Add CSRF checks to login.

handlers/app.go

@@ -20,6 +20,7 @@ type PageData struct {
 	CurrentURL       string
 	ContentTemplate  string
 	BodyClass        string
+	CSRFToken        string
 }
 
 type Config struct {

handlers/csrf.go

@@ -0,0 +1,55 @@
+package handlers
+
+import (
+	"crypto/rand"
+	"crypto/subtle"
+	"encoding/base64"
+	"net/http"
+
+	"github.com/gorilla/sessions"
+)
+
+const csrfSessionKey = "csrf_token"
+
+func (app *App) ensureCSRFToken(session *sessions.Session) (string, error) {
+	if token, ok := session.Values[csrfSessionKey].(string); ok && token != "" {
+		return token, nil
+	}
+
+	raw := make([]byte, 32)
+	if _, err := rand.Read(raw); err != nil {
+		return "", err
+	}
+
+	token := base64.RawURLEncoding.EncodeToString(raw)
+	session.Values[csrfSessionKey] = token
+	return token, nil
+}
+
+func (app *App) csrfToken(session *sessions.Session) string {
+	token, err := app.ensureCSRFToken(session)
+	if err != nil {
+		return ""
+	}
+	return token
+}
+
+func (app *App) validateCSRFToken(r *http.Request, session *sessions.Session) bool {
+	expected, ok := session.Values[csrfSessionKey].(string)
+	if !ok || expected == "" {
+		return false
+	}
+
+	provided := r.Header.Get("X-CSRF-Token")
+	if provided == "" {
+		provided = r.FormValue("csrf_token")
+	}
+	if provided == "" {
+		provided = r.URL.Query().Get("csrf_token")
+	}
+	if len(provided) != len(expected) {
+		return false
+	}
+
+	return subtle.ConstantTimeCompare([]byte(provided), []byte(expected)) == 1
+}

handlers/login.go

@@ -13,6 +13,11 @@ func LoginHandler(app *App) http.HandlerFunc {
 	return func(w http.ResponseWriter, r *http.Request) {
 		session := r.Context().Value("session").(*sessions.Session)
 		if r.Method == http.MethodPost {
+			if !app.validateCSRFToken(r, session) {
+				http.Error(w, "Invalid CSRF token", http.StatusForbidden)
+				return
+			}
+
 			username := r.FormValue("username")
 			password := r.FormValue("password")
 			user, err := models.GetUserByUsername(app.DB, username)
@@ -53,6 +58,7 @@ func LoginHandler(app *App) http.HandlerFunc {
 				BasePath:   app.Config.ThreadrDir,
 				StaticPath: app.Config.ThreadrDir + "/static",
 				CurrentURL: r.URL.RequestURI(),
+				CSRFToken:  app.csrfToken(session),
 			},
 			Error: "",
 		}

templates/pages/login.html

@@ -17,6 +17,7 @@
             <p class="field-error" style="text-align: center; font-size: 1em;">{{.Error}}</p>
             {{end}}
             <form method="post" action="{{.BasePath}}/login/">
+                <input type="hidden" name="csrf_token" value="{{.CSRFToken}}">
                 <label for="username">Username:</label>
                 <input type="text" id="username" name="username" required autocomplete="username"><br>
                 <label for="password">Password:</label>
@@ -28,4 +29,4 @@
     {{template "cookie_banner" .}}
 </body>
 </html>
-{{end}}
+{{end}}