AWS CloudFront Error: 403, 404, and 502 — What CloudFront Is Really Telling You

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AWS CloudFront Error: 403, 404, and 502 — What CloudFront Is Really Telling You

#aws#cloudfront#devops#cloud

How CloudFront requests work correctly at the origin but fail at the edge due to permission boundaries, path resolution mismatches, and origin communication failures





Problem

Requests routed through Amazon CloudFront return 403, 404, or 502 errors, even though the same resource may work when accessed directly at the origin. The error responses are brief and inconsistent, often leading to repeated configuration changes without resolving the root cause.

Clarifying the Issue

CloudFront is a layered system.

When a request fails, CloudFront returns an error on behalf of the layer that failed, not necessarily the component you were inspecting.

These error codes are not application messages. They are infrastructure signals.

Correct interpretation depends on identifying which layer rejected or failed the request:

  • The edge
  • The origin
  • The trust or execution boundary between them

Why It Matters

CloudFront sits directly between users and your backend. When it is misconfigured:

  • Fixes appear not to work
  • Cached failures persist
  • IAM or routing edits feel random
  • Time is lost invalidating, redeploying, and guessing

Understanding where the failure occurs prevents trial-and-error debugging and makes CloudFront behavior predictable.

Key Terms

  • Edge – CloudFront locations that receive viewer requests
  • Origin – The backend CloudFront forwards requests to (S3, ALB, EC2, etc.)
  • 403 (Forbidden) – Request was received and explicitly rejected
  • 404 (Not Found) – Requested path does not exist at the origin
  • 502 (Bad Gateway) – CloudFront could not successfully communicate with the origin
  • Trust Boundary – The permission or execution boundary between CloudFront and the origin

Steps at a Glance

  1. Identify the CloudFront error code
  2. Map the error to the failing layer
  3. Test the origin directly
  4. Compare direct access vs CloudFront access
  5. Fix the issue at the correct layer

Detailed Steps

Step 1: Diagnose a 403 Error (Permission or Trust Failure)

A 403 means the request reached a layer that explicitly rejected it.

This rejection may occur at the edge or at the origin.

Typical causes:

  • S3 bucket policy does not allow CloudFront
  • AWS WAF rules or geo-restrictions blocking the request at the edge
  • OAI or OAC configured incorrectly
  • Required headers or HTTP methods not permitted
  • Signed URLs or cookies expected but missing
  • S3 returning 403 instead of 404 for missing objects due to permission structure

Actions:

  • Access the origin directly, bypassing CloudFront
  • If direct access works, inspect origin policies and WAF rules
  • Confirm CloudFront is explicitly trusted
  • Check CloudFront logs or reproduce the request path using curl

Step 2: Diagnose a 404 Error (Path or Routing Failure)

A 404 means CloudFront requested a path that does not exist at the origin.

This is usually a routing or mapping issue, not a missing deployment.

Typical causes:

  • Incorrect default root object
  • Single Page Application (SPA) routing without an index.html fallback
  • Misconfigured origin path
  • Assumed path rewrites that do not exist
  • Cached 404 responses

Actions:

  • Review the origin path configuration
  • Compare requested paths at the edge vs the origin
  • Request a known-good object explicitly
  • Invalidate cached error responses after fixing the root cause

Step 3: Diagnose a 502 Error (Connectivity or Execution Failure)

A 502 means CloudFront could not complete a successful request to the origin.

This is not an application-level error.

Typical causes:

  • Origin is unavailable or timing out
  • TLS certificate domain name mismatch at the origin
  • Listener or protocol misconfiguration
  • Unsupported HTTP versions
  • Lambda@Edge or CloudFront Function runtime errors

Actions:

  • Verify origin availability and response times
  • Confirm TLS certificate matches the origin domain
  • Check listener ports and protocols
  • Review CloudFront and Lambda@Edge logs for execution failures

Pro Tips

  • Always test the origin first, then CloudFront
  • Change one layer at a time, not multiple
  • A working origin plus a failing CloudFront almost always indicates a trust, routing, or execution issue
  • Treat CloudFront errors as layer diagnostics, not bugs
  • Avoid invalidations until the underlying problem is fixed

Conclusion

CloudFront error messages are concise because they assume familiarity with the system:

  • 403 indicates a permission, trust, or edge-level rejection
  • 404 indicates a routing or path mismatch
  • 502 indicates a connectivity or execution failure

When you diagnose by layer instead of guessing, CloudFront becomes predictable — and predictable infrastructure is the goal.

Aaron Rose is a software engineer and technology writer at tech-reader.blog and the author of Think Like a Genius.

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