• Comments & Sensitive information embedded in the HTML source code
• Error messages generated at the server and HTTP response returned.
• Application error message 

During web application development, it is very important to think about these aspects. Comments or sensitive information can be very useful for you while developing and maintaining the code, but if it is accessed by malicious user it can be dangerous. Similarly, detailed error messages given to improve usability can results in the security loophole.

HTML source present on the client side can be an excellent source of information for the attacker. It is very easy for everyone to view HTML source code and since it is not compiled, there is no way to hide HTML comments. For testing web applications for security, you should look for the sensitive information like passwords, usernames, database names, connection strings etc. As a person responsible for security testing you need to make sure that sensitive information is not present in the HTML source code.

In order to start attack on any web application, it is important to know how its pages can be accessed, what kind of data and parameters are passed from one page to another. You can keep an eye on the URL for this purpose and look for key-value pairs.  You should always consider creating page map of your site containing this information. You can use tool or can create it manually by navigating to all the pages and making appropriate maps. After you have created this page map, you can search HTML source for specific strings containing information related to HTML comments, Application Comments, IP Address, E-Mail Address, SQL Queries, Database Connection Strings, Hidden input fields etc.

If you try to change parameters selected during this process and resubmit the request, you can find interesting information in error messages. Sometimes server or application throws overly helpful error messages which can give subtle hints to the attackers.  For example on supplying invalid password during login process if system throws error like 'invalid password' , it essentially means that username is proper.

Page map created earlier can also help you in guessing file names and directory structure present on the web server. Using this technique you can access files for which there is no link present on the pages, or which is not intended to be visible to the user. You must always check for the presence of any pattern in file names and location. Files which should not be viewed by clients should be located in places not accessible to the clients. Using this technique of educated guessing, malicious users can even access admin or control panel of the website, which usually runs as a separate sub-site or run on a different port. These type of loopholes can be identified easily by tools like port scanners and other brute-force tools.

Vulnerability of the web application can also be exposed by manipulating UI controls. For example, you might have used websites containing list boxes to take user input. Reason for providing list box is to make sure that client do not choose any other option apart from whatever is supplied in the list. It is very easy for development team to make this assumption and not do any other form of validation. What they fail to realize is that, these values can be changed by making changes in the page source even request can be tampered on transit using appropriate tools. Also if validations are present at the client-side, it is still possible to bypass those validations. This could be achieved either by disabling the java scripts or saving a local copy of the file and removing those validations. To safeguard from these vulnerabilities, it is essential to make sure that validations are present on the server side as well.

Another thing that should be checked at the client side is cookies. For people not familiar with cookies, cookies are small files of textual data that a web application writes on a client's hard drive. Web application can reuse this data on subsequent visits. Cookies can be delivered by web application using either persistent/non-persistent and secure/non-secure mode.  Cookies can be used for personalization or making sure that information is not accessed after it is expired. There are many ways in which cookies can be used. Cookies are normally stored at predefined location with predefined formats. If your application relies on cookies for any functionality, it is essential for you to make sure that tempered cookies can not be used with your application.

Hope after understanding the different ways in which security can be compromised, you will appreciate  the importance of security testing of web applications. In the next article, we will explore how data supplied by client can be tempered and servers can be attacked.

You can read more articles on software testing in our article section. You can suggest topics of your interest here , we will try to provide information on those topics as well.

These articles are influenced by the book ( “How to Break Web Software” from Mike Andrews and James A. Whittaker ) I have recently read and should be a good read for you if you need information on web application security testing.