Mutiny on the Bridge: The Signal K Deception (CVE-2025-69203)

In the modern maritime world, the wooden wheel has been replaced by iPads, and the sextant by NMEA 2000 networks. At the heart of this digital revolution sits Signal K Server, an open-source hub that centralizes data from every sensor on a boat—GPS, depth, engine diagnostics, and autopilot controls. It is the central nervous system of a connected vessel.

Now, imagine if someone could walk onto the bridge, wearing a mask that says "I am the harmless Depth Sensor," and ask the Captain for the keys to the engine room. The Captain, seeing the trusted label, hands them over without checking if the person is actually a pirate holding a cutlass behind their back.

That is essentially CVE-2025-69203. It isn't a complex buffer overflow or a heap spray. It is a logic flaw that turns the Signal K Server into an accomplice for social engineering. By combining three distinct weaknesses—IP spoofing, UI deception, and information leakage—an attacker can trick a server administrator into unknowingly handing over the highest privileges available. This isn't just about stealing data; on a boat, this is about potentially hijacking navigation and control systems.

The vulnerability relies on a "Trifecta of Trust" failure. The developers built a system that trusted the client too much, trusted the network too much, and trusted that the UI would tell the truth. Here is how the house of cards collapses:

1. The X-Forwarded-For Sin The oldest sin in web security is trusting the X-Forwarded-For header without verification. Signal K used this header to determine if a request came from localhost or a trusted internal network. An attacker simply adds X-Forwarded-For: 127.0.0.1 to their request, and the server nods along, marking the request as coming from the server itself. This bypasses the initial "stranger danger" filter.

2. The Bait and Switch (UI Deception) When a device requests access to Signal K, it sends a description (e.g., "iPad at Helm") and permissions (e.g., read_only or admin). In versions prior to 2.19.0, the Admin UI committed a cardinal design sin: it highlighted the description but obscured or hid the requested permissions. An attacker could request admin rights but set the description to "Benign Read-Only Sensor". The administrator sees the safe description, assumes the permissions match, and clicks Approve.

3. The Leak (Information Disclosure) To make the lie convincing, you need to sound like a local. A separate flaw allowed unauthenticated users to listen to the serverevent stream. This stream broadcasted the names and IDs of legitimate devices on the network. The attacker uses this to copy the name of a known, trusted device (e.g., "Captain's Cabin Display"), making the spoofed request nearly indistinguishable from reality.

The root of the issue lay in how events were propagated and how authorization was (not) checked. The fix in version 2.19.0 (specifically commit 221aff6) introduces segregation of duties.

Previously, the server broadcasted sensitive access requests to the generic serverevent channel, which was too accessible. The patch moves these critical alerts to a restricted serverAdminEvent channel and enforces strict checks on who can subscribe to it.

Here is the critical logic introduced to stop the bleeding:

// src/tokensecurity.js - The new Sheriff in town
function hasAdminAccess(req) {
  return (
    req.skIsAuthenticated &&
    req.skPrincipal &&
    req.skPrincipal.permissions === 'admin'
  )
}
strategy.hasAdminAccess = hasAdminAccess

By enforcing hasAdminAccess on the event stream, the developers ensured that an unauthenticated attacker can no longer "sniff" the network for valid device names to impersonate. Furthermore, the X-Forwarded-For trust issue was mitigated by requiring proper proxy configuration, rather than blindly accepting the header from any source.

Let's walk through a realistic attack scenario. Our attacker has gained access to the boat's Wi-Fi (often protected by weak passwords like 12345678 or SignalK).

Step 1: Reconnaissance The attacker connects to the Signal K websocket endpoint and subscribes to serverevent. They wait. Suddenly, they see a legitimate device connect: "deviceId": "urn:mrn:imo:mmsi:234567890", "name": "NMEA Gateway". They now have a valid identity to clone.

Step 2: The Trojan Horse The attacker crafts a POST request to /signalk/v1/access/requests. They inject the spoofed header and the malicious payload:

POST /signalk/v1/access/requests HTTP/1.1
Host: signal-k-server.local
X-Forwarded-For: 127.0.0.1
Content-Type: application/json
 
{
  "clientId": "urn:mrn:imo:mmsi:234567890", 
  "description": "NMEA Gateway Re-connection (Read Only)",
  "permissions": "admin"
}

Step 3: The Social Engineering The Administrator is sitting at the helm. A notification pops up: "Access Request: NMEA Gateway Re-connection (Read Only) from 127.0.0.1".

Thinking the gateway just rebooted and needs to reconnect locally, the Admin hits Approve. Because the UI hid the fact that permissions was set to admin, the attacker is instantly granted a token with full root privileges over the Signal K server.

Step 4: Impact With an Admin token, the attacker can install malicious plugins (leading to Remote Code Execution), manipulate NMEA data streams (altering course headers or depth readings), or simply shut down the server during critical navigation maneuvers.

The mitigation for CVE-2025-69203 is straightforward but requires action.

1. Update Immediately Upgrade to Signal K Server v2.19.0 or later. This version separates the event streams (serverAdminEvent) and patches the UI to clearly display requested permissions, regardless of the description provided.

2. Trust No One (Network Config) If your Signal K server is exposed to a wider network or the internet (which it shouldn't be without a VPN), ensure you are using a reverse proxy (Nginx, Traefik). Configure the proxy to strip incoming X-Forwarded-For headers from the client and replace them with the actual remote IP. Configure Signal K to only trust X-Forwarded-For headers originating from the specific IP of your reverse proxy.

3. Vigilance For administrators: Treat every access request like a stranger knocking on your door at midnight. Verify the clientId and, most importantly, scrutinize the requested permissions. If a "Read Only" display is asking for admin rights, sound the alarm.