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🔐 Use the AWS CLI Without Access Keys, and Prove a Policy Before You Attach It (Hands-on)

#aws #beginners #security #tutorial

🎤 The interview question

"How do you use the AWS CLI without an access key, and how do you know a policy works before you attach it?"

The strong answer is two things you have actually run: sign the CLI in with aws login, and prove the policy in the IAM policy simulator first. 20 minutes. 👇

👉 Flow: Sign in without a key → Write the policy → Prove it in the simulator → Store it, look, delete

🧰 Before you start

  • 💻 AWS CLI v2, version 2.32.0 or later (aws login needs it). Check with aws --version.
  • 👤 Signed in to the console as your everyday admin IAM user (not root). It needs the SignInLocalDevelopmentAccess managed policy or broader, e.g. AdministratorAccess.
  • 🔑 Permissions used: iam:SimulateCustomPolicy, iam:CreatePolicy, iam:GetPolicyVersion, iam:DeletePolicy.
  • 🌍 Region: ap-south-1, or your usual one.
  • 💵 Cost: $0. IAM and STS are free, and the simulator sends no real request to any service. No Free plan credits used.

🔐 Step 1: Sign the CLI in without a key

Why: an access key works until someone deletes it; one pushed to GitHub is a stolen account. aws login reuses your console sign-in and gives the CLI temporary credentials (refreshed every 15 minutes, 12 hours max).

aws --version
aws login
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At AWS Region [us-east-1]: type ap-south-1. In the browser, pick your user, then return.

✅ Expected: aws-cli/2.32.0 or newer, then Updated profile default to use arn:aws:....

Now ask "who am I?" (it needs no permissions, so run it first whenever something is denied):

aws sts get-caller-identity
aws configure list
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✅ Expected: an Arn ending in your user name, and login in the TYPE column.

⚠️ If aws login is rejected as an unknown command → your CLI is older than 2.32.0. Update: curl -fsSL https://awscli.amazonaws.com/v2/install.sh | bash. If you see an old user or ExpiredToken → an access key in ~/.aws/credentials wins. Delete it and log in again.

📝 Step 2: Write a read-only policy

Why: interviewers listen for "least privilege": list one bucket, read its objects, nothing else. The bucket needn't exist yet.

BUCKET=hoc-site-ar-4821
mkdir -p ~/hoc-unit2 && cd ~/hoc-unit2
cat > hoc-read-policy.json << EOF
{
  "Version": "2012-10-17",
  "Statement": [
    {"Sid": "ListTheBucket", "Effect": "Allow", "Action": "s3:ListBucket", "Resource": "arn:aws:s3:::${BUCKET}"},
    {"Sid": "ReadItsObjects", "Effect": "Allow", "Action": "s3:GetObject", "Resource": "arn:aws:s3:::${BUCKET}/*"}
  ]
}
EOF
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👀 Note the two ARNs: ListBucket acts on the bucket, GetObject on the objects (/*).

🧪 Step 3: Prove it in the simulator

Why: the simulator answers "would this be allowed?" without touching anything or saving the policy. A helper makes each test one line:

sim() { aws iam simulate-custom-policy --policy-input-list "$(cat "$1")" \
  --action-names "$2" --resource-arns "$3" \
  --query 'EvaluationResults[0].EvalDecision' --output text; }

sim hoc-read-policy.json s3:ListBucket arn:aws:s3:::$BUCKET
sim hoc-read-policy.json s3:GetObject  arn:aws:s3:::$BUCKET/index.html
sim hoc-read-policy.json s3:PutObject  arn:aws:s3:::$BUCKET/index.html
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✅ Expected: allowed, allowed, implicitDeny (the only other answer is explicitDeny).

📚 Why implicitDeny? The full chapter explains how AWS decides, with an eight-request test list for this policy.

🧨 Break it on purpose

Put ListBucket on the object ARN, a classic beginner bug:

sed "s#\"arn:aws:s3:::${BUCKET}\"}#\"arn:aws:s3:::${BUCKET}/*\"}#" hoc-read-policy.json > broken.json
sim broken.json s3:ListBucket arn:aws:s3:::$BUCKET
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✅ Expected: implicitDeny. Wrong resource ARN, so listing fails, and you caught it before a real user did.

📦 Step 4: Store it, look at it, delete it

Why: the clean-up habit. Created, never attached, deleted.

POLICY_ARN=$(aws iam create-policy --policy-name hoc-read-site-bucket \
  --policy-document "$(cat hoc-read-policy.json)" \
  --tags Key=project,Value=hands-on-cloud \
  --query Policy.Arn --output text)
echo "$POLICY_ARN"
aws iam get-policy-version --policy-arn "$POLICY_ARN" --version-id v1 \
  --query 'PolicyVersion.Document.Statement[].Sid'
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✅ Expected: arn:aws:iam::<your account ID>:policy/hoc-read-site-bucket, then ["ListTheBucket", "ReadItsObjects"].

🧹 Cleanup 

aws iam delete-policy --policy-arn "$POLICY_ARN"
rm -f ~/hoc-unit2/broken.json
aws logout
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✅ delete-policy prints nothing. After aws logout, get-caller-identity fails: no credentials left. 🎉

💬 Say it in the interview

"I never create access keys for my own CLI: aws login gives it temporary credentials that expire on their own. Before a policy goes anywhere, I run the requests that must be allowed and must be denied through the IAM policy simulator, and fix it until they match."

🎯 What you can now say

✅ "My CLI signs in with aws login, not a key"
✅ "First command when denied: aws sts get-caller-identity"
✅ "ListBucket needs the bucket ARN, GetObject the object ARN"
✅ "I prove a policy in the simulator before I attach it"

📚 Go deeper

This is the hands-on cut of Session 2 of our free Hands-on Cloud for Freshers course: the three rules AWS decides by, and the model answer.

👉 Read the full chapter on TechEazy Consulting

🚀 Real projects and a real internship certificate get you hired. Start at TechEazy Consulting.

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