Articles / Desarrollo Web

Mitigating CVE-2018-6389 WordPress DoS attack with lighttpd

Early in 2018, Barak Tawily published a possible DoS attack for WordPress, that basically works by requesting all possible scripts on the /wp-admin/load-scripts.php, a script that fetches and concatenates javascript files — there’s also a load-styles.php file that does the same for styles.

His vulnerability report was rejected by the WordPress team, on the account that this type of attack should be mitigated at the server or network level… so how do you do that using lighttpd?

Actually it’s pretty easy using mod_evasive, a “very simplistic module to limit connections per IP”, as advertised on the lighttpd docs.

First, you must make sure that mod_evasive it’s enabled on the server.modules block:

server.modules = (
  "mod_access",
  "mod_alias",
  "mod_compress",
  "mod_redirect",
  "mod_rewrite",
  "mod_accesslog",
  "mod_evasive"
)

Then, on the main lighttpd config file you can add the following:

$HTTP["url"] =~ "/wp-admin/load-(styles|scripts).php(.*)" {
  evasive.max-conns-per-ip = 8
}

This will effectively limit the amount of allowed connections to 8 by IP. Of course, you can adjust that value to whatever you need; 8 connections by IP it’s plenty enough for a “normal” editor use.

You can test if it’s working by “attacking” your server with siege or ab or your favourite benchmarking/load testing tool and checking your lighttpd error log, where this should appear:

2018-03-22 21:05:53: (mod_evasive.c.183) 192.168.33.1 turned away. Too many connections.

After testing, you might also want to add this to the lighttpd config:

evasive.silent = "enabled"

… that way, blocked IPs won’t be logged on the errors.log (which, on its own could trigger a DoS by repeatedly writing to the log file)

If you’re using nginx with HTTP/2, there’s an even better way.