Web Applications

The web app world is extremely heterogeneous. Every web application is different from others because developers have many ways to accomplish the same task.

  • Having flexibility in web app development also means having flexibility in creating insecure code.

  • Fundamental aspects: HTTP protocol basics, Cookies, Sessions, Same Origin Policy.

HTTP Protocol Basics

HTTP works on top of TCP protocol, so when the connection is established, the client sends a request and waits for the answer. The server processes the request and sends back its answer, along with status code and data:

  • Client -> HTTP request -> Server

  • Client <- HTTP response <- Server

HTTP Connection establishment:

  • Client -> SYN -> Server

  • Client <- SYN/ACK <- Server

  • Client -> ACK + GET /html -> Server

  • Client <- HTML response + Close connection <- Server

The format of an HTTP message is:

Message Body\r


 # carriage return
\n newline


Some status codes:

Status codeMeaning

200 OK

the resource is found

301 Moved Permanently

the requested resource has been assigned a new permanent URI

302 Found

the resource is temporarily under another URI

403 Forbidden

the client doesn't have enough privileges, server refuses req.

404 Not Found

the server cannot find the resource matching the request

500 Internal Server Error

the server does not support the functionality required

GET / HTTP/1.1                                    # VERB path protocol version
Host: www.elarnsecurity.com                       # Specifies the internet hostname and port number, obtained from the URI of the resource
User-Agent: Mozilla/5.0 (X11; Linux x86_64 ...)   # Tells the server what client software is issuing the request
Accept: text/html                                 # Specifies which document type is expected in the response
Accept-Language: en-US,en;q=0.5                   # Browser can as for a specific human language in the response
Accept-Encoding: gzip, deflate                    # Restricts the content encoding
Connection: keep-alive                            # Future communications with the server will reuse the current connection

HTTPS: HTTP over SSL/TLS as an encryption layer

HTTPS: HTTP over SSL/TLS is a method to run HTTP which is a clear-text protocol over SSL/TLS, a cryptographic protocol:

  • Provides: Confidentiality, Integrity Protection and Authentication to the HTTP protocol.

  • An attacker cannot neither sniff the application layer communication nor alter the application layer data.

  • Traffic can be sniffed, but any adjacent user will not know request/response headers, request/response body, request target domain.

  • The client can tell the real identity of the server and, sometimes, vice-versa.

  • When inspecting HTTPS, one cannot know what domain is contacted and what data is exchanged.

  • HTTPS does not protect against web application flaws.

  • All the attacks against an application happen regardless of SSL/TLS, such as XSS and SQL injection will still work.


netcat opens a raw connection to a service port:

$ nc -v www.ferrari.com 80

GET / HTTP/1.1
Host: www.ferrari.com # 2 lines after inputting this line

Host: www.ferrari.com

HEAD /en_en/ HTTP/1.1
Host: www.ferrari.com
$ nc -v hack.me 443

GET / HTTP/1.1
Host: hack.me

# Nothing is displayed as netcat cannot open SSL websites
$ openssl s_client -connect hack.me:443
$ openssl s_client -connect hack.me:443 -debug
$ openssl s_client -connect hack.me:443 -state
$ openssl s_client -connect hack.me:443 -quiet

# Certificate is spat out

GET / HTTP/1.1
Host: hack.me

Host: hack.me

Burp Suite repeater tool also allows same operation in order to inspect raw response given a HTTP verb. It can also configure a HTTPS connection right away.

HTTP Cookies (1994 - Netscape)

  • HTTP is a stateless protocol, therefore HTTP cannot keep the state of a visit across different HTTP requests.

  • HTTP requests are unrelated to the preceding and following ones.

  • Often exploits rely on stealing cookies: Cookies / Cookie jar, just textual information installed by a website into a web browser.

A server can set a cookie via Set-Cookie HTTP header field in a response message:

HTTP/1.1 200 OK
Date: Wed, 19 Nov 2020 10:10:10 GNT
Cache-Control: private, max-age=0
Content-Type: text/html; charset=UTF=8
Content-Encoding: gzip
Server: Apache/2.2.15 (CentOS)
Set-Cookie: ID=Value; expires=Thu, 21-May-2015 15:25:20 GMT; path=/; domain=.example.site; Http
Content-Length: 99043


A cookie contains the following attributes: the actual content, an expiration date, path, domain and optional flags (Http only flag, Secure flag).


Cookie Content


Expiration Date

expires=Thu, 21-May-2015 15:25:20 GMT;





Flag-setting Attribute


  • Browsers use domain, path, expires and flags attributes to choose whether or not to send a cookie in request.

  • Cookies are sent only to the valid domain/path when they are not expired and according to their flags.

  • The domain field and the path field set the scope of the cookie.

  • The browser sends the cookie only if the request is for the right domain.

  • When a web server installs a cookie, it sets the domain field.

  • Then the browser will use the cookie for every request sent to that domain and all its subdomains.

  • If the server does not specify the domain attribute, the browser will automatically set the domain as the server domain and set the cookie 'host-only' flag,meaning that this cookie will be sent only to that precise hostname.

  • Respectively, when a cookie has the path attribute set, the browser will send the cookie to the right domain and to the resources in that path and not any other.

  • A browser will not send an expired cookie to the server, session cookies will expire with the HTTP session.

  • http-only flag is a mechanism that prevents JavaScript or any other non-HTML technology from reading the cookie (preventing a XSS robbery).

  • Secure flag creates secure cookies that will only be sent over an HTTPS connection.


Sessions are a mechanism that lets the website store variables specific for a given visit on the server side:

  • Sometimes the web developer prefers to store some information on the server side.

  • This avoids the back and forth data transmission and hides the application logic.

  • Each session is identified by a session id, where the client presents this ID for each subsequent request.

  • With that ID, the server is able to retrieve the state of the client.

Session cookies

Session cookies allow to install a session ID on a web browser

PHPSESSID=1992maiwr2H     # PHP
JSESSIONID=W8234mSfsw3    # JSP
  • The browser then uses the cookie in subsequent requests.

  • A session could contain multiple variables, so sending a small cookie keeps the bandwidth usage low.

  • The browser will send back the cookie according to the cookie protocol,thus sending the session ID.

  • Session IDs can also be transmitted via GET requests.

Session cookies and Cookies can be inspected and manipulated via Firebug for Firefox or any other web developer tools. Your session will be forwarded to re-authenticate if you delete the session cookie after login process.

// If HttpOnly isn't enabled, we'd be able to access the cookie jar from JavaScript

Same Origin Policy (SOP)

  • SOP / Same Origin Policy is a critical point of web application security.

  • Prevents JavaScript code from getting/setting properties on a resource coming from a different origin.

  • To determine if JS can access a resource, hostname, port and protocol must match.

  • SOP only applies to the actual code of a script.

  • It is still possible to include external resources by using HTML, like IMG, script, iFrame...

  • If a script on Domain A was able to read content on Domain B, it would be possible to steal clients' information and mount a number of dangerous attacks.

Burp Suite

  • Any web application contains many objects like scripts, images, stylesheets, client and server-side intelligence.

  • Having tools that help in the study and analysis of web application behavior is critical.

  • An intercepting proxy is a tool that lets you analyze and modify any request and any response exchanged between an HTTP client and a server.

  • Intercepting proxy != web proxy (as Squid)

BurpSuite will let you:

  • Intercept request/responses between your browser and web server.

  • Build requests manually.

  • Crawl a website by automatically visiting every page in a website.

  • Fuzz webapps by sending them patterns of a valid and invalid inputs to test their behavior.

  • You can modify the header and the body of a message by hand or automatically.

  • Burp Repeater lets you manually build raw HTTP requests.

  • Same can be achieved with nc or telnet.


  • Proxy: let's you view, intercept and modify traffic between browser/

  • Spider: web crawler.

  • Repeater: manipulate and reissue HTTP requests.

  • Learn how to find hidden resources on web application.

  • Learn how to configure your browser's proxy to work with Burp Suite, apart of setting 'scope'.

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