diff --git a/CMakeLists.txt b/CMakeLists.txt
index 203f8b9..403e091 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -284,7 +284,7 @@ configure_file (tools/openvas-migrate-to-postgres.in tools/openvas-migrate-to-po
 
 ## 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 -D_FILE_OFFSET_BITS=64")