diff --git a/main.go b/main.go
index 36b179e..ee0fa42 100644
--- a/main.go
+++ b/main.go
@@ -11,11 +11,14 @@ import (
 	"path/filepath"
 	"strconv"
 	"strings"
+	"sync"
 	"time"
 
 	"github.com/joho/godotenv"
 )
 
+var mu sync.Mutex
+
 func ipAddrFromRemoteAddr(s string) string {
 	idx := strings.LastIndex(s, ":")
 	if idx == -1 {
@@ -64,6 +67,10 @@ func uploadHandler(w http.ResponseWriter, r *http.Request) {
 
 	files := r.MultipartForm.File["file"]
 
+	// Serialize access to the YubiKey
+	mu.Lock()
+	defer mu.Unlock()
+
 	for _, fileHeader := range files {
 		file, err := fileHeader.Open()
 		if err != nil {