Business Logic Decision During testing it is crucial to identify the key parameters related to business logic and understand how application handles them. This section will focus on insecure business logic decisions that are based on such parameters. Two such cases are listed below, it is important to look for such scenarios in the application while testing. 1. Use of non-editable controls – Applications may use the values of non-editable controls, drop-down menus, hidden fields or query string parameters for business logic processing. If such fields contain values like the type of the user, nature of the request, status of the transaction, etc. the attackers will get a chance to manipulate them and perform unauthorized operations. The application developers must understand that such fields are non-editable only in the context of the proxy tool. The attackers can easily modify their values using a proxy editor tool and try to manipulate business logic. 2. Business logic decision based on presence or absence of certain parameters - This is especially observed in ASP.NET applications where there is provision to make the server side controls hidden/invisible for certain users. However, in most cases it has been observed that if the users add the parameters corresponding to the UI elements that are kept hidden/invisible to them into the request, they are able to change the behaviour of the server side logic. Consider a scenario where only admin user can change password of other users of the system, as a result the field to enter username is only made visible to the admin user. However, if a normal a user tries to add username parameter in the request he/she will be able to trick the server in believing that the request has come from an admin user and try to change password of other users. Thus there exists a hole in such applications where the server side behaviour can be influenced with request parameters. Users can perform unauthorized operations in the application by supplying the values for the inputs fields that are hidden from them. Secure Design Recommendation: ● The application must not expose such parameters to the client. ● If they are exposed, the application must not rely on request parameters for logical decisions. It must maintain a separate copy of such values at the server side and use the same for business logic processing. ● Apply proper authorization checks on the server side for all transactions, wherever necessary. Do not depend on presence of a user input for such decisions.
Business Logic Invocation Technique Introduction In most of the design techniques the request parameters or the URL’s serve as sole factors to determine the processing logic. In such a scenario the elements in the request which are used for such identifications may be subject to manipulation attacks to obtain access to restricted resources or pages in the application. Consider a design below; here the business logic class is identified based on a configuration file that keeps the mapping of the request URL and the business logic class i.e. action class. Image two
What is the flaw? A flaw in such a design could be unused configurations present in the configuration file. Such configurations that are not exposed as valid features in the application and could serve as a potential backdoor to it. An unused configuration present in the configuration file of the application is shown below: Image three Observe that the “TestAction” has an insecure logic to delete records from the system. This can act as a potential backdoor to the application. Image four. Consider another scenario In the some designs request parameters are used to identify business logic methods. In the figure shown below a request parameter named “event” is used to identify and invoke the corresponding event handling methods of the business logic/action class. Image five What is the flaw? Here, the user can attempt to invoke the methods of the events that are not visible to the user. Secure Design Recommendation: The applications must ensure to: ● Remove ALL redundant/test/unexposed business logic configurations from the file ● Apply necessary authorization check before processing business logic method ● Maintain a mapping of method/class/view names with the privilege level of the users, wherever applicable and restrict access of the users to restricted URLs/methods/views. Review Criteria Understand the business logic invocation technique used in the design of any application. Check if the user inputs are directly (i.e. without any restriction) used to determine any of the following elements (as applicable): ● Business logic class ● Method names ● View component
Data Binding Technique Introduction Another popular feature seen in most of the design frameworks today is data binding, where the request parameters get directly bound to the variables of the corresponding business/command object. Binding here means that the instance variables of such classes get automatically initialized with the request parameter values based on their names. Consider a sample design given below; observe that the business logic class binds the business object with the request parameters. Image five What is the flaw? The flaw in such design is that the business objects may have variables that are not dependent on the request parameters. Such variables could be key variables like price, max limit, role etc. having static values or dependent on some server side processing logic. A threat in such scenarios is that an attacker may supply additional parameters in request and try to bind values for unexposed variable of business object class. As illustrated in the figure below, the attacker sends an additional “price” parameter in the request and binds with the unexposed variable “price” in business object, thereby manipulating business logic. Image six Secure Design Recommendation: ● An important point to be noted here is that the business/form/command objects must have only those instance variables that are dependent on the user inputs. ● If additional variables are present those must not be vital ones like related to the business rule for the feature. ● In any case the application must accept only desired inputs from the user and the rest must be rejected or left unbound. And initialization of unexposed of variables, if any must take place after the binding logic.
Review Criteria Review the application design and check if it incorporates a data binding logic. In case it does, check if business objects/beans that get bound to the request parameters have unexposed variables that are meant to have static values. If such variables are initialized before the binding logic this attack will work successfully.
Execution Flow Introduction Execution flow is another important consideration of design. The execution flow must terminate appropriately in case of an error condition. However, due to mishandling of some programming entities there could be a big hole in the application which would allow unrestricted access to applications. One such flaw is related to the “sendRedirect” method in J2EE applications.
This method is used to send a redirection response to the user who then gets redirected to the desired web component whose URL is passed an argument to the method. One such misconception is that execution flow in the Servlet/JSP page that is redirecting the user stops after a call to this method. Take a look at the code snippet below, it checks for authenticated session using an “if” condition. If the condition fails the response.sendRedirect() is used to redirect the user to an error page. Image 11 Note that there is code present after the If condition, which continues to fetch request parameters and processes business logic for instance adding a new branch entry of a bank in this case. What is the flaw? This flaw manifests as a result of the misconception that the execution flow in the JSP/Servlet page stops after the “sendRedirect” call. However it does not; in this case the execution of the servlet would continue even if an invalid session is detected by the “if” condition and thus the business logic will get processed for unauthenticated requests. Note: The fact that execution of a servlet or JSP continues even after sendRedirect() method, also applies to Forward method of the RequestDispatcher Class. However, <jsp:forward> tag is an exception, it is observed that the execution flow stops after the use of <jsp:forward> tag.
Secure Design Recommendation: Since this flaw results from the assumption made by developers that control flow execution terminates after a sendRedirect call, the recommendation would be to terminate the flow using a “return” statement. Review criteria Check if there is an appropriate logic to terminate the execution flow is present in case of an error condition. Check for similar instances of insecure security controls built using “sendRedirect” method. References: http://artechtalks.blogspot.in/ http://packetstormsecurity.com/files/119129/Insecure-Authentication-Control-In-J2EE.html