Identify application entry points (OTG-INFO-006)

This article is part of the new OWASP Testing Guide v4.

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   https://www.owasp.org/index.php/OWASP_Testing_Guide_v4_Table_of_Contents
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Summary

Enumerating the application and its attack surface is a key precursor before any thorough testing can be undertaken, as it allows the tester to identify likely areas of weakness. This section aims to help identify and map out areas within the application that should be investigated once enumeration and mapping have been completed.

Test Objectives

Understand how requests are formed and typical responses from the application

How to Test

Before any testing begins, the tester should always get a good understanding of the application and how the user and browser communicates with it. As the tester walks through the application, they should pay special attention to all HTTP requests (GET and POST Methods, also known as Verbs), as well as every parameter and form field that is passed to the application. In addition, they should pay attention to when GET requests are used and when POST requests are used to pass parameters to the application. It is very common that GET requests are used, but when sensitive information is passed, it is often done within the body of a POST request.

Note that to see the parameters sent in a POST request, the tester will need to use a tool such as an intercepting proxy (for example, OWASP: Zed Attack Proxy (ZAP)) or a browser plug-in. Within the POST request, the tester should also make special note of any hidden form fields that are being passed to the application, as these usually contain sensitive information, such as state information, quantity of items, the price of items, that the developer never intended for you to see or change.

In the author's experience, it has been very useful to use an intercepting proxy and a spreadsheet for this stage of the testing. The proxy will keep track of every request and response between the tester and the application as they walk through it. Additionally, at this point, testers usually trap every request and response so that they can see exactly every header, parameter, etc. that is being passed to the application and what is being returned. This can be quite tedious at times, especially on large interactive sites (think of a banking application). However, experience will show what to look for and this phase can be significantly reduced.

As the tester walks through the application, they should take note of any interesting parameters in the URL, custom headers, or body of the requests/responses, and save them in a spreadsheet. The spreadsheet should include the page requested (it might be good to also add the request number from the proxy, for future reference), the interesting parameters, the type of request (POST/GET), if access is authenticated/unauthenticated, if SSL is used, if it's part of a multi-step process, and any other relevant notes. Once they have every area of the application mapped out, then they can go through the application and test each of the areas that they have identified and make notes for what worked and what didn't work. The rest of this guide will identify how to test each of these areas of interest, but this section must be undertaken before any of the actual testing can commence.

Below are some points of interests for all requests and responses. Within the requests section, focus on the GET and POST methods, as these appear the majority of the requests. Note that other methods, such as PUT and DELETE, can be used. Often, these more rare requests, if allowed, can expose vulnerabilities. There is a special section in this guide dedicated for testing these HTTP methods.

Requests:

• Identify where GETs are used and where POSTs are used.
• Identify all parameters used in a POST request (these are in the body of the request).
• Within the POST request, pay special attention to any hidden parameters. When a POST is sent all the form fields (including hidden parameters) will be sent in the body of the HTTP message to the application. These typically aren't seen unless a proxy or view the HTML source code is used. In addition, the next page shown, its data, and the level of access can all be different depending on the value of the hidden parameter(s).
• Identify all parameters used in a GET request (i.e., URL), in particular the query string (usually after a ? mark).
• Identify all the parameters of the query string. These usually are in a pair format, such as foo=bar. Also note that many parameters can be in one query string such as separated by a &, ~, :, or any other special character or encoding.
• A special note when it comes to identifying multiple parameters in one string or within a POST request is that some or all of the parameters will be needed to execute the attacks. The tester needs to identify all of the parameters (even if encoded or encrypted) and identify which ones are processed by the application. Later sections of the guide will identify how to test these parameters. At this point, just make sure each one of them is identified.
• Also pay attention to any additional or custom type headers not typically seen (such as debug=False).

Responses:

• Identify where new cookies are set (Set-Cookie header), modified, or added to.
• Identify where there are any redirects (3xx HTTP status code), 400 status codes, in particular 403 Forbidden, and 500 internal server errors during normal responses (i.e., unmodified requests).
• Also note where any interesting headers are used. For example, "Server: BIG-IP" indicates that the site is load balanced. Thus, if a site is load balanced and one server is incorrectly configured, then the tester might have to make multiple requests to access the vulnerable server, depending on the type of load balancing used.

Black Box Testing

Testing for application entry points:
The following are two examples on how to check for application entry points.

EXAMPLE 1

This example shows a GET request that would purchase an item from an online shopping application.

GET https://x.x.x.x/shoppingApp/buyme.asp?CUSTOMERID=100&ITEM=z101a&PRICE=62.50&IP=x.x.x.x 
Host: x.x.x.x
Cookie: SESSIONID=Z29vZCBqb2IgcGFkYXdhIG15IHVzZXJuYW1lIGlzIGZvbyBhbmQgcGFzc3dvcmQgaXMgYmFy

Result Expected:

Here the tester would note all the parameters of the request such as CUSTOMERID, ITEM, PRICE, IP, and the Cookie (which could just be encoded parameters or used for session state).

EXAMPLE 2

This example shows a POST request that would log you into an application.

POST https://x.x.x.x/KevinNotSoGoodApp/authenticate.asp?service=login
Host: x.x.x.x
Cookie: SESSIONID=dGhpcyBpcyBhIGJhZCBhcHAgdGhhdCBzZXRzIHByZWRpY3RhYmxlIGNvb2tpZXMgYW5kIG1pbmUgaXMgMTIzNA==
CustomCookie=00my00trusted00ip00is00x.x.x.x00

Body of the POST message:

user=admin&pass=pass123&debug=true&fromtrustIP=true

Result Expected:

In this example the tester would note all the parameters as they have before but notice that the parameters are passed in the body of the message and not in the URL. Additionally, note that there is a custom cookie that is being used.

Gray Box Testing

Testing for application entry points via a Gray Box methodology would consist of everything already identified above with one addition. In cases where there are external sources from which the application receives data and processes it (such as SNMP traps, syslog messages, SMTP, or SOAP messages from other servers) a meeting with the application developers could identify any functions that would accept or expect user input and how they are formatted. For example, the developer could help in understanding how to formulate a correct SOAP request that the application would accept and where the web service resides (if the web service or any other function hasn't already been identified during the black box testing).

Tools

Intercepting Proxy:

• OWASP: Zed Attack Proxy (ZAP)
• OWASP: WebScarab
• Burp Suite
• CAT

Browser Plug-in:

• TamperIE for Internet Explorer
• Tamper Data for Firefox

References

Whitepapers

• RFC 2616 – Hypertext Transfer Protocol – HTTP 1.1 -

http://tools.ietf.org/html/rfc2616