Signal K Server: OAuth Authorization Code Theft via Unvalidated Host Header in OIDC Flow

Summary

SignalK Server contains a code-level vulnerability in its OIDC login and logout handlers where the unvalidated HTTP Host header is used to construct the OAuth2 redirect_uri. Because the redirectUri configuration is silently unset by default, an attacker spoof the Host header to steal OAuth authorization codes and hijack user sessions in realistic deployments as The OIDC provider will then send the authorization code to whatever domain was injected.

The OIDC specification requires redirect_uri to be pre-registered and not derived from untrusted input. Constructing it from the Host header violates this requirement and introduces a trust boundary break. This risk is actively amplified by SignalK's official documentation, which instructs administrators to deploy an Nginx configuration that forwards the vulnerable Host header, exposing production environments.

Vulnerability Root Cause

Two factors combine to create this vulnerability:

Factor 1: redirectUri is optional with an unsafe fallback In types.ts:30, redirectUri is declared as optional

export interface OIDCConfig {
  // ...
  redirectUri?: string   // ← Optional, no default value
  // ...
}

The defaults in types.ts:175-185 do not include a redirectUri: never checks or warns about a missing redirectUri. This means a fully "valid" OIDC configuration can exist without redirectUri, silently activating the vulnerable fallback path.

export const OIDC_DEFAULTS: Omit<OIDCConfig, 'issuer' | 'clientId' | 'clientSecret'> = {
  enabled: false,
  scope: 'openid email profile',
  defaultPermission: 'readonly',
  autoCreateUsers: true,
  providerName: 'SSO Login',
  autoLogin: false
  // ← No redirectUri default
}

Factor 2: Unsafe Host header usage in two locations Location 1 — Login handler in oidc-auth.ts:278-282:

const protocol = req.secure ? 'https' : 'http'
const host = req.get('host')                          // ← Attacker-controlled
const redirectUri =
  oidcConfig.redirectUri ||                            // ← Only safe if explicitly set
  `${protocol}://${host}${skAuthPrefix}/oidc/callback` // ← Uses attacker's Host

This redirectUri flows into createAuthState() → buildAuthorizationUrl() → OIDC provider's redirect_uri parameter. The OIDC provider will then send the authorization code to whatever domain was injected.

Location 2 — Logout handler in oidc-auth.ts:513-515:

const protocol = req.secure ? 'https' : 'http'
const host = req.get('host')                            // ← Same pattern
const fullPostLogoutUri = `${protocol}://${host}${postLogoutRedirect}`

This constructs the post_logout_redirect_uri sent to the OIDC provider's end_session_endpoint, allowing an attacker to redirect the user to an attacker controlled domain after logout.

Official Documentation Enables the Attack

SignalK's own security documentation at docs/security.md:222-228 provides the recommended nginx reverse proxy configuration: The proxy_set_header Host $host; directive forwards the client-supplied Host header to the backend unmodified. Without this directive, nginx would replace the Host header with the upstream address (localhost:3000), which would neutralize the injection.

location / {
    proxy_pass http://localhost:3000;
    proxy_set_header X-Forwarded-For $remote_addr;
    proxy_set_header X-Forwarded-Proto $scheme;
    proxy_set_header Host $host;   # ← Forwards client's Host header to SignalK
}

Administrators who follow the official documentation are directly enabling this vulnerability behind their reverse proxy.

Proof of Concept

Tested against SignalK Server v2.23.0 in Docker with OIDC enabled .

Step 1 — Send login request with injected Host header: $response = Invoke-WebRequest -Uri "http://localhost:3000/signalk/v1/auth/oidc/login" -Headers @{"Host"="evil.com"} -MaximumRedirection 0 -ErrorAction SilentlyContinue -UseBasicParsing

Step 2: Decode and print the injected redirect URL [uri]::UnescapeDataString($response.Headers.Location) <img width="1259" height="211" alt="Screenshot 2026-03-25 171251" src="https://github.com/user-attachments/assets/6e4a9655-639e-48c2-a7f0-06e17ad471ff" />

Impact

• Authorization Code Theft: The OIDC provider sends the OAuth authorization code to the attacker's domain instead of the legitimate server.
• Session Hijack: The attacker can exchange the stolen code for tokens and create a session as the victim user.
• Logout Redirect Hijack: The logout handler has the same pattern, allowing post-logout redirection to an attacker domain (phishing opportunity).