HackSkills is an organized knowledge base of installable skills that gives AI agents practical security knowledge across areas such as web security, privilege escalation, reverse engineering, and digital forensics. It is intended for bug bounty work, penetration testing, CTF competitions, and authorized security research. The catalogue entries are the project's own master, category, and topic skills.
Getting it into your agent
One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.
npx skills add yaklang/hack-skills --skill subdomain-takeovergit clone --depth 1 https://github.com/yaklang/hack-skillsWrote this? Show the measurements
A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.
[](https://agentmods.dev/skills/yaklang/hack-skills/subdomain-takeover)<a href="https://agentmods.dev/skills/yaklang/hack-skills/subdomain-takeover"><img src="https://agentmods.dev/badge/skills/yaklang/hack-skills/subdomain-takeover.svg" alt="Measured on agentmods" height="20"></a>- Socket warn
- Snyk fail
- NVIDIA SkillSpector pass
What it costs to keep this loaded
Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.
| Model | Per session | Once invoked |
|---|---|---|
| Fable 5.1 | $0.00061 | $0.02780 |
| Opus 5 | $0.00030 | $0.01390 |
| Sonnet 5 | $0.00012 | $0.00556 |
| Haiku 4.5 | $0.00006 | $0.00278 |
Grade A, and why
subdomain-takeover scanned grade A with 1 finding against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 4d ago.
A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.
Makes network callslowCapability
Not a fault in itself. Listed so you know the mod talks to something, and to what.
1. Confirm: curl -s http://sub.target.com → "NoSuchBucket" Copies of this mod
3 near-identical copies found in the catalogue:
- subdomain-takeover — 100% identical, 0 lines differ
- subdomain-takeover — 100% identical, 0 lines differ
- subdomain-takeover — 95% identical, 7 lines differ
How it starts
The opening of the file, as written. The whole thing — 248 lines — stays where its author put it; the contents beside it link to each section on GitHub.
SKILL: Subdomain Takeover — Detection & Exploitation Playbook
AI LOAD INSTRUCTION: Covers CNAME/NS/MX takeover, per-provider fingerprint matching, claim procedures, and defensive monitoring. Base models often confuse "CNAME exists" with "takeover possible" — the key is whether the resource behind the CNAME is unclaimed and claimable.
0. RELATED ROUTING
- ssrf-server-side-request-forgery when a subdomain takeover is used to bypass SSRF allowlists trusting
*.target.com - cors-cross-origin-misconfiguration when CORS trusts
*.target.com— takeover → full cross-origin read - xss-cross-site-scripting takeover gives you script execution under target origin (cookie theft, OAuth redirect abuse)
- http-host-header-attacks when Host routing leads to subdomain-scoped cache or auth issues
- web-cache-deception when a taken-over subdomain shares cache with the main domain
1. CORE CONCEPT
Subdomain takeover occurs when:
sub.target.comhas a DNS record (CNAME, NS, A) pointing to an external service- The external resource is no longer provisioned (deleted S3 bucket, removed Heroku app, etc.)
- The attacker can register/claim that exact resource name on the provider
- The attacker now controls content served under
sub.target.com
Impact: cookie theft (parent domain cookies), OAuth token interception, phishing under trusted domain, CORS bypass, CSP bypass via whitelisted subdomain.
2. DETECTION METHODOLOGY
2.1 CNAME Enumeration
1. Collect subdomains (amass, subfinder, assetfinder, crt.sh, SecurityTrails)
2. Resolve DNS for each:
dig CNAME sub.target.com +short
3. For each CNAME → check if the CNAME target returns NXDOMAIN or a provider error
4. Match error response against fingerprint table (Section 3)
What this file has done since we first saw it
Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.
- 4d ago First seen · 248 lines · 61 tokens per session scan A cc79ff339a65
subdomain-takeover is a skill published in the GitHub repository yaklang/hack-skills (2,133 stars, last pushed 2mo ago), licensed MIT. It adds 61 tokens to every session and 2,780 once invoked, about $0.0003 per session on Opus 5. A static security scan graded it A with 1 finding (makes network calls). No closer match exists in the catalogue, so it is treated as the original; first seen 2026-09-03.
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