https://github.com/apache/httpd/pull/578

Index: modules/md/md_crypt.c
--- modules/md/md_crypt.c.orig
+++ modules/md/md_crypt.c
@@ -1222,7 +1222,7 @@ const char *md_cert_get_serial_number(const md_cert_t 
         serial = BN_bn2hex(bn);
         s = apr_pstrdup(p, serial);
         OPENSSL_free((void*)serial);
-        OPENSSL_free((void*)bn);
+        BN_free(bn);
     }
     return s;
 }
