Slowloris is a denial-of-service (DoS) tool developed by the grey hat hacker “RSnake” that causes DoS by using a very slow HTTP request. By sending HTTP headers to the target site in tiny chunks as slow as possible (waiting to send the next tiny chunk until just before the server would time out the request), the server is forced to continue to wait for the headers to arrive. If enough connections are opened to the server in this fashion, it is quickly unable to handle legitimate requests.

Slowloris is cross-platform, except due to Windows’ ~130 simultaneous socket use limit, it is only effective from UNIX-based systems which allow for more connections to be opened in parallel to a target server (although a GUI Python version of Slowloris dubbed PyLoris was able to overcome this limiting factor on Windows).