feat(auth): implement OIDC token exchange to get real username from Authelia

src/core/auth/oidc.go

@@ -1,22 +1,21 @@
 package auth
 
 import (
+	"encoding/base64"
+	"encoding/json"
 	"fmt"
+	"io"
 	"net/http"
 	"net/url"
+	"strings"
 )
 
 // OIDCConfig holds the configuration for the Authelia OIDC client.
 type OIDCConfig struct {
-	// Authelia's OIDC issuer URL (e.g., http://127.0.0.1:9091)
 	IssuerURL    string
-	// Client ID registered in Authelia
 	ClientID     string
-	// Client secret (if required)
 	ClientSecret string
-	// Redirect URL after OIDC login (e.g., https://sechpoint.app/auth/callback)
 	RedirectURL  string
-	// The public-facing domain for cookie domain
 	Domain       string
 }
 
@@ -39,12 +38,11 @@ func (h *OIDCHandler) LoginRedirect(w http.ResponseWriter, r *http.Request) {
 	state := generateToken(16)
 	nonce := generateToken(16)
 
-	// Store state in a short-lived cookie for CSRF protection
 	http.SetCookie(w, &http.Cookie{
 		Name:     "oidc_state",
 		Value:    state,
 		Path:     "/",
-		MaxAge:   300, // 5 minutes
+		MaxAge:   300,
 		HttpOnly: true,
 		SameSite: http.SameSiteLaxMode,
 	})
@@ -62,18 +60,14 @@ func (h *OIDCHandler) LoginRedirect(w http.ResponseWriter, r *http.Request) {
 }
 
 // Callback handles the OIDC authorization code callback from Authelia.
-// For now, this validates state and creates a session.
-// Full token exchange requires an HTTP client to Authelia's token endpoint.
 func (h *OIDCHandler) Callback(w http.ResponseWriter, r *http.Request) {
-	// Get state from cookie for CSRF check
 	stateCookie, err := r.Cookie("oidc_state")
 	if err != nil {
 		http.Error(w, "missing state cookie", http.StatusBadRequest)
 		return
 	}
 
-	// Verify state parameter matches
-	// Get state from URL query (GET) or form body (POST form_post mode)
+	// Get state from URL query (GET) or form body (POST)
 	stateParam := r.URL.Query().Get("state")
 	if stateParam == "" {
 		r.ParseForm()
@@ -93,7 +87,7 @@ func (h *OIDCHandler) Callback(w http.ResponseWriter, r *http.Request) {
 		HttpOnly: true,
 	})
 
-	// Get code from URL query (GET) or form body (POST form_post mode)
+	// Get code from URL query (GET) or form body (POST)
 	code := r.URL.Query().Get("code")
 	if code == "" {
 		code = r.Form.Get("code")
@@ -103,17 +97,11 @@ func (h *OIDCHandler) Callback(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	// TODO: Exchange code for tokens using Authelia's token endpoint.
-	// For now, we create a session with the authorization code as a placeholder.
-	// In production, you would:
-	// 1. POST to /api/oidc/token with the code
-	// 2. Validate the ID token
-	// 3. Extract the user's subject (sub) claim
-	// 4. Create a session with that subject
-
-	username := r.URL.Query().Get("sub")
-	if username == "" {
-		username = "authenticated-user" // placeholder until token exchange
+	// Exchange code for tokens
+	username, err := h.exchangeCode(code)
+	if err != nil {
+		http.Error(w, "token exchange failed: "+err.Error(), http.StatusInternalServerError)
+		return
 	}
 
 	token, err := h.store.CreateSession(username, 60)
@@ -127,22 +115,89 @@ func (h *OIDCHandler) Callback(w http.ResponseWriter, r *http.Request) {
 		Name:     "nextwks_session",
 		Value:    token,
 		Path:     "/",
-		MaxAge:   3600, // 1 hour
+		MaxAge:   3600,
 		HttpOnly: true,
 		SameSite: http.SameSiteStrictMode,
 	})
 
-	// Redirect to the workspace
 	http.Redirect(w, r, "/", http.StatusFound)
 }
 
+// exchangeCode exchanges an OIDC authorization code for an ID token
+// and extracts the username (sub claim) from it.
+func (h *OIDCHandler) exchangeCode(code string) (string, error) {
+	tokenURL := h.config.IssuerURL + "/api/oidc/token"
+
+	data := url.Values{
+		"grant_type":    {"authorization_code"},
+		"code":          {code},
+		"redirect_uri":  {h.config.RedirectURL},
+		"client_id":     {h.config.ClientID},
+		"client_secret": {h.config.ClientSecret},
+	}
+
+	resp, err := http.PostForm(tokenURL, data)
+	if err != nil {
+		return "", fmt.Errorf("token request failed: %w", err)
+	}
+	defer resp.Body.Close()
+
+	body, _ := io.ReadAll(resp.Body)
+	if resp.StatusCode != http.StatusOK {
+		return "", fmt.Errorf("token endpoint returned %d: %s", resp.StatusCode, string(body))
+	}
+
+	var tokenResp struct {
+		IDToken string `json:"id_token"`
+	}
+	if err := json.Unmarshal(body, &tokenResp); err != nil {
+		return "", fmt.Errorf("parse token response: %w", err)
+	}
+
+	if tokenResp.IDToken == "" {
+		return "", fmt.Errorf("no id_token in response")
+	}
+
+	// Decode JWT payload (without signature verification for now)
+	username, err := decodeJWTSub(tokenResp.IDToken)
+	if err != nil {
+		return "", fmt.Errorf("decode id_token: %w", err)
+	}
+
+	return username, nil
+}
+
+// decodeJWTSub extracts the "sub" (subject/username) from a JWT without verifying the signature.
+func decodeJWTSub(token string) (string, error) {
+	parts := strings.Split(token, ".")
+	if len(parts) != 3 {
+		return "", fmt.Errorf("invalid JWT format")
+	}
+
+	payload, err := base64.RawURLEncoding.DecodeString(parts[1])
+	if err != nil {
+		return "", fmt.Errorf("decode JWT payload: %w", err)
+	}
+
+	var claims struct {
+		Sub string `json:"sub"`
+	}
+	if err := json.Unmarshal(payload, &claims); err != nil {
+		return "", fmt.Errorf("parse JWT claims: %w", err)
+	}
+
+	if claims.Sub == "" {
+		return "", fmt.Errorf("missing sub claim in id_token")
+	}
+
+	return claims.Sub, nil
+}
+
 // AuthGateMiddleware protects routes behind OIDC authentication.
-// If the user has no valid session, redirect to Authelia login.
 func (h *OIDCHandler) AuthGateMiddleware(next http.Handler) http.Handler {
 	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		_, ok := GetUserID(r)
 		if !ok {
-			// Not authenticated — redirect to login
 			h.LoginRedirect(w, r)
 			return
 		}