HTTP is a ubiquitous protocol and is one of the cornerstones of the web. If you are a newcomer to web application security, a sound knowledge of the HTTP protocol will make your life easier when interpreting findings by automated security tools, and it’s a necessity if you want to take such findings further with manual testing. What follows is a security-focused introduction to the HTTP protocol to help you get started.

HTTP is a message-based (request, response), stateless protocol comprised of headers (key-value pairs) and an optional body. Three versions of HTTP have been released so far — HTTP/1.0 (released in 1996, rare usage), HTTP 1.1 (released in 1997, wide usage) and HTTP/2 (released in 2015, increasing usage).

The HTTP protocol works over the Transmission Control Protocol (TCP). TCP is one of the core protocols within the Internet protocol suite and it provides a reliable, ordered, and error-checked delivery of a stream of data, making it ideal for HTTP. The default port for HTTP is 80, or 443 if you’re using HTTPS (an extension of the HTTP over TLS).

HTTP is a line-based protocol, meaning that each header is represented on its own line, with each line ending in a Carriage Return Line Feed (CRLF) with a blank line separating the head from the optional body of the request or response.

Up to HTTP/1.1, HTTP was a text-based protocol, however, with HTTP/2 this has changed — HTTP/2, unlike its predecessors is a binary protocol with most implementations requiring TLS encryption. It’s worth noting that for the vast majority of cases (and certainly, for this article) interacting with the HTTP/2 protocol won’t be any different. It’s also worth mentioning that HTTP/1.1 isn’t going away anytime soon, and it’s still early days for HTTP/2 (as such, HTTP/1.1 will be referenced throughout this article).