diff --git a/libwvdrmengine/cdm/util/include/string_format.h b/libwvdrmengine/cdm/util/include/string_format.h
index 62f9fd91..49ac3810 100644
--- a/libwvdrmengine/cdm/util/include/string_format.h
+++ b/libwvdrmengine/cdm/util/include/string_format.h
@@ -13,6 +13,11 @@ namespace wvutil {
 #endif
 bool FormatString(std::string* out, const char* fmt, ...);
 
+#ifdef __GNUC__
+[[gnu::format(printf, 2, 0)]]
+#endif
+bool VFormatString(std::string* out, const char* fmt, va_list vlist);
+
 }  // namespace wvutil
 
 #endif  // WVCDM_UTIL_STRING_FORMAT_H_
diff --git a/libwvdrmengine/cdm/util/src/string_format.cpp b/libwvdrmengine/cdm/util/src/string_format.cpp
index 91cce37e..3926fc41 100644
--- a/libwvdrmengine/cdm/util/src/string_format.cpp
+++ b/libwvdrmengine/cdm/util/src/string_format.cpp
@@ -16,20 +16,27 @@ namespace wvutil {
 bool FormatString(std::string* out, const char* fmt, ...) {
   if (out == nullptr || fmt == nullptr) return false;
 
-  va_list ap1;
-  va_start(ap1, fmt);
-  const int desired_size = vsnprintf(nullptr, 0, fmt, ap1);
-  va_end(ap1);
+  va_list vlist;
+  va_start(vlist, fmt);
+  const bool result = VFormatString(out, fmt, vlist);
+  va_end(vlist);
+  return result;
+}
+
+bool VFormatString(std::string* out, const char* fmt, va_list vlist) {
+  if (out == nullptr || fmt == nullptr) return false;
+
+  va_list vlist_copy;
+  va_copy(vlist_copy, vlist);
+  const int desired_size = vsnprintf(nullptr, 0, fmt, vlist_copy);
+  va_end(vlist_copy);
 
   if (desired_size < 0) return false;
   const size_t buffer_size =
       static_cast<size_t>(desired_size) + 1;  // +1 for null
   std::unique_ptr<char[]> buffer(new char[buffer_size]);
 
-  va_list ap2;
-  va_start(ap2, fmt);
-  const int actual_size = vsnprintf(buffer.get(), buffer_size, fmt, ap2);
-  va_end(ap2);
+  const int actual_size = vsnprintf(buffer.get(), buffer_size, fmt, vlist);
 
   if (actual_size != desired_size) return false;