HTTP DELETE flood is a layer 7 DDoS attack that targets web servers and applications.

Layer 7 is the application layer of the OSI model. The HTTP protocol – is an Internet protocol which is the basis of browser-based Internet requests, and is commonly used to send form contents over the Internet or to load web pages.

HTTP DELETE floods are designed to overwhelm web servers’ resources by continuously requesting single or multiple URL’s from many source attacking machines, which simulate HTTP clients, such as web browsers (Though the attack analyzed here, does not use browser emulation).

An HTTP DELETE Flood consists of DELETE requests. Unlike other HTTP floods that may include other request methods such as POST, PUT, GET, etc.

When the server’s limits of concurrent connections are reached, the server can no longer respond to legitimate requests from other clients attempting to connect, causing a denial of service.

HTTP DELETE flood attacks use standard URL requests, hence it may be quite challenging to differentiate from valid traffic. Traditional rate-based volumetric detection is ineffective in detecting HTTP DELETE flood attacks since traffic volume in HTTP DELETE floods is often under detection thresholds. However, HTTP DELETE flood uses the less common DELETE method. As such, it may be beneficial to review network traffic carefully when witnessing many such incoming requests.

Technical analysis

To send an HTTP DELETE request, a client first establishes a TCP connection with a server, using the TCP 3-Way Handshake (SYN, SYN-ACK, ACK), seen in packets 30,55,56 in Image 1. The HTTP request will be in a PSH, ACK packet.

Image 1 – Example of TCP connection

An attacker (IP sends HTTP/1.1 DELETE requests, while the target responds with HTTP/1.1 405 Method Not Allowed as seen in Image 2.

While in this flow we see an HTTP/1.1 405 Method Not Allowed response, that might change depending on the web server settings.

Image 2 – Example of HTTP packets exchange between an attacker and a target

The capture analyzed is around 3 seconds long while it contains an average of 71 PPS (packets per second), with an average traffic rate of 0.07 Mbps (considered low, the attack you are analyzing could be significantly higher).

Image 3 – HTTP DELETE Flood stats

Analysis of HTTP DELETE Flood in WireShark – Filters

“http” filter – Will show all http related packets.

“http.request.method == DELETE” – Will show HTTP DELETE requests.

It will be important to review the user agent and other HTTP header structures as well as the timing of each request to understand the attack underway.

Download example PCAP of HTTP DELETE Flood attack

*Note: IP’s have been randomized to ensure privacy.