AWS 403 proxy versus origin split response

When this happens

Use this when a cdn, load balancer, reverse proxy, or waf sits in front of the service. Do not stop at the screen message; validate whether the edge/proxy or the origin server creates the failure first.

Symptom checklist

• 403 appears repeatedly in the AWS UI or logs.
• CloudTrail, IAM policy, region, VPC, target health differs between successful and failed requests.
• The issue appears only after separating AWS API failure from application failure.
• It often follows deploys, permission changes, configuration edits, or data refreshes.

Likely causes

• 403 specifically changes the investigation surface for AWS: verify the exact failing object, route, user, and timestamp before applying the broader pattern.
• The actor lacks the permission required by the target object.
• Ownership or group membership changed after the data was created.
• A key, signature, role, or inherited policy is evaluated differently than expected.
• A deny policy or WAF rule overrides a later allow rule.
• The application reports a generic error while the authorization layer has the real reason.
• For the proxy versus origin split case, the first useful clue is whether the edge/proxy or the origin server creates the failure.

First 1-minute checks

1. Write down the first failure time, latest change, affected user, path, and object ID.
2. Compare CloudTrail, IAM policy, region, VPC, target health for success and failure in the same window.
3. Test the hypothesis: authorization failure caused by missing role, stale owner, key mismatch, inherited deny, WAF policy, or object-level permission.
4. Classify this as proxy versus origin split: whether the edge/proxy or the origin server creates the failure.
5. Capture current values before changing configuration.

First evidence

Treat 403 as a proxy versus origin split case. First collect evidence for whether the edge/proxy or the origin server creates the failure.

Output examples

Normal output

Direct origin and routed proxy return equivalent status and headers.

Failing output

Origin succeeds but proxy fails, or proxy succeeds while direct origin exposes the real backend error.

Output-to-action branches

• A CDN, load balancer, reverse proxy, or WAF sits in front of the service.
  Test direct origin and routed traffic separately, then change only the layer that differs.
• The working and failing outputs differ.
  Act on the differing layer first: For 403, apply the fix only after reproducing the same condition and saving the before/after evidence for this exact code.
• Command output is normal but users still fail.
  Separate browser cache, cookies, permissions, and network location before declaring it fixed.

Do not do this

• Do not assume the CDN error code is the application root cause.
• Do not change multiple layers before identifying the failing layer.
• Do not delete production data, grant broad permissions, or disable security controls as a first response.

Evidence quality

Auto-generated operator draft: includes issue-specific causes, commands, output branches, and unsafe-action warnings. Official-source links and real incident validation are queued for enrichment.

Commands to run first

grep -R "403" ./logs

aws sts get-caller-identity

aws cloudtrail lookup-events --max-results 5

aws elbv2 describe-target-health --target-group-arn ARN

aws iam simulate-principal-policy --policy-source-arn ARN --action-names ACTION

grep -R "permission\|denied\|forbidden\|AccessDenied\|403" ./logs

whoami

ls -la

ssh -v [email protected]

curl -Iv https://example.com --resolve example.com:443:ORIGIN_IP

Fix order

1. Record the full 403 message, failing URL, user, object ID, and latest change.
2. Collect issue-specific evidence for authorization failure caused by missing role, stale owner, key mismatch, inherited deny, WAF policy, or object-level permission.
3. Compare the failing case with a successful case before editing settings.
4. If this is the proxy versus origin split branch, Test direct origin and routed traffic separately, then change only the layer that differs.
5. Re-check with the same command and URL, then record the normal output.

Actions by cause

• For 403, apply the fix only after reproducing the same condition and saving the before/after evidence for this exact code.
• Identify the actor, target object, action, and policy that denied the request.
• Restore ownership or grant the smallest required permission.
• Refresh keys, signatures, sessions, or role assumptions.
• Remove conflicting deny rules before adding broad allow rules.
• Verify using the affected account, not an administrator session.
• For the proxy versus origin split branch, Test direct origin and routed traffic separately, then change only the layer that differs.

Evidence links

• AWS Elastic Load Balancing troubleshooting official
• AWS CloudFront troubleshooting official

Verification metadata

• operator-draft
• official-reference-linked
• 2026-07-23

Update queue

• Review cadence
  weekly-source-review
• Next enrichment
  Add one official-source check and one real output example for AWS 403.

Environment-specific checks

• Shared hosting, proxies, VPNs, or CDN layers can change AWS API failure from application failure results.
• Do not trust only the AWS UI; compare command output.
• Japanese hosting panels may show completion before DNS or SSL fully propagates.
• Test from both office and external networks.

Prevent it next time

• Store normal examples for CloudTrail, IAM policy, region, VPC, target health.
• Add IAM least privilege, region match, security groups, target groups to the release checklist.
• Keep recurring errors in the same note format.
• Split alerts by error rate, latency, certificates, disk, and permission changes.

Related notes

• AWS ALB 502 target response error
• S3 AccessDenied fix note
• CloudFront 403 fix note
• EC2 Connection refused fix note
• S3 AccessDenied fix note 2
• CloudFront 403 fix note 2
• EC2 Connection refused fix note 2
• S3 AccessDenied fix note 3