HTTP Headers

General Headers

General headers are used in both HTTP requests and responses. They are contextual and are used to describe the message rather than its contents.

Entity Headers

Similar to general headers, Entity Headers can be common to both the request and response. These headers are used to describe the content (entity) transferred by a message. They are usually found in responses and POST or PUT requests.

Header	Example	Description
Content-Type	Content-Type: text/html	Used to describe the type of resource being transferred. The value is automatically added by the browsers on the client-side and returned in the server response. The charset field denotes the encoding standard, such as UTF-8.
Media-Type	Media-Type: application/pdf	The media-type is similar to Content-Type, and describes the data being transferred. This header can play a crucial role in making the server interpret our input. The charset field may also be used with this header.
Boundary	boundary="b4e4fbd93540"	Acts as a marker to separate content when there is more than one in the same message. For example, within a form data, this boundary gets used as --b4e4fbd93540 to separate different parts of the form.
Content-Length	Content-Length: 385	Holds the size of the entity being passed. This header is necessary as the server uses it to read data from the message body, and is automatically generated by the browser and tools like cURL.
Content-Encoding	Content-Encoding: gzip	Data can undergo multiple transformations before being passed. For example, large amounts of data can be compressed to reduce the message size. The type of encoding being used should be specified using the Content-Encoding header.

Request Headers

Header	Example	Description
Host	Host: www.inlanefreight.com	Used to specify the host being queried for the resource. This can be a domain name or an IP address. HTTP servers can be configured to host different websites, which are revealed based on the hostname. This makes the host header an important enumeration target, as it can indicate the existence of other hosts on the target server.
User-Agent	User-Agent: curl/7.77.0	The User-Agent header is used to describe the client requesting resources. This header can reveal a lot about the client, such as the browser, its version, and the operating system.
Referer	Referer: http://www.inlanefreight.com/	Denotes where the current request is coming from. For example, clicking a link from Google search results would make https://google.com the referer. Trusting this header can be dangerous as it can be easily manipulated, leading to unintended consequences.
Accept	Accept: */*	The Accept header describes which media types the client can understand. It can contain multiple media types separated by commas. The */* value signifies that all media types are accepted.
Cookie	Cookie: PHPSESSID=b4e4fbd93540	Contains cookie-value pairs in the format name=value. A cookie is a piece of data stored on the client-side and on the server, which acts as an identifier. These are passed to the server per request, thus maintaining the client's access. Cookies can also serve other purposes, such as saving user preferences or session tracking. There can be multiple cookies in a single header separated by a semi-colon.
Authorization	Authorization: BASIC cGFzc3dvcmQK	Another method for the server to identify clients. After successful authentication, the server returns a token unique to the client. Unlike cookies, tokens are stored only on the client-side and retrieved by the server per request. There are multiple types of authentication types based on the webserver and application type used.

Response Headers

Header	Example	Description
Content-Security-Policy	Content-Security-Policy: script-src 'self'	Dictates the website's policy towards externally injected resources. This could be JavaScript code as well as script resources. This header instructs the browser to accept resources only from certain trusted domains, hence preventing attacks such as Cross-site scripting (XSS).
Strict-Transport-Security	Strict-Transport-Security: max-age=31536000	Prevents the browser from accessing the website over the plaintext HTTP protocol, and forces all communication to be carried over the secure HTTPS protocol. This prevents attackers from sniffing web traffic and accessing protected information such as passwords or other sensitive data.
Referrer-Policy	Referrer-Policy: origin	Dictates whether the browser should include the value specified via the Referer header or not. It can help in avoiding disclosing sensitive URLs and information while browsing the website.

cURL

If we were only interested in seeing the response headers, then we can use the -I flag to send a HEAD request and only display the response headers. Furthermore, we can use the -i flag to display both the headers and the response body (e.g. HTML code).

eldeim@htb[/htb]$ curl -I https://www.inlanefreight.com

Host: www.inlanefreight.com
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_5) AppleWebKit/605.1.15 (KHTML, like Gecko)
Cookie: cookie1=298zf09hf012fh2; cookie2=u32t4o3tb3gg4
Accept: text/plain
Referer: https://www.inlanefreight.com/
Authorization: BASIC cGFzc3dvcmQK

Date: Sun, 06 Aug 2020 08:49:37 GMT
Connection: keep-alive
Content-Length: 26012
Content-Type: text/html; charset=ISO-8859-4
Content-Encoding: gzip
Server: Apache/2.2.14 (Win32)
Set-Cookie: name1=value1,name2=value2; Expires=Wed, 09 Jun 2021 10:18:14 GMT
WWW-Authenticate: BASIC realm="localhost"
Content-Security-Policy: script-src 'self'
Strict-Transport-Security: max-age=31536000
Referrer-Policy: origin

eldeim@htb[/htb]$ curl https://www.inlanefreight.com -A 'Mozilla/5.0'

<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN">
<html><head>
...SNIP...

Exercise: Try to use the -I or the -v flags with the above example, to ensure that we did change our User-Agent with the -A flag.