This tool has been hitting the news, including some mentions in the SANS ISC Diary.

It’s not actually a new attack (it’s been around since 2005) but this is the first time a packaged tool has been released for the attack.

Slowloris holds connections open by sending partial HTTP requests. It continues to send subsequent headers at regular intervals to keep the sockets from closing. In this way webservers can be quickly tied up. In particular, servers that have threading will tend to be vulnerable, by virtue of the fact that they attempt to limit the amount of threading they’ll allow.

Slowloris must wait for all the sockets to become available before it’s successful at consuming them, so if it’s a high traffic website, it may take a while for the site to free up it’s sockets. So while you may be unable to see the website from your vantage point, others may still be able to see it until all sockets are freed by them and consumed by Slowloris. This is because other users of the system must finish their requests before the sockets become available for Slowloris to consume. If others re-initiate their connections in that brief time-period they’ll still be able to see the site.

So it’s a bit of a race condition, but one that Slowloris will eventually always win – and sooner than later.

Slowloris lets the webserver return to normal almost instantly (usually within 5 seconds or so). That makes it ideal for certain attacks that may just require a brief down-time.

This affects a number of webservers that use threaded processes and ironically attempt to limit that to prevent memory exhaustion – fixing one problem created another. This includes but is not necessarily limited to the following:

• Apache 1.x
• Apache 2.x
• dhttpd
• GoAhead WebServer
• Squid

There are a number of webservers that this doesn’t affect as well, in the authors testing:

• IIS6.0
• IIS7.0
• lighttpd
• nginx
• Cherokee (verified by user community)

You can download Slowloris here:

slowloris.pl

Or read more here.