diff --git a/passphrase_cache.cc b/passphrase_cache.cc
index e6413f9..d11a2f1 100644
--- a/passphrase_cache.cc
+++ b/passphrase_cache.cc
@@ -48,6 +48,7 @@ namespace pEp {
 
     const char *PassphraseCache::add_stored(const std::string& passphrase)
     {
+        std::lock_guard<std::mutex> lock(_shared_mtx);
         _stored = passphrase;
         return _stored.c_str();
     }
diff --git a/passphrase_cache.hh b/passphrase_cache.hh
index 99ab574..3a82655 100644
--- a/passphrase_cache.hh
+++ b/passphrase_cache.hh
@@ -26,6 +26,7 @@ namespace pEp {
         cache _cache;
         std::string _stored;
         std::mutex _mtx;
+        std::mutex _shared_mtx;
         size_t _max_size;
         duration _timeout;