by Jody Melbourne and David Jorm
This is the first in a series of three articles on penetration testing for Web applications. The first installment provides the penetration tester with an overview of Web applications - how they work, how they interact with users, and most importantly how developers can expose data and systems with poorly written and secured Web application front-ends.
Note: It is assumed that the reader of this article has some knowledge of the HTTP protocol - specifically, the format of HTTP GET and POST requests, and the purpose of various header fields. This information is available in RFC2616.
Web applications are becoming more prevalent and increasingly more sophisticated, and as such they are critical to almost all major online businesses. As with most security issues involving client/server communications, Web application vulnerabilities generally stem from improper handling of client requests and/or a lack of input validation checking on the part of the developer.
The very nature of Web applications - their ability to collate, process and disseminate information over the Internet - exposes them in two ways. First and most obviously, they have total exposure by nature of being publicly accessible. This makes security through obscurity impossible and heightens the requirement for hardened code. Second and most critically from a penetration testing perspective, they process data elements from within HTTP requests - a protocol that can employ a myriad of encoding and encapsulation techniques.
Most Web application environments (including ASP and PHP, which will both be used for examples throughout the series), expose these data elements to the developer in a manner that fails to identify how they were captured and hence what kind of validation and sanity checking should apply to them. Because the Web "environment" is so diverse and contains so many forms of programmatic content, input validation and sanity checking is the key to Web applications security. This involves both identifying and enforcing the valid domain of every user-definable data element, as well as a sufficient understanding of the source of all data elements to determine what is potentially user definable.
The Root of the Issue: Input Validation
Input validation issues can be difficult to locate in a large codebase with lots of user interactions, which is the main reason that developers employ penetration testing methodologies to expose these problems. Web applications are, however, not immune to the more traditional forms of attack. Poor authentication mechanisms, logic flaws, unintentional disclosure of content and environment information, and traditional binary application flaws (such as buffer overflows) are rife. When approaching a Web application as a penetration tester, all this must be taken into account, and a methodical process of input/output or "blackbox" testing, in addition to (if possible) code auditing or "whitebox" testing, must be applied.
What exactly is a Web application?
A Web application is an application, generally comprised of a collection of scripts, that reside on a Web server and interact with databases or other sources of dynamic content. They are fast becoming ubiquitous as they allow service providers and their clients to share and manipulate information in an (often) platform-independent manner via the infrastructure of the Internet. Some examples of Web applications include search engines, Webmail, shopping carts and portal systems.
How does it look from the users perspective?
Web applications typically interact with the user via FORM elements and GET or POST variables (even a 'Click Here' button is usually a FORM submission). With GET variables, the inputs to the application can be seen within the URL itself, however with POST requests it is often necessary to study the source of form-input pages (or capture and decode valid requests) in order to determine the users inputs.
An example HTTP request that might be provided to a typical Web application is as follows:
Every element of this request can potentially be used by the Web application processing the request. The REQUEST-URI identifies the unit of code that will be invoked along with the query string: a separated list of &variable=value pairs defining input parameters. This is the main form of Web applications input. The Session-ID header provides a token identifying the client's established session as a primitive form of authentication. The Host header is used to distinguish between virtual hosts sharing the same IP address and will typically be parsed by the Web server, but is, in theory, within the domain of the Web application.
As a penetration tester you must use all input methods available to you in order to elicit exception conditions from the application. Thus, you cannot be limited to what a browser or automatic tools provide. It is quite simple to script HTTP requests using utilities like curl, or shell scripts using netcat. The process of exhaustive blackbox testing a Web application is one that involves exploring each data element, determining the expected input, manipulating or otherwise corrupting this input, and analysing the output of the application for any unexpected behaviour.
The Information Gathering Phase
Fingerprinting the Web Application Environment
One of the first steps of the penetration test should be to identify the Web application environment, including the scripting language and Web server software in use, and the operating system of the target server. All of these crucial details are simple to obtain from a typical Web application server through the following steps:
Hidden form elements and source disclosure
In many cases developers require inputs from the client that should be protected from manipulation, such as a user-variable that is dynamically generated and served to the client, and required in subsequent requests. In order to prevent users from seeing and possibly manipulating these inputs, developers use form elements with a HIDDEN tag. Unfortunately, this data is in fact only hidden from view on the rendered version of the page - not within the source.
There have been numerous examples of poorly written ordering systems that would allow users to save a local copy of order confirmation pages, edit HIDDEN variables such as price and delivery costs, and resubmit their request. The Web application would perform no further authentication or cross-checking of form submissions, and the order would be dispatched at a discounted price!
This practice is still common on many sites, though to a lesser degree. Typically only non-sensitive information is contained in HIDDEN fields, or the data in these fields is encrypted. Regardless of the sensitivity of these fields, they are still another input to be manipulated by the blackbox penetration tester.
All source pages should be examined (where feasible) to determine if any sensitive or useful information has been inadvertently disclosed by the developer - this may take the form of active content source within HTML, pointers to included or linked scripts and content, or poor file/directory permissions on critical source files. Any referenced executables and scripts should be probed, and if accessible, examined.
This suggests that the application is trying to protect the form handler from quantity values of 255 of more - the maximum value of a
Determining Authentication Mechanisms
Besides the obvious problem of clear text credentials when using Basic, there is nothing inherently wrong with HTTP authentication, and this clear-text problem be mitigated by using HTTPS. The real problem is twofold. First, since this authentication is applied by the Web server, it is not easily within the control of the Web application without interfacing with the Web server's authentication database. Therefore custom authentication mechanisms are frequently used. These open a veritable Pandora's box of issues in their own right. Second, developers often fail to correctly assess every avenue for accessing a resource and then apply authentication mechanisms accordingly.
Given this, penetration testers should attempt to ascertain both the authentication mechanism that is being used and how this mechanism is being applied to every resource within the Web application. Many Web programming environments offer session capabilities, whereby a user provides a cookie or a Session-ID HTTP header containing a psuedo-unique string identifying their authentication status. This can be vulnerable to attacks such as brute forcing, replay, or re-assembly if the string is simply a hash or concatenated string derived from known elements.
Every attempt should be made to access every resource via every entry point. This will expose problems where a root level resource such as a main menu or portal page requires authentication but the resources it in turn provides access to do not. An example of this is a Web application providing access to various documents as follows. The application requires authentication and then presents a menu of documents the user is authorised to access, each document presented as a link to a resource such as:
Although reaching the menu requires authentication, the showdoc.asp script requires no authentication itself and blindly provides the requested document, allowing an attacker to simply insert the docid GET variable of his desire and retrieve the document. As elementary as it sounds this is a common flaw in the wild.
In this article we have presented the penetration tester with an overview of web applications and how web developers obtain and handle user inputs. We have also shown the importance of fingerprinting the target environment and developing an understanding of the back-end of an application. Equipped with this information, the penetration tester can proceed to targeted vulnerability tests and exploits. The next installment in this series will introduce code and content-manipulation attacks, such as PHP/ASP code injection, SQL injection, Server-Side Includes and Cross-site scripting.
This article originally appeared on SecurityFocus.com -- reproduction in whole or in part is not allowed without expressed written consent.