prometheus-scraper` is the fastest way to verify what a ServiceAccount is allowed to do without deploying anything. Always test this before going to production.

COMMON MISTAKE / WARNING
**Common Mistake:** Creating a ServiceAccount but forgetting to add `serviceAccountName` to the pod spec. The pod silently falls back to the `default` SA instead of the scoped one — and you won't notice until a permission error surfaces in production.

Quick Reference

Command	Purpose
`kubectl get serviceaccounts -n <ns>`	List all SAs in a namespace
`kubectl describe sa <name> -n <ns>`	See mounted secrets and token references
`kubectl create token <sa-name> -n <ns>`	Generate a short-lived token for testing
`kubectl auth can-i get pods --as=system:serviceaccount:<ns>:<sa>`	Test what an SA is allowed to do
`kubectl get rolebindings -n <ns> -o wide`	See which SAs are bound to which roles