diff --git a/dlls/ntdll/virtual.c b/dlls/ntdll/virtual.c
index ff947daf0b4..479ca795e93 100644
--- a/dlls/ntdll/virtual.c
+++ b/dlls/ntdll/virtual.c
@@ -1320,7 +1320,8 @@ static NTSTATUS map_image( HANDLE hmapping, int fd, char *base, SIZE_T total_siz
 
     loadcfg = RtlImageDirectoryEntryToData( (HMODULE)ptr, TRUE,
                                             IMAGE_DIRECTORY_ENTRY_LOAD_CONFIG, &loadcfg_size );
-    if (loadcfg && loadcfg_size >= sizeof(*loadcfg))
+    if (loadcfg &&
+        loadcfg_size >= offsetof(IMAGE_LOAD_CONFIG_DIRECTORY, SecurityCookie) + sizeof(loadcfg->SecurityCookie))
         set_security_cookie((ULONG_PTR *)loadcfg->SecurityCookie);
 
     /* set the image protections */