Researchers at last month's OWASP 2010 Application Security Conference in Washington, D.C., demonstrated how the HTTP protocol leaves the door open for attackers to wage a new form of distributed denial-of-service (DDoS) attack that floods Web servers with very slow HTTP "POST" traffic.
Researcher Wong Onn Chee, who first discovered the attack in 2009 with a team of researchers in Singapore, and Tom Brennan, a security researcher with Proactive Risk, also demonstrated how an online game could be used as a means of recruiting bots for an "agentless" botnet that executes this HTTP POST DDoS attack. Chee says HTTP is broken and that all Web-based servers or systems with a Web interface are vulnerable to this attack. "If it has a Web interface, we can knock it down [with this attack]," Chee said earlier this month.
Ryan Barnett, senior security researcher with Trustwave's SpiderLabs, has been testing the tools and devised some mitigation strategies using the open-source ModSecurity Web application firewall and Apache, namely specifying different thresholds for receiving data. Barnett uses the timely analogy of airport security screening to explain what happens in a HTTP POST attack: Unlike a traditional denial-of-service attack where there are just too many business-type frequent flyers in line to get the traffic through quickly, he says, this new "connection consumption" type of attack is like a relatively short line of travelers going through security -- most of whom are families with kids, strollers, and other items that take longer to screen.
"And just when you think the group is going to make it through, the metal detector sounds and the whole group has to go back through the process again. This is essentially what is happening with these slow HTTP requests," Barnett says. "They are sending data very slowly, and just when the Web server's timeout thresholds are about to be exceeded, they send a little more data."
An HTTP POST attack sends POST headers that let the server know how much data is arriving, but when it does the message body is sent a very slow speed to jam the connection and sap server resources. According to Chee, this attack can take down a website within minutes using just tens of thousands of slow HTTP POST connections.
Click here to view Onn Chee and Brennan's slides from their recent OWASP presentation.
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