diff --git a/backend/main.go b/backend/main.go
index 00057bd1..04c8757e 100644
--- a/backend/main.go
+++ b/backend/main.go
@@ -99,19 +99,31 @@ func main() {
 	limiter := middleware.NewRateLimiter(30*time.Second, 50)
 	rateLimitedHandler := middleware.RateLimitMiddleware(limiter)
 
+	// Auth middleware validates session and injects session credentials into request body
+	// This prevents credential manipulation and allows frontend to not store sensitive secrets
+	authHandler := middleware.AuthMiddleware(store)
+
+	// Helper to compose rate limiting + auth middleware
+	authenticatedHandler := func(h http.Handler) http.Handler {
+		return rateLimitedHandler(authHandler(h))
+	}
+
+	// Auth endpoints (no auth middleware - these handle authentication)
 	mux.Handle("/auth/oauth", rateLimitedHandler(http.HandlerFunc(app.OAuthHandler)))
 	mux.Handle("/auth/callback", rateLimitedHandler(http.HandlerFunc(app.OAuthCallbackHandler)))
 	mux.Handle("/api/user", rateLimitedHandler(http.HandlerFunc(app.UserInfoHandler)))
 	mux.Handle("/auth/logout", rateLimitedHandler(http.HandlerFunc(app.LogoutHandler)))
-	mux.Handle("/tasks", rateLimitedHandler(http.HandlerFunc(controllers.TasksHandler)))
-	mux.Handle("/add-task", rateLimitedHandler(http.HandlerFunc(controllers.AddTaskHandler)))
-	mux.Handle("/edit-task", rateLimitedHandler(http.HandlerFunc(controllers.EditTaskHandler)))
-	mux.Handle("/modify-task", rateLimitedHandler(http.HandlerFunc(controllers.ModifyTaskHandler)))
-	mux.Handle("/complete-task", rateLimitedHandler(http.HandlerFunc(controllers.CompleteTaskHandler)))
-	mux.Handle("/delete-task", rateLimitedHandler(http.HandlerFunc(controllers.DeleteTaskHandler)))
+
+	// Task endpoints - require authentication, credentials injected from session
+	mux.Handle("/tasks", authenticatedHandler(http.HandlerFunc(controllers.TasksHandler)))
+	mux.Handle("/add-task", authenticatedHandler(http.HandlerFunc(controllers.AddTaskHandler)))
+	mux.Handle("/edit-task", authenticatedHandler(http.HandlerFunc(controllers.EditTaskHandler)))
+	mux.Handle("/modify-task", authenticatedHandler(http.HandlerFunc(controllers.ModifyTaskHandler)))
+	mux.Handle("/complete-task", authenticatedHandler(http.HandlerFunc(controllers.CompleteTaskHandler)))
+	mux.Handle("/delete-task", authenticatedHandler(http.HandlerFunc(controllers.DeleteTaskHandler)))
 	mux.Handle("/sync/logs", rateLimitedHandler(http.HandlerFunc(controllers.SyncLogsHandler)))
-	mux.Handle("/complete-tasks", rateLimitedHandler(http.HandlerFunc(controllers.BulkCompleteTaskHandler)))
-	mux.Handle("/delete-tasks", rateLimitedHandler(http.HandlerFunc(controllers.BulkDeleteTaskHandler)))
+	mux.Handle("/complete-tasks", authenticatedHandler(http.HandlerFunc(controllers.BulkCompleteTaskHandler)))
+	mux.Handle("/delete-tasks", authenticatedHandler(http.HandlerFunc(controllers.BulkDeleteTaskHandler)))
 
 	mux.HandleFunc("/health", controllers.HealthCheckHandler)
 
diff --git a/backend/middleware/auth.go b/backend/middleware/auth.go
new file mode 100644
index 00000000..8349fe69
--- /dev/null
+++ b/backend/middleware/auth.go
@@ -0,0 +1,89 @@
+package middleware
+
+import (
+	"bytes"
+	"ccsync_backend/utils"
+	"encoding/json"
+	"io"
+	"net/http"
+
+	"github.com/gorilla/sessions"
+)
+
+// credentialsPayload represents the common credential fields in request bodies
+type credentialsPayload struct {
+	Email            string `json:"email"`
+	EncryptionSecret string `json:"encryptionSecret"`
+	UUID             string `json:"UUID"`
+}
+
+// AuthMiddleware validates that the user is authenticated and injects session
+// credentials into the request body. This ensures:
+// 1. Only authenticated users can access protected endpoints
+// 2. Users cannot manipulate credentials to access other users' data
+// 3. Frontend doesn't need to store/send sensitive encryption secrets
+func AuthMiddleware(store *sessions.CookieStore) func(http.Handler) http.Handler {
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			// Get session
+			session, err := store.Get(r, "session-name")
+			if err != nil {
+				utils.Logger.Warnf("Auth middleware: failed to get session: %v", err)
+				http.Error(w, "Authentication required", http.StatusUnauthorized)
+				return
+			}
+
+			// Check if user is authenticated
+			userInfo, ok := session.Values["user"].(map[string]interface{})
+			if !ok || userInfo == nil {
+				http.Error(w, "Authentication required", http.StatusUnauthorized)
+				return
+			}
+
+			// Extract session credentials
+			sessionEmail, _ := userInfo["email"].(string)
+			sessionUUID, _ := userInfo["uuid"].(string)
+			sessionSecret, _ := userInfo["encryption_secret"].(string)
+
+			if sessionEmail == "" || sessionUUID == "" || sessionSecret == "" {
+				utils.Logger.Warnf("Auth middleware: incomplete session credentials")
+				http.Error(w, "Authentication required", http.StatusUnauthorized)
+				return
+			}
+
+			// For POST requests with JSON body, inject session credentials
+			if r.Method == http.MethodPost && r.Body != nil {
+				// Read the body
+				bodyBytes, err := io.ReadAll(r.Body)
+				if err != nil {
+					http.Error(w, "Failed to read request body", http.StatusBadRequest)
+					return
+				}
+
+				// Parse the body as a generic map
+				var bodyMap map[string]interface{}
+				if err := json.Unmarshal(bodyBytes, &bodyMap); err == nil {
+					// Inject/overwrite credentials from session
+					// This ensures users can't manipulate credentials to access other users' data
+					bodyMap["email"] = sessionEmail
+					bodyMap["UUID"] = sessionUUID
+					bodyMap["encryptionSecret"] = sessionSecret
+
+					// Re-encode the modified body
+					modifiedBody, err := json.Marshal(bodyMap)
+					if err != nil {
+						http.Error(w, "Failed to process request body", http.StatusInternalServerError)
+						return
+					}
+					r.Body = io.NopCloser(bytes.NewBuffer(modifiedBody))
+					r.ContentLength = int64(len(modifiedBody))
+				} else {
+					// If we can't parse the body, restore original and let handler deal with it
+					r.Body = io.NopCloser(bytes.NewBuffer(bodyBytes))
+				}
+			}
+
+			next.ServeHTTP(w, r)
+		})
+	}
+}