diff --git a/config.h.in b/config.h.in
index b363fd3..3d48991 100644
--- a/config.h.in
+++ b/config.h.in
@@ -21,3 +21,6 @@
 
 /* The default name of the authorities list file. */
 inline constexpr char defaultAuthoritiesListFileName[] = "@authorities_list_name@";
+
+/* Whether to allow expired certificates. */
+inline constexpr bool allowExpired = @allow_expired@;