diff --git a/CMakeLists.txt b/CMakeLists.txt
index 588f5d8..a98929f 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -34,6 +34,7 @@ if (POLICY CMP0005)
 endif (POLICY CMP0005)
 
 include (FindPkgConfig)
+include(CheckIncludeFile)
 
 if (NOT PKG_CONFIG_FOUND)
   message(FATAL_ERROR "pkg-config executable not found. Aborting.")
@@ -225,7 +226,7 @@ configure_file (tools/greenbone-nvt-sync.in tools/greenbone-nvt-sync @ONLY)
 
 ## Program
 
-set (HARDENING_FLAGS            "-Wformat -Wformat-security -O2 -D_FORTIFY_SOURCE=2 -fstack-protector -Wl,-z,relro -Wl,-z,now")
+set (HARDENING_FLAGS            "-Wformat -Wformat-security -O2 -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=2 -fstack-protector -Wl,-z,relro -Wl,-z,now")
 
 set (CMAKE_C_FLAGS_DEBUG        "${CMAKE_C_FLAGS_DEBUG} -Werror")
 set (CMAKE_C_FLAGS              "${CMAKE_C_FLAGS} ${HARDENING_FLAGS} -Wall -D_BSD_SOURCE -D_ISOC99_SOURCE -D_SVID_SOURCE -D_DEFAULT_SOURCE")