Introduction to Hacking Thick Clients is a series of blog posts that will outline many of the tools and methodologies used when performing thick client security assessments. In conjunction with these posts, NetSPI has released two vulnerable thick clients: BetaFast, a premier Betamax movie rental service, and Beta Bank, a premier finance application for the elite. Many examples in this series will be taken directly from these applications, which can be downloaded from the BetaFast GitHub repo. A brief overview is covered in a previous blog post.
The thick clients we come across most often at NetSPI are written in C# and a bit long in the tooth. In these applications, there are two main GUI platforms: Windows Forms and Windows Presentation Foundation (WPF).
Windows Forms was state of the art in 2002. And it’s not half bad in 2020! Basically, a form is composed of controls, which are your typical Windows objects such as text boxes, labels, buttons, etc. And each of these objects has its own properties, methods, and events.
Consider the following Windows Forms login form:
An administrator’s credentials are saved. Nice! Let’s steal those for later!
Darn! Thankfully, there are some tools at our disposal that allow us to view and modify controls. The first is WinSpy. Simply target a control and note that the masked value is stored in plain text.
Another similar tool for targeting Windows Forms objects is Window Detective. Selecting a control will highlight the object in the desktop window hierarchy. There, you can view its properties, including the elusive password.
Here’s another Windows Forms example. This application verifies the Role on the client-side and has the text box disabled so it cannot be changed.
But with WinSpy and Window Detective, we can set the text property to Admin.
And why not just change the Enabled property to true?
Some applications attempt to ward off SQL injections or other attacks by limiting the length of input or the types of input characters. But relying on any client-side control is a clear security risk.
Windows Presentation Foundation
WPF was released a day before Tenacious D in The Pick of Destiny and has aged just as well. It’s more complicated to work with than Windows Forms, but allows for a much more customizable design. A WPF-focused tool such as Snoop is needed for this platform.
Authenticate to BetaFast as a typical user and note that there are four menu items: Home, Shopping Cart, Settings, Logout.
By connecting Snoop to the application process, the design tree can be expanded to reveal an AdminVisibility property set to Hidden.
Changing this value to Visible reveals additional menu items restricted to administrators.
By itself, this is not a vulnerability. Any functionality in these restricted pages should have server-side authorization checks. However, this is often not the case. Below, a User is able to create an Admin account and view all of the accounts in the database.
The payment page of this application verifies the total rental cost in the client, not the server.
That means we can use Snoop to get a nice discount on these Betamax tapes – perhaps store credit?
Modifying GUI elements in thick clients is rather straightforward, especially with a set of polished tools. The remediation steps for any of these vulnerabilities are simple as well:
Perform authorization and input validation on the server
Do not hide sensitive data behind GUI designs
I’ll add another link to our BetaFast GitHub repo here. Be sure to try some of these examples out yourself, and be sure to tune in for our next installment of Introduction to Hacking Thick Clients.
Necessary cookies help make a website usable by enabling basic functions like page navigation and access to secure areas of the website. The website cannot function properly without these cookies.
YouTube session cookie.
Marketing cookies are used to track visitors across websites. The intention is to display ads that are relevant and engaging for the individual user and thereby more valuable for publishers and third party advertisers.
Analytics cookies help website owners to understand how visitors interact with websites by collecting and reporting information anonymously.
Preference cookies enable a website to remember information that changes the way the website behaves or looks, like your preferred language or the region that you are in.
Unclassified cookies are cookies that we are in the process of classifying, together with the providers of individual cookies.
Cookies are small text files that can be used by websites to make a user's experience more efficient. The law states that we can store cookies on your device if they are strictly necessary for the operation of this site. For all other types of cookies we need your permission. This site uses different types of cookies. Some cookies are placed by third party services that appear on our pages.