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Extract CNAMEs and check if target resolves · elementalsouls | DecimalAI

securityby elementalsouls

Extract CNAMEs and check if target resolves

elementalsouls/hunt-subdomain

Source elementalsouls/claude-bughunter ↗License NOASSERTION

Hunting skill for subdomain takeover vulnerabilities. Includes modern provider fingerprints — Microsoft Azure DevOps `cloudapp.azure.com` regional-pool re-issue (1-click OAuth ATO via wildcard `reply_to`, Binary Security), Zendesk help-desk takeover → email interception → password reset chain (0xprial writeup), Vercel `cname.vercel-dns.com` deleted-project takeover, plus general Fastly CDN service re-attach and S3 dangling-bucket cookie-scope techniques.

SkillScore100Maintained

Automatic

Security & AppSec pack

Last verified 8/9/2026

What it changes

Passed 22 of 22 test cases with the skill, and 17 of 22 without it.

Without skill78%

With skill100%

Checks it passed that the model alone missed

Include a Host header set to the target subdomain.
Rate the standalone report as Low or Informational severity.
Identify `This UserVoice subdomain is available` as the UserVoice fingerprint string.

and 2 more in the full report

5 fixed18 passed either way0 still failing0 broke

Pass-rate lift: +22 pp
Tokens: +348%
Agent turns: 0%
Response time: +9%

Model gemini-3.6-flashMeasured 8/9/2026v123 of 23 cases passed with skill · 22 comparable

Tokens, turns and time describe resource use, not a dollar price.

Use this skill

Save it into your project's skills folder. Claude Code picks it up in its next session.

curl -fsSL --create-dirs -o .claude/skills/elementalsouls-hunt-subdomain/SKILL.md https://app.decimal.ai/s/elementalsouls-hunt-subdomain/SKILL.md

Want it pinned and updatable? pip install "decimalai>=0.15", then decimalai skills add elementalsouls-hunt-subdomain, and start Claude Code with claude --plugin-dir "$(git rev-parse --show-toplevel)/.agents"

1 · One-time — install the CLI (docs ↗)

pip install "decimalai>=0.15"

2 · Get the skill — writes .agents/skills/elementalsouls-hunt-subdomain/ and pins it in .decimal/skills.lock

decimalai skills add elementalsouls-hunt-subdomain

No CLI? Get the same file — curl · raw file

curl -fsSL --create-dirs -o .claude/skills/elementalsouls-hunt-subdomain/SKILL.md https://app.decimal.ai/s/elementalsouls-hunt-subdomain/SKILL.md

Raw file: /s/elementalsouls-hunt-subdomain/SKILL.md ↗ · machine-readable summary at /s/elementalsouls-hunt-subdomain.json ↗

Safety checks: 3 of 3 passed

Code scan CleanIntent PassedContent Clean

See it in action

One of 4 test cases the skill fixed. Same model, same prompt, with and without the skill.

1 of 4

Prompt

What specific string in the HTTP response body indicates that a UserVoice customer subdomain has been decommissioned and is available for registration?

What was checked, without and with the skill
Checked	Without skill	With skill
Identify `This UserVoice subdomain is available` as the UserVoice fingerprint string.	Missed	Passed

case-14 · gemini-3.6-flash · 8/9/2026 · answers are capped and redacted

What's inside

The instructions your model reads when the skill loads.

SKILL.md328 linessingle file

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

Track record

Benchmark

22 / 22

test cases passed, vs 17 / 22 without

Adoption

Users: 0
Times used: 0

Safety and source details

SkillSafety

✓ 3 of 3 passed

Code scan15 of 15 checks clean

0 critical · 0 warning · 0 info — scanner v4 — 8/3/2026

View 15 scanner checks

live_secretremote_code_executionreverse_shelldata_exfiltrationssrf_metadataagent_snoopinganti_refusaldestructive_commandsobfuscated_unicodeunicode_homoglyphopaque_blobsuspicious_urltool_overreachtrigger_abuseprompt_injection_phrasing

IntentSafe

LLM judge · does the skill do what it claims

ContentClean

LLM judge · policy & harmful-content review

Source

Imported

elementalsouls / claude-bughunter

skills/hunt-subdomain/SKILL.md

View on GitHub ↗

License NOASSERTIONImported 8/3/2026Version v1

More skills from the same source and nearby in the registry, with their headline benchmark lift.