hunt-subdomain— agent skill

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

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

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

1. 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

1. 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/

1. Manual verification for each flagged subdomain:

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

1. 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

1. 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

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

1. 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?

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

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 -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 -Isk "https://target.com" | grep -i "set-cookie" | grep "domain=.target.com"

Fastly-specific detection:

curl -sI "https://subdomain.target.com" -H "Host: subdomain.target.com" | grep -i "fastly\|x-served-by\|x-cache"
curl -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

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.

1. 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.

1. 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.

1. 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.

1. 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.

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

1. 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.

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 postMessage targetOrigin checks in JS.

Gate 0 Validation

1. What can the attacker DO right now?

Can you register the unclaimed resource (GitHub repo, S3 bucket, Heroku app, UserVoice account) and serve arbitrary content — including phishing pages, credential harvesters, or malicious scripts — under the target's trusted domain name?

1. What does the victim LOSE?

Users lose trust and safety: they see a company-branded URL serving attacker content. The company loses brand integrity, potentially leaks session cookies if the subdomain is in domain=.target.com scope, and may have OAuth/SSO flows hijacked. Depending on CSP configuration, XSS against the main application may be possible.

1. Can it be reproduced in 10 minutes from scratch?

• dig CNAME subdomain.target.com → confirms CNAME to provider
• curl -sk https://subdomain.target.com → confirms provider error string
• Visit provider registration page → confirms namespace is available
• Screenshots of all three steps = reproducible in under 10 minutes

If you cannot show the provider resource is currently unclaimed and claimable, it is not a valid report.

Real Impact Examples

Scenario A — Trusted Brand Phishing via Abandoned SaaS (Snapchat/UserVoice) An attacker finds feedback.snapchat.com CNAME pointing to a UserVoice subdomain. The UserVoice account was cancelled but the DNS record remained. The attacker registers the matching UserVoice subdomain for free, gaining control of feedback.snapchat.com. Any user navigating to that URL — perhaps from old bookmarks or Google results — sees attacker-controlled content on a Snapchat-branded domain. Since the domain is trusted by browsers, phishing campaigns sent from this subdomain bypass email security filters that check domain reputation.

Scenario B — CDN Origin Takeover Enabling Same-Origin Attacks (Mozilla/Fastly) addons-preview-cdn.mozilla.net had a CNAME pointing to a Fastly origin hostname that was no longer registered to Mozilla's Fastly account. An attacker could create a Fastly service claiming that origin hostname, causing all requests to addons-preview-cdn.mozilla.net to be routed to attacker-controlled Fastly infrastructure. Since the subdomain shares the mozilla.net domain, it could be leveraged to serve malicious CDN assets that appear to come from Mozilla's infrastructure, potentially bypassing CSP rules that allowlist *.mozilla.net.

Scenario C — Staging Subdomain Abandoned Post-Product Migration (Rails/GitHub Pages) new.rubyonrails.org was pointed at a GitHub Pages deployment for a website redesign project. After the new site launched and the old GitHub repo was deleted or made private, the DNS CNAME remained. An attacker could fork or create a matching GitHub Pages repository and claim the namespace, serving content under new.rubyonrails.org. Because this is the official Ruby on Rails domain, any content served there — including fake download links or malicious gems — carries the full trust of the Rails brand.

Disclosed Report Citations (2022-2023)

The following coordinated-disclosure / writeup cases extend this skill with modern provider fingerprints (Vercel/Azure cloudapp/Zendesk era) and explicit ATO-chain examples. Citations #12 and #13 are external writeups (not disclosed HackerOne reports) — treat their payout figures as unverified; only #14 has a public H1 report ID.

1. *Microsoft Azure DevOps — Two `cloudapp.azure.com` subdomains + wildcard `.visualstudio.com` OAuth reply_to → 1-click ATO** (Binary Security writeup)

• Subclass: Azure cloudapp.azure.com regional-pool dangling CNAME — chained to ATO
• ATO chain: YES — app.vssps.visualstudio.com/_signin?reply_to=https://feedsprodwcus0dr.feeds.visualstudio.com/ whitelisted any *.visualstudio.com. Attacker claimed the dangling Azure VM hostnames, then crafted sign-in URLs that returned JWT + FedAuth tokens to attacker-controlled endpoints
• Claim flow: identify dangling cloudapp.azure.com CNAME, deploy a free-tier VM in the same Azure region requesting the exact released hostname, Azure re-issues the name first-come-first-serve
• Year: reported Feb 2021, disclosed Nov 2022 — MSRC explicitly out-of-scope (relied on subdomain takeover), $0

1. Anonymous H1 — `admin-support.xyz.com` → unclaimed Zendesk → email interception → ATO (Writeup by 0xprial)

• Subclass: Zendesk help-desk takeover via xyzdocs.zendesk.com host-mapping
• ATO chain: YES — researcher configured email forwarding on the hijacked Zendesk instance, intercepted [email protected] tickets containing payment info + password-reset emails, then triggered password resets on customer accounts that delivered reset links into the attacker's Zendesk inbox
• Claim flow: dig CNAME returns xyzdocs.zendesk.com (unregistered) → register free Zendesk trial → add xyzdocs as subdomain → enable host-mapping for admin-support.xyz.com
• Year: 2023 — payout per the 0xprial writeup (blog source; no public HackerOne report ID, treat as unverified)

1. Vercel deleted-project takeover — a dangling cname.vercel-dns.com CNAME pointing to a removed Vercel project can be re-claimed by deploying a new project under that name.

• Subclass: Vercel dangling CNAME (cname.vercel-dns.com) after project deletion
• Impact: crypto-DEX phishing — proxies. subdomain trusted for RPC proxy routing; attacker could serve malicious wallet-drain JS under a "trusted" subdomain
• Claim flow: subdomain returns Vercel 404 DEPLOYMENT_NOT_FOUND → create free Vercel project → Settings → Domains → add proxies.sifchain.finance — Vercel verifies the existing CNAME and auto-issues TLS without out-of-band ownership proof
• Year: 2022 — Sifchain treated as Critical (web3 phishing vector)

Fastly CDN dangling-service technique (general, not a single disclosed report):

• Fingerprint: subdomain returns Fastly error: unknown domain. Please check that this domain has been added to a service
• Claim flow: sign up for Fastly free trial → create new CDN service → attach the dangling hostname as the service domain
• Root cause: Fastly historically does not verify CNAME ownership on service-creation; any CNAME pointing into Fastly's anycast can be attached to a fresh service
• Potential ATO chain: chains to CSP-bypass + JS-injection if the parent domain trusts the assets./CDN host for script-src

Chains & Compositions (Senior Hunting)

Subdomain takeover by itself is Low-Medium / Informational on most mature programs — defacement of a non-business-critical subdomain is unsexy. The chain payout is 10-100x the standalone. Every takeover should be evaluated against the five chains below before submission. If none apply, you have a Low; if one applies, you have a High; if two compose, you have a Critical.

Chain 1 — Takeover + OAuth redirect_uri Whitelist → Auth-Code Theft → 1-Click ATO

• A. Enumerate the OAuth redirect_uri allowlist via /oauth/authorize flow. Look for any wildcard *.target.com or any takeover-candidate hostname in the static list (e.g., feedsprod.feeds.visualstudio.com, legacy.target.com).
• B. Find a takeover-able subdomain in that allowlist. Claim it via the provider's onboarding (Vercel project, Azure cloudapp regional pool, S3 bucket, Heroku app, Zendesk, Shopify storefront — see the Disclosed Report Citations above).
• C. Host an OAuth callback receiver on the claimed subdomain. Send victim to /oauth/authorize?redirect_uri=https://legacy.target.com/cb&response_type=code&client_id=<legit>. Victim's browser already has session → auth happens transparently → auth code lands on attacker host. Exchange via token endpoint → ATO.
• Impact: Persistent 1-click ATO across every user of the target. OAuth flow is implicit-to-the-user (no consent screen if previously consented), so requires only a single click on attacker's link.
• Real shape: Microsoft Azure DevOps cloudapp.azure.com + wildcard *.visualstudio.com reply_to chain (Binary Security, Nov 2022 — Disclosed Report Citation #12). Multiple H1 disclosures on SaaS programs with permissive OAuth allowlists.

Chain 2 — Takeover at Sibling Subdomain + Cookie-Domain Wildcard → Session Fixation on Parent App

• A. Inspect cookies set by the main app (app.target.com). If Set-Cookie has Domain=.target.com (parent-scoped) instead of host-only, cookies bleed to every sibling subdomain — including taken-over ones.
• B. Take over any sibling (legacy.target.com, feedback.target.com, assets.target.com). The taken-over host can now Set-Cookie for the parent domain.
• C. Plant Set-Cookie: SESSIONID=<attacker_session>; Domain=.target.com via a script on the taken-over host. Victim visits app.target.com with attacker's session cookie attached. Server treats them as the attacker's account → session-fixation ATO.
• Impact: ATO without OAuth or password reset — pure cookie-domain bleed. Especially effective when the parent app uses a non-__Host- prefixed session cookie.
• Real shape: Discussed extensively in hunt-auth-bypass Duende BFF Attack Class 2 (cookie-domain wildcarding turns subdomain takeover into session fixation). Pattern class: S3-bucket-takeover combined with parent-scoped (Domain=.target.com) cookies.

Chain 3 — Takeover + CSP script-src Includes the Taken-Over Host → Persistent Stored XSS on Main App

• A. Inspect CSP header on the main app's HTML response. Look for script-src 'self' assets.target.com cdn.target.com legacy.target.com ....
• B. Take over one of the CSP-allowlisted subdomains (especially common: stale CNAMEs to deleted CDNs, deleted Vercel/Netlify projects, archived analytics services).
• C. Host attacker-controlled JavaScript at the takeover host. Every page load on the main app fetches <script src="//taken-over-host/x.js"> because the host is on the CSP allowlist. JS executes with main-app origin — full session access, can call any same-origin API.
• Impact: Stored XSS-equivalent on every page of the main app, persistent until the CSP is updated. Bypasses every input sanitiser because the JS source is "trusted" per CSP.
• Real shape: Sifchain proxies.sifchain.finance Vercel takeover — Disclosed Report Citation #14 (web3 phishing); pattern documented in multiple H1 disclosures 2020-2024.

Chain 4 — Takeover + CORS Access-Control-Allow-Origin Regex Match → Credentialed Cross-Origin API Read

• A. Inspect the API's CORS configuration. Look for any regex / wildcard / suffix-match in Access-Control-Allow-Origin that includes *.target.com or target.com.* (the second is a common bug).
• B. Take over any subdomain that the CORS regex would accept (or register a new target.com.attacker.com host if suffix-match is broken).
• C. Attacker page hosted on the taken-over subdomain issues fetch('https://api.target.com/account', {credentials:'include'}). CORS preflight passes. Server returns credentialed response. Attacker's JS reads it.
• Impact: Mass cross-tenant API read with credentials — sessions, PATs, account data, billing records — all reachable from a single attacker page.
• Real shape: Multiple disclosed cases; cross-refs hunt-api-misconfig CORS subsection. Pairs with hunt-misc step 1 (CORS regex enumeration).

Chain 5 — Takeover at Email DNS (DKIM / SPF / MX) → Email Spoofing → Phishing Trusted by Parent Brand

• A. Enumerate the target's email DNS — DKIM selectors (selector1._domainkey.target.com), SPF includes (include:_spf.takeover-candidate.com), MX records (mx.target.com → defunct-provider.example).
• B. Take over any DKIM-selector or SPF-include host. Now the attacker can publish DKIM/SPF records that authorise their own server to send mail "from" @target.com.
• C. Send phishing email From: [email protected] to victim. Recipient mail server passes SPF + DKIM checks (because the takeover server is now authorised). Email lands in inbox with target.com brand, no security warning.
• Impact: Highly-effective phishing campaign exploiting the parent brand. Victims trust the email because every authentication check passes. Credential harvesting, BEC fraud, supply-chain access.
• Real shape: Multiple historical disclosures on DKIM selector takeover / SPF include chain hijacking. Cross-refs DMARC / SPF / DKIM section in offensive-osint.

Operator-level pattern

The five chains above are exhaustive in practice — virtually every senior-tier subdomain-takeover payout maps to one of them. Before reporting any takeover, run through the checklist:

1. OAuth `redirect_uri` allowlist — does the taken-over host appear? → Chain 1, Critical.
2. Parent-domain cookies — does the main app set Domain=.target.com? → Chain 2, High.
3. CSP `script-src` — does the taken-over host appear in the allowlist? → Chain 3, Critical.
4. CORS allowlist — does any regex match the taken-over host? → Chain 4, High.
5. Email DNS (DKIM selector / SPF include) — does the taken-over host appear? → Chain 5, High.

If none apply, file at Low/Informational. Do not file at Critical without demonstrating one of these chains — triagers downgrade fast otherwise.

Cross-references:

• hunt-oauth — Chain 1 (redirect_uri bypass class)
• hunt-auth-bypass Duende BFF Attack Class 2 — Chain 2 (cookie scoping)
• hunt-xss Chain 4 — Chain 3 (CSP bypass via trusted-origin JS)
• hunt-api-misconfig CORS section — Chain 4
• offensive-osint email-security section — Chain 5

Related Skills & Chains

• `hunt-cloud-misconfig` — Most stale CNAMEs point at deleted cloud assets (S3, CloudFront, Heroku). Chain primitive: Cloud misconfig (S3 deleted) + hunt-subdomain → unclaimed CNAME points to bucket → claim bucket name → full subdomain control.
• `hunt-oauth` — A takeover on an OAuth redirect_uri host = persistent ATO across the entire SSO surface. Chain primitive: Subdomain takeover at auth.target.com + OAuth redirect_uri allowlist → auth code theft → ATO every user that re-authenticates.
• `hunt-api-misconfig` — CORS regexes routinely allowlist a takeoverable subdomain. Chain primitive: Subdomain takeover + CORS *.target.com with credentials → credentialed cross-origin API read → mass IDOR.
• `hunt-xss` — A claimed subdomain is same-origin to session-cookie-domain siblings. Chain primitive: Subdomain takeover at feedback.target.com + cookie scope .target.com → JS hosted on takeover host reads main-app cookies → session hijack.
• `security-arsenal` — Load the 27+ Subdomain Takeover Fingerprint Table (NoSuchBucket, "no such app", GitHub Pages 404 strings, Heroku, Shopify, Fastly) and the subzy/subjack automation patterns.
• `triage-validation` — Apply the Unique-Marker gate: takeover claim is informational on its own; submit only after publishing a unique HTML marker on the claimed host AND demonstrating a downstream impact (cookie read, OAuth chain, CSP bypass).