Update on using realtime blacklists with ModSecurity

A few days ago I posted a blog article about stopping comment spam with ModSecurity using realtime blacklists (rbl). While the approach was working, I noted having problems with rules when I tried to match on POST methods in HTTP requests.

Luckily, ModSecurity creator Ivan Ristic was quick to point out where the problem is. I’m using the Core Ruleset for ModSecurity, and one thing that ruleset does is use the ‘lowercase’ transformation. This converts all text from arguments to lowercase, so my ^POST$ match would never be able to match. So like Ivan suggested, using ^post$ solved this part.

Next Ivan pointed out a weakness in the rules. My rules looked for /blog/wp-comment-post.php, and would be easily evaded by just using /blog//wp-comment-post.php. He suggested using the ‘normalisePath’ transformation. I did this, but I also slightly changed the rules to not look for the /blog/ part at all (maybe this makes normalisePath useless, but I decided to rather be safe than sorry).

The rules I’m using now look like this:

SecRule REQUEST_METHOD “^post$” “log,deny,chain,msg:’LOCAL comment spammer at rbl list.dsbl.org'”
SecRule REQUEST_URI “wp-(comments-post|trackback).php$” “chain,t:normalisePath”
SecRule REMOTE_ADDR “@rbl list.dsbl.org”

SecRule REQUEST_METHOD “^post$” “log,deny,chain,msg:’LOCAL comment spammer at rbl bl.spamcop.net'”
SecRule REQUEST_URI “wp-(comments-post|trackback).php$” “chain,t:normalisePath”
SecRule REMOTE_ADDR “@rbl bl.spamcop.net”

SecRule REQUEST_METHOD “^post$” “log,deny,chain,msg:’LOCAL comment spammer at rbl sbl-xbl.spamhaus.org'”
SecRule REQUEST_URI “wp-(comments-post|trackback).php$” “chain,t:normalisePath”
SecRule REMOTE_ADDR “@rbl sbl-xbl.spamhaus.org”

Thanks a lot Ivan Ristic for your comments!