Denial of Service Attacks, Layer 7

This is a brief reposted post (one I wrote ages back) on how Layer 7, or "Application Layer" Denial of Service Attacks work.

Application Layer DoS attacks are a newer form of DoS attack. They work by not simply flooding/saturating the servers bandwidth, but by attacking a specific service, or application, running on the server. They often require far less bandwidth to accomplish, and are far more "efficient" an attack method. No massive botnets are required for an attacker to be able to effectively take out the target service.

I first got interested in Layer 7 DoS after realizing that LOIC and such "packet flooders" were essentially next to useless. TCP flooding was bandwidth intensive and required either a horde of fools, a large botnet, or a fucking huge datapipe to accomplish (a few cheap VPS's, however, made the job easier...). Sure, you could use spoofing and such attacks to enhance your "firepower", but when up against *big* targets with fairly impressive filtering, you were essentially wasting resources.

For those of you unfamiliar with Layer 7 DoS attacks, have a read of this paper from OWASP - it was what gave me my head start in understanding it all.
Layer 7 DoS - OWASP

Now. Onward to the "attack techniques". The series I wrote on DoS attacks was all about explaining what attack methods are used where, and seeing as the only edits done in this repost were a quick spellcheck and this comment, I do not plan on editing very much else. Yes, I am lazy.

HTTP GET DoS / SlowLoris Attack.
Wikipedia on Slowloris
Original page explaining it

Now, first off, list of affected target webservers:

• Apache 2.x 
• Apache 1.x
• dhttpd
• GoAhead WebServer
• WebSense "block pages" (unconfirmed)
• Trapeze Wireless Web Portal (unconfirmed)
• Verizon's MI424-WR FIOS Cable modem (unconfirmed)
• Verizon's Motorola Set-Top Box (port 8082 and requires auth - unconfirmed)
• BeeWare WAF (unconfirmed)
• Deny All WAF (unconfirmed) 

All of these are (according to RSnake), affected by the SlowLoris technique.

How it works is simple, it asks the server to wait. The server, being nice, waits. It does this simulating over9000 clients. The server keeps on waiting, being nice. Server dies, pretty much.

Now, to the interesting part. Attack Tools.
Original Slow Loris
TOR Loris - SlowLoris w/ Multiple TOR Proxies
PyLoris - Python SlowLoris
In Development: FluxLoris (Rapid SOCKS switching SlowLoris implementation)

HTTP POST DoS Attack (SlowPost)
This one was inspired by the OWASP paper I referred to above, and we released a PoC tool to exploit the bug around Christmas 2010. I worked on developing the tool and learned a LOT. Basically you are uploading (POST-ing) data to the server and saying "Hey, you! I am on a laggy connect! Please wait!". The server waits... And waits... You keep the connection open.
You do this with a literal shitload of threads.

It requires bugger all bandwidth and has a very destructive effect, rendering most webservers 404-ed within a few minutes.

So, here was our initial PoC tool:
POST-it v1

We had a more "lethal" variant but it is lost long ago, maybe some day I will dig it up and re-implement it, but given current climate, no point.

Now, onward, there are far better attack tools!
SlowPost by NEC - This one is VERY nice. Uses Proxy lists to anonymize the attack. Was written by the current mantainer of the "LOIC" package I believe.

OWASP HTTP POST DoS - This one is from OWASP, and seems to be moreso for testing.

R U Dead Yet - This one is considered the "industry standard" for HTTP POST DoS attacks. It works. Most of the time. I know of some unusual errors it has thrown in the past, but it has TOR support.

And finally, the well known and loved TORSHAMMER . This one is incredibly effective, known to drop servers within minutes. Anecdotal evidence has it one guy on a DSL line took out a bunch of Iranian government websites for a half hour a year or so ago, and then ate the Libyan .gov servers for second helpings! It works fairly reliably, and uses TOR.

There are a great deal of other attack tools out there exploiting these weaknesses, but the best bet (for now) to avoid the embarassment of someone taking you down a peg with some of these is to use the NGINX platform for a webserver. It works fairly well and seems to just blatantly ignore these attacks.

Further Reading...
Testing Webservers for Slow HTTP Attacks
http://en.wikipedia.org/wiki/Denial-of-service_attack
http://www.acunetix.com/blog/web-security-zone/articles/http-post-denial-service/
http://www.us-cert.gov/cas/tips/ST04-015.html
http://isc.sans.edu/diary.html?storyid=6601
http://www.funtoo.org/wiki/Slowloris_DOS_Mitigation_Guide
http://www.checkpoint.com/defense/advisories/public/announcement/071409-slowloris-dos-attack.html
http://www.bullten.com/what-is-slowiris-ddos-attack-and-how-to-mitigate-its-effect/