Signal K Server CVE-2026-41893

Summary

The HTTP login endpoints (POST /login and POST /signalk/v1/auth/login) are protected by express-rate-limit (default: 100 attempts per 10-minute window, configurable via HTTP_RATE_LIMITS). The WebSocket login path - sending {login: {username, password}} messages over an established WebSocket connection - calls app.securityStrategy.login() directly without any rate limiting.

An attacker can bypass HTTP rate limiting entirely by opening a WebSocket connection and attempting unlimited password guesses at the speed bcrypt allows (~20 attempts/sec with 10 salt rounds).

Details

Vulnerable code: src/interfaces/ws.ts, function processLoginRequest (lines 753-780)

The function directly calls app.securityStrategy.login(msg.login.username, msg.login.password) with no throttling or attempt tracking.

Rate-limited HTTP path for comparison: src/tokensecurity.ts lines 609-617 apply loginLimiter middleware to the HTTP login routes at line 637.

Steps to Reproduce

1. Start Signal K server with security enabled
2. Open a WebSocket connection to ws://server:3000/signalk/v1/stream?subscribe=none
3. Wait for the hello message
4. Send login attempts in rapid succession:

   {"requestId": "1", "login": {"username": "admin", "password": "guess1"}}
   {"requestId": "2", "login": {"username": "admin", "password": "guess2"}}

1. Observe that all attempts are processed without any 429 response or throttling
2. For comparison, send 100+ HTTP POST requests to /signalk/v1/auth/login - the 101st returns 429

A POC script is available that demonstrates both the HTTP rate limiting working correctly and the WebSocket path accepting unlimited attempts.

Impact

• Credential brute-forcing via the WebSocket protocol at ~20 attempts/sec (bcrypt-limited)
• Complete bypass of the HTTP rate limiting defense
• A single WebSocket connection is sufficient for unlimited attempts
• With multiple parallel connections, throughput multiplies
• A 10,000-word dictionary attack completes in ~8 minutes over a single connection

Signal K servers are commonly deployed on boat networks where they may be accessible to other devices on the same LAN.

CWE

CWE-307: Improper Restriction of Excessive Authentication Attempts

Suggested Fix

Track failed login attempts per remote IP in a shared store (or reuse the existing express-rate-limit store) that is checked in both the HTTP login middleware and the processLoginRequest WebSocket handler.

Context

Found while building an open source maritime security scanner. Verified on v2.24.0 (current master).

Discovered by Mark Curphey