Hypertext Transfer Protocol (HTTP) is an application level network protocol that is used for the transfer of content on the World Wide Web.

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Vary is one of the most powerful HTTP response headers. Used correctly, it can do wonderful things. Unfortunately, this header is frequently used incorrectly, which can lead to abysmal hit ratios. Worse still, if it's not used when it should be, the wrong content could be delivered.

Because HTTP is an extensible protocol browsers have pioneered some useful headers to prevent or increase the difficulty of exploiting these vulnerabilities. Knowing what they are and when to apply them can help you increase the security of your system.