hunt-subdomain – DevOps & Infrastructure skill for AI agents

Crown Jewel Targets

Subdomain takeover is high-value because it allows an attacker to serve content from a trusted, company-owned domain — bypassing browser same-origin trust, phishing filters, and user skepticism simultaneously.

Highest payout contexts:

• Subdomains of major SaaS brands (Shopify, Snapchat, Mozilla, Yelp) where the trusted domain has user session context
• CDN-backed subdomains (Fastly, CloudFront) where CNAME points to unclaimed origins
• Third-party service integrations: UserVoice, WordPress.com, GitHub Pages, GitLab Pages, Heroku, Zendesk
• Preview/staging/dev subdomains (new., preview., course., delivery., addons-preview.) — abandoned after feature launches
• Subdomains used for OAuth redirect URIs or SSO endpoints — these pay highest

Asset types that matter most:

• CNAME records pointing to deprovisioned third-party services
• NS delegations to abandoned zones
• A records pointing to unallocated cloud IPs (less common)
• GitLab/GitHub Pages with unclaimed project namespaces

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Attack Surface Signals

DNS signals:

• CNAME pointing to *.github.io, *.gitlab.io, *.fastly.net, *.herokudns.com, *.wordpress.com, *.uservoice.com, *.zendesk.com, *.s3.amazonaws.com, *.azurewebsites.net, *.netlify.app
• NXDOMAIN or SERVFAIL on the CNAME target while the parent record still exists
• NS records delegating to registrars where the zone is no longer registered

HTTP response signals:

• "There isn't a GitHub Pages site here"
• "NoSuchBucket" (S3)
• "The specified bucket does not exist"
• "No such app" (Heroku)
• "Sorry, this shop is currently unavailable" (Shopify)
• "This UserVoice subdomain is available"
• "Do you want to register" (any domain parking page)
• HTTP 404 with provider-specific error templates
• Fastly: "Fastly error: unknown domain"
• "404 Web Site not found" (Azure App Service)

Tech stack signals:

• Response headers: X-Served-By: cache-* (Fastly), X-GitHub-Request-Id, Server: Netlify
• CNAME chain resolving to provider infrastructure but returning provider 404
• SSL cert issued to provider wildcard (*.fastly.net) rather than company domain

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Step-by-Step Hunting Methodology

1. Enumerate all subdomains for the target using passive + active sources:

• subfinder -d target.com -all
• amass enum -passive -d target.com
• assetfinder --subs-only target.com
• Certificate transparency: crt.sh/?q=%.target.com

2. Resolve all subdomains and flag those with:

• NXDOMAIN responses
• CNAME pointing to a third-party provider

   cat subdomains.txt | dnsx -a -cname -o resolved.txt
   

3. Cross-reference CNAMEs against known vulnerable provider fingerprints using nuclei or subjack:

   subjack -w subdomains.txt -t 100 -timeout 30 -ssl -c fingerprints.json
   nuclei -l subdomains.txt -t takeovers/
   

4. Manual verification for each flagged subdomain:

• dig CNAME subdomain.target.com — confirm CNAME exists
• dig A <cname-target> — confirm NXDOMAIN or no resolution
• curl --max-time 30 --connect-timeout 10 -sk https://subdomain.target.com — check for provider error string

5. Confirm claimability — attempt to register the resource:

• GitHub Pages: check if <username>.github.io/<repo> or org page is unclaimed
• GitLab Pages: check project namespace
• S3: attempt aws s3api create-bucket --bucket <bucketname>
• UserVoice/Zendesk/WordPress: visit registration URL
• Fastly: check if origin hostname is unregistered

6. Claim the resource (only enough to prove control — do NOT serve malicious content):

• Create a minimal index page with your HackerOne username and a timestamp
• Take screenshot showing your content served on subdomain.target.com

7. Document the chain: CNAME record → provider target → unclaimed resource → your content

8. Assess impact escalation:

• Does the subdomain appear in OAuth redirect allowlists?
• Does it share cookies with parent domain (domain=.target.com)?
• Is it referenced in the app's CSP?
• Can it receive authenticated API calls?

9. Write report before releasing the claim (some programs want to verify first)

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Payload & Detection Patterns

Bulk CNAME extraction and NXDOMAIN detection:

# Extract CNAMEs and check if target resolves
while read sub; do
  cname=$(dig +short CNAME "$sub" | head -1)
  if [ -n "$cname" ]; then
    result=$(dig +short A "$cname")
    if [ -z "$result" ]; then
      echo "[POTENTIAL] $sub -> $cname (NXDOMAIN)"
    fi
  fi
done < subdomains.txt

Nuclei takeover scan:

nuclei -l subdomains.txt -t ~/nuclei-templates/http/takeovers/ -severity medium,high,critical

subjack with SSL:

subjack -w subdomains.txt -t 100 -timeout 30 -ssl -c $GOPATH/src/github.com/haccer/subjack/fingerprints.json -v

Provider fingerprint grep patterns:

curl --max-time 30 --connect-timeout 10 -sk "https://$subdomain" | grep -iE \
  "there isn't a github pages|no such bucket|no such app|this uservoice|fastly error: unknown domain|do you want to register|sorry, this shop|project not found|404 not found|unclaimed"

Check if subdomain is in scope for cookies (shared parent domain):

curl --max-time 30 --connect-timeout 10 -Isk "https://target.com" | grep -i "set-cookie" | grep "domain=.target.com"

Fastly-specific detection:

curl --max-time 30 --connect-timeout 10 -sI "https://subdomain.target.com" -H "Host: subdomain.target.com" | grep -i "fastly\|x-served-by\|x-cache"
curl --max-time 30 --connect-timeout 10 -sk "https://subdomain.target.com" | grep -i "fastly error"

S3 unclaimed bucket check:

aws s3api head-bucket --bucket <extracted-bucket-name> 2>&1 | grep -i "NoSuchBucket\|403\|404"

GitLab Pages specific:

dig CNAME sub.target.com
# If pointing to *.gitlab.io — visit the gitlab.io URL directly
# 404 from gitlab.io project = claimable

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Common Root Causes

1. Service offboarding without DNS cleanup — Developer removes a Heroku app, UserVoice account, or WordPress site but never deletes the CNAME record. DNS lives forever; service does not.

2. Staging/preview infrastructure abandoned post-launch — course., new., preview., beta. subdomains provisioned for a product launch, pointed at a third-party, then forgotten when the campaign ends.

3. Subdomain provisioned by a third-party team — Marketing sets up a UserVoice or Zendesk subdomain via IT, product sunset kills it, but DNS is owned by engineering who doesn't know.

4. CDN misconfiguration without origin validation — Fastly and similar CDNs historically allowed any domain to "claim" a backend hostname by creating a service pointing to it. Unregistered origin hostnames become claimable.

5. GitHub/GitLab Pages namespace not reserved — Organization renames, user accounts deleted, or repos made private/deleted while the Pages CNAME still points to the old namespace.

6. Wildcard DNS entries — *.target.com pointing to a cloud provider means *any* unclaimed subdomain potentially resolves to claimable infrastructure.

7. Acquired/divested company DNS not cleaned — Post-acquisition, former brand subdomains (like oberlo.com under Shopify) retain CNAMEs to services that are no longer paid for.

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Bypass Techniques

Defense: Manual fingerprint review before publishing

• Bypass: Use alternative error strings — providers change their 404 pages. Maintain an up-to-date fingerprint list. Some providers show *different* errors on HTTP vs HTTPS. Test both.

Defense: Scope restrictions (only main domain in scope)

• Bypass: Check program's asset list carefully — *.target.com wildcards often include subdomains implicitly. Escalate impact to get it in scope.

Defense: "Can't reproduce" responses due to timing

• Bypass: Screenshot immediately after claiming. Record a video walkthrough. The window can be short for popular subdomains.

Defense: HTTPS certificate mismatch blocking proof

• Bypass: Some providers (GitHub Pages, Netlify) auto-provision TLS for claimed domains. Others don't — show HTTP takeover and note TLS would be resolved by provider on claim.

Defense: Provider-side validation (Fastly verifying domain ownership)

• Bypass: Some Fastly configurations don't validate origin hostnames. Check if the CNAME target is a generic Fastly backend hostname vs. a customer-verified one. Try claiming anyway and observe behavior.

Defense: Rate limiting on subdomain enumeration

• Bypass: Use passive-only sources (SecurityTrails, Shodan, crt.sh, VirusTotal) to avoid triggering WAF/IDS. DNS resolution doesn't touch the web server.

Defense: Program claims "low severity / no impact"

• Bypass: Demonstrate same-origin cookie theft, OAuth redirect abuse, or CSP bypass to escalate. Find if the subdomain is listed in any `po

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