diff --git a/lib/efi_loader/efi_capsule.c b/lib/efi_loader/efi_capsule.c
index ea22ee7..4ef2546 100644
--- a/lib/efi_loader/efi_capsule.c
+++ b/lib/efi_loader/efi_capsule.c
@@ -73,8 +73,8 @@
 	struct efi_time time;
 	efi_status_t ret;
 
-	efi_create_indexed_name(variable_name16, "Capsule", index);
-
+	efi_create_indexed_name(variable_name16, sizeof(variable_name16),
+				"Capsule", index);
 	result.variable_total_size = sizeof(result);
 	result.capsule_guid = capsule->capsule_guid;
 	ret = EFI_CALL((*efi_runtime_services.get_time)(&time, NULL));
@@ -896,7 +896,8 @@
 	free(files);
 
 	/* CapsuleLast */
-	efi_create_indexed_name(variable_name16, "Capsule", index - 1);
+	efi_create_indexed_name(variable_name16, sizeof(variable_name16),
+				"Capsule", index - 1);
 	efi_set_variable_int(L"CapsuleLast", &efi_guid_capsule_report,
 			     EFI_VARIABLE_READ_ONLY |
 			     EFI_VARIABLE_NON_VOLATILE |
diff --git a/lib/efi_loader/efi_string.c b/lib/efi_loader/efi_string.c
index 3de721f..9627242 100644
--- a/lib/efi_loader/efi_string.c
+++ b/lib/efi_loader/efi_string.c
@@ -23,13 +23,19 @@
  * Return: A pointer to the next position after the created string
  *	   in @buffer, or NULL otherwise
  */
-u16 *efi_create_indexed_name(u16 *buffer, const char *name, unsigned int index)
+u16 *efi_create_indexed_name(u16 *buffer, size_t buffer_size, const char *name,
+			     unsigned int index)
 {
 	u16 *p = buffer;
 	char index_buf[5];
+	size_t size;
 
+	size = (utf8_utf16_strlen(name) * sizeof(u16) +
+		sizeof(index_buf) * sizeof(u16));
+	if (buffer_size < size)
+		return NULL;
 	utf8_utf16_strcpy(&p, name);
-	sprintf(index_buf, "%04X", index);
+	snprintf(index_buf, sizeof(index_buf), "%04X", index);
 	utf8_utf16_strcpy(&p, index_buf);
 
 	return p;