S3 Bucket Misconfigurations: How to Find Cloud Storage Bugs

S3 Bucket Misconfigurations: How to Find Cloud Storage Bugs

Object storage like Amazon S3 (and equivalents on other clouds) powers uploads, backups, and static assets. When buckets are left public, listable, or writable, hunters can read private files or even overwrite site content. These bugs are common because misconfiguration is easy and defaults change over time.

Only test buckets that belong to your in-scope target, and prove impact with markers — not real customer data.

Find buckets tied to the target

Recon sources:

• URLs and asset references in the app and JavaScript (image/CDN hosts)
• Bucket names derived from the company/app name and environment suffixes
• Content-Security-Policy and CORS entries pointing at storage origins
• Public code, mobile apps, and error messages that leak bucket names

Keep a list of candidate buckets with the asset that revealed each one.

Test permissions safely

For each candidate, check what anonymous access allows:

1. Listing: can you enumerate objects? A public listing that reveals private file names is already a finding.
2. Read: can you fetch a non-public object directly by key?
3. Write: can you upload a harmless marker object (for example bugflare-poc.txt with your report ID)?
4. Overwrite: can an existing asset be replaced? Prove this concept without actually defacing production — a writable test key is enough.

Always upload only benign markers you can later point to, and request cleanup in your report.

Impact ladder

• Public listing of sensitive object names → information disclosure
• Readable private files (backups, PII, credentials) → high severity
• Writable bucket serving site assets → potential stored XSS or defacement
• Exposed credentials inside objects → possible account/cloud compromise

Match severity to what the access actually exposes.

Prove it without harm

• Read: fetch one object that is clearly private and screenshot the metadata, not the sensitive contents in bulk.
• Write: upload a uniquely named marker and show it is retrievable.
• Never delete, mass-download, or alter real assets.

Common false positives

• Buckets intended to be public (static assets, marketing images)
• “Access denied” that looks like a bug but is correct
• Third-party buckets not owned by the target (out of scope)

Report structure

Include the bucket name, how you tied it to the target, the exact permission (list/read/write), the marker object or screenshot, and the data at risk. Recommend blocking public access, tightening bucket policies and ACLs, enabling account-level public-access blocks, and rotating any exposed secrets.

Defensive checklist

• Enable account-wide “block public access” and review exceptions.
• Use least-privilege bucket policies; deny anonymous list/read/write by default.
• Separate public asset buckets from private data buckets.
• Scan for secrets stored in objects and rotate any found.
• Monitor for unexpected public buckets in CI/CD.

Cloud storage bugs reward careful recon and disciplined proof. Enumerate candidate buckets, test permissions with markers, and report the exact access — high impact with zero harm to real data.

Original Bugflare guide informed by OWASP configuration and deployment testing guidance.