CVE-2026-34083

## 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).