This project highlights some of the vital areas of design security. We are all aware of “secure coding” and practice it to great extent while developing applications. But do we give equal attention to – “Secure Design”? Most of us would probably say, NO. Design level flaws are lesser known concepts but their presence is a very big risk to the applications. Such flaws are hard to find in static or dynamic application scans and instead require deep understanding of application architecture and layout to uncover them manually. With increasing business needs the complexities in application design and architecture are also increasing. There is a rise in the use of custom design techniques and diverse technologies in the applications today, which makes the need for design reviews imperative. This project focuses on highlighting some important secure design principles that developers and architects must adapt to build a secure application design. With the help of some design flaws we will see the areas of design that are exposed to security risks and what measures can be taken to avoid them in our design. It also includes mitigation techniques that can be implemented in the applications to prevent them.
Understanding the design
What is an application design?
A design is a blueprint of an application; it lays a foundation for its development. It illustrates the layout of the application and identifies different application components needed for it. It is a structure that determines execution flow of the application. Most of the application designs are based on a concept of MVC. In such designs different components interact with each other in an ordered sequence to serve any user request.
Why should be review the design?
Design review should be an integral part of secure software development process. If the application is reviewed for security at the design level many inherent backdoors can be uncovered. Design reviews also help to implementing the security requirements in a better way.
The methodology to be followed for design reviews is explained below:
Collection of Design Documents:
This phase involves collecting required information of the proposed design. It would involve all kinds of documentation maintained by the development team about the design like flow charts, sequence diagrams, class diagrams etc. Requirements documents are also needed to understand the objective of the proposed design.
In this phase the design is thoroughly studied mainly with respect to the data flow, different application components and their interactions, data handling etc. This is achieved through manual analysis and discussions with the design or technical architect’s team. The design and the architecture of the application must be understood thoroughly to analyse vulnerable areas that can lead to security breaches in the application. The key areas of the design that must be considered during threat analysis are given below.
- Data Flow/Code Layout
- Access control
- Existing/Built-in Security controls
- Entry points of non-user inputs
- Integrations with external services
- Location of configurations file and data sources
- Add-ons and customization present (in case of built-in design framework)
This will help in identifying the trust boundaries for an application and thus aid in taking decisions about the vulnerabilities and their risk levels posed to the application.
After understanding the design the next phase is to analyse the threats for the design. This phase involves “threat modelling” the design.
The threats must be identified for different design areas that were identified in the previous step. It involves observing the design from an attacker’s perspective and uncovering the backdoors and insecure areas present in it. The analysis can be done broadly on the basis of 2 important criteria: 1. Insecure Implementation – This would mean the design has a loophole which can lead to a security breach in the application. For instance, insecure reference to business logic functions. 2. Lack of secure implementation – This would mean the design has not incorporated secure practices. For instance, in connection to external server different security requirements to protect confidentiality and integrity of the data are not present.
Similar instances are listed below to illustrate the points that should be broadly considered while analysing different design areas:
- Data Flow -
a. Are user inputs used to directly reference a business logic class/function b. Is there a data binding flaw? c. Does it expose any backdoor parameter to invoke business logic? d. Is the execution flow of the application correct?
- Authentication and access control -
a. Does the design implement access control for all the files? b. Does it handle session securely? c. Is there SSO, does it leave any backdoor?
- Existing/built-in Security Controls -
a. Weakness in any existing security control b. Is the placement of the security controls correct?
- Architecture –
a. Is there validation for all input sources? b. Is the connection to external servers secure?
- Configuration/code files and datastores -
a. Are sensitive data present in configuration files? b. Does it support any insecure data source?
A detailed checklist is available here - Excel Doc was Here At the end of this activity we get a list of threats or insecure areas applicable to the design.
Propose Security Requirements:
After analysing the insecure areas in the design in this step a list of security requirements corresponding to them must be created. Requirements are high level changes or additions to be incorporated in the design, for instance: Validate the inputs fetched from the webservice response before processing them. Any protection that is needed for resolving the vulnerable area identified in the design would go as a security requirement for the design. This phase involves listing all the security requirements for the design along with security risk associated with them. This risk based approach would help the development teams in prioritizing the security requirements.
Recommend Design Changes:
In this phase every security requirement must be associated with a security control. A security control best suited for the design is proposed and documented. These security controls are an elaborate view of the security requirements. Here, we would identify exact changes or additions to be incorporated in the design that are needed to meet any requirement or mitigate a threat. The changes or controls recommended for the design should be clear and detailed, as given in the instances below: a. Elaborate validation strategy with respect to: a. Identifying right application component like servlet filters, interceptors, validator classes etc. b. Placement of check c. Validation mechanism d. Use of 3rd party security API’s or inbuilt design features of the frameworks b. Encryption techniques c. Design Patterns And so on depending on the control to be built in the design.
Discussion with the design team:
The list of security requirements and proposed controls must be then discussed with the development teams. The queries of the teams should be addressed and feasibility of incorporating the controls must be determined. Exceptions, if any must be taken into account and alternate recommendations should be proposed. In this phase a final agreement on the security controls is achieved.
The final design incorporated by the development teams must be reviewed again and finalized for further development process.
This section describes some of the important design flaws that can leave a backdoor in the application to access it without authentication or manipulate its business logic. We will understand such flaws and secure design recommendations in detail.
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 users.
- 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
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.
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
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
- 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 the application design and check it is 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.
Placement of Security Controls
Placement of security checks is a vital area of review in an application design. Incorrect placements can render the applied security controls null and void. So, it is important to study the application design and spot the correctness of such checks in the overall execution flow of the design. Most of the application designs are based on the concept of Model-View-Controller (MVC). They have a central controller, which listens to all incoming request and delegates control to appropriate form/business processing logic. And ultimately the user is rendered with a view. In such a layered design, when there are many entities involved in processing a request, developers often go wrong in placing the security controls at the right place. Most application developers feel “view” is the right place to have the security checks like authentication check etc. Image seven
What is the flaw?
It thus seems logical that if you restrict the users at the page/view level they won’t be able to perform any operation in the application. But what if instead of requesting for a page/view an unauthorized user tries to request for an internal action like to action to add/modify any data in the application? It will get processed but the resultant view will be denied to the user; because the flaw lies in just having a view based access control in the applications. I am sure you will agree that a lot of processing for a request is done before the “view” comes into picture in any design. So the request to process any action will get processed successfully without authorization.
Consider a MVC based given in the figure below. Observe in the figure that the authentication check is present only in the view pages.
Observe that neither the controller servlet (central processing entity) nor the action classes have any access control checks. So here, if the user requests for an internal action like add user details, etc. without authentication it will get processed, but the only difference is that the user will be shown an error page as resultant view will be disallowed to the user. Image nine
Insecure POST-BACK’s in ASP.NET A similar flaw is predominantly observed in ASP.NET applications where the developers tend to mix the code for handling POSTBACK’s and authentication checks. Usually it is observed that the authentication check in the ASP.NET pages are not applied for POSTBACKs, as indicated below. Here, if an attacker tries to access the page without authentication an error page will be rendered. Instead, if the attacker tries to send an internal POSTBACK request directly without authentication it would succeed. A detailed explanation is present here. Image ten
Secure Design Recommendation:
It is imperative to place all validation checks before processing any business logic and in case of ASP.NET applications independent of the POSTBACKs.
Check if the placement of the security checks is correct. The security controls like authentication check must be place before processing any request.
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 – “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 who’s URL is passesd 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.
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.