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Issue of March 2007
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Application security starts with the developer

Building secure applications is hard. Software can automate parts of it for you. Sachin Khanna, Sales Director, Alternate Channels, Compuware Asia Pacific Pte Ltd. Asia South Region.


Sachin Khanna

Building secure applications requires a certain mindset that doesn’t come naturally to most developers. First we try to get our logic correct and our algorithms working, and build the application so that it does fundamentally what it’s supposed to do. As we address the minor functionality issues and bugs, we also strive to streamline the code to improve performance and give the end user a better experience.

Last, if at all, we turn our attention to how secure that code actually is. And at that point, there are only a limited number of things we can do to improve it. This is where another mindset prevents us from identifying weaknesses in code. Developers want to see their applications do well, and unconsciously don’t delve into the more difficult and obscure areas of the problem. Security, after all, doesn’t get people anywhere near as excited as a cool user interface or useful features no one had thought of before. Let’s concentrate on the visible parts of our applications, because that’s where the rewards are.

And when we do turn to security, we typically practice the maxim “security through obscurity,” not addressing the problem, but rather hiding it in unexpected parts of the code or surrounding it with difficult-to-comprehend code. This may not be intentional, but often it serves as the best security we can offer.

This mindset is a prescription for continuing along the same path applications have always taken. It’s an easy trap to fall into, especially because we simply can’t believe anyone would take the time and effort to hack our application. All developers have busy lives, filled with development deadlines, outside activities, and career and skills development. It’s difficult to imagine that someone can make the effort to painstakingly sit and pick at an application for days or even weeks until he or she encounters a weakness, then write code to specifically exploit that weakness.

But there is no denying that there are very talented, dedicated and patient hackers who have ready access to the equipment and information they need to disrupt an application for the expectation of theft, or for no reason at all except to demonstrate their skills. Whether or not they target your application is simply a matter of time and luck. It will happen, sooner or later.

The only defense is to not make it easy for them. If it’s easy, hackers can do real damage to the application and possibly the business in general. If it’s not very easy, it’s likely they will move on to other applications, on other servers, that present better opportunities. At the very least, it may take them so long that their attempts at compromise are noticed by those responsible for monitoring applications in production.

No matter what you do, making an application completely safe from compromise will be impossible. There is no application that is hackproof. Even if you take reasonable precautions with the application, it must execute within the environment of an operating system, hardware, network, and most important, end users. The most talented and determined hacker can compromise just about any commercial application.

Consider also that there is an inverse relationship between the security of an application and its applicability for a particular use. The only fully secure application is one running on a computer in a locked vault, with no I/O. Such an application, however, isn’t typically very useful. The easiest-to-use application is one that requires no authentication and provides the user with full access to everything within its scope, without requiring passwords or denying any information. This application, while potentially very useful, is fully open to compromise.

The application developer has to strike a balance by providing a reasonable set of access restrictions in an application while making useful its intended tasks. This balance varies depending on the likelihood of a security threat, the consequences of a breach, and the duties and sophistication of the user.

Setting out and working toward that balance is largely a function of application design. Building an application that reflects the specified level of security is more challenging. The primary problem is that just because code is accurate, doesn’t necessarily ensure its security. Secure code requires extra effort, along with a detailed knowledge of Windows vulnerabilities and how to properly code to protect against them. Most developers simply lack the experience to pinpoint areas of vulnerability and how to fix them.

Ideally, these types of vulnerabilities, which are known but often obscure and difficult to find and repair, could be identified and addressed automatically. There is yet no silver bullet for that task, however, it is possible to ease the process of writing code and analysing running code for weaknesses.

There are several stages in the development process where security can be applied. Writing code is the most obvious and important place, for a couple of reasons. First, it is the least expensive stage of the application life cycle in which to get security right. Second, during coding the developer is in the best position to identify and address potential security flaws.

Writing code with security in mind is a significant challenge. The problem is that many constructs have potential vulnerabilities and can be used in ways that either expose those vulnerabilities or hide them. Perfectly correct and generally accepted code can be compromised, given the right circumstances. Developers may not even be aware of many of the subtle nuances and side effects of the algorithms and constructs they use.

One possible difficulty is finding back doors allowed by the language and platform. For example, if you declare FileIOPermissionAttribute like this: FileIOPermissionAttribute (SecurityAction. Deny, Read=@”c:\passwords”)

You will block access to the passwords directory, but only if it’s accessed in that specific way. You can still use the UNC path (\\machine_name\passwords), or another route to that same directory, unless you also identify all possible routes and protect each of them.

These and many more detailed rules are difficult to remember, and to know when to apply.

By default, signed assemblies assign FullTrust permissions to public and private methods in public classes, so they can’t be executed by code that isn’t specifically given those permissions. By not signing your code or marking your assembly with the Allow Partially Trusted Callers Attribute, you expose your code to potential use by partially trusted or untrusted code. This could result in your code being hijacked by code inserted in your process, or executing in another process.

Analysing security at runtime

Once code is written and the application built, looking for security holes during runtime is also a significant challenge. Here, the problems tend to be better understood. The Microsoft .NET platform offers a sandbox approach to security, so that issues with one application can be contained within its sandbox. That doesn’t solve the problem entirely, but it does mitigate it.

Larger problems arise when a .NET application calls native Windows code, through a P/Invoke, COM callable wrapper, or other approach. The application’s flow of control leaves that sandbox, and is vulnerable to any weaknesses in the code it executes outside of the .NET Framework.

These types of weaknesses tend to be better known, because they have been found and exploited by hackers and virus writers over the years. But finding them in code, especially code that was written years ago, is a tedious and error-prone process. And developers writing new .NET applications often lack the time and experience needed to find and fix these weaknesses.

Possibly the most infamous of these errors is the buffer overrun. In C and C++, there’s no internal consistency check to ensure that there is enough memory allocated for the sise of a value. A developer can write those checks into the code, but many neglect to. A user, operating from the command line or on a Web page, can intentionally overrun the memory to try to gain control of the application. If it’s local memory, it overwrites the stack. This often crashes the application or at least the component, but occasionally it can allow malicious code to execute once the stack is cleared.

There are other weaknesses to native Windows code that operate in a similar way, by hijacking and using runtime memory. These include overwriting an array, using uninitialised memory and assigning a pointer out of range. Even a COM interface leak can be a weakness, as it can enable hostile code to gain control of the interface and pass bad data back into the calling .NET application.

Finding these weaknesses means going over every line of code, listing every declared variable, determining its allocated memory and putting error-checking code in places that are vulnerable. In addition to the time and expense involved, the potential holes are difficult to find in an application that may do millions of allocations and deallocations during its lifetime.

The process of identifying many memory-related security holes can be automated using automatic error detection tools. Simply running error detection can identify the potential for overrunning buffers, overwriting stack value and similar potential holes. A stack overrun error occurs in a running application and you will only find this if such an error is injected into the application. This means that this process should be combined with an active attempt at hacking to be completely effective.

Data in the clear

A third place where checking for weaknesses is vital involves passing data between components in a distributed application. Many developers believe that data passed within the application is secure because the user doesn’t have access to it. However, a virus or hostile program can observe the application and either determine its weaknesses in an attack, or how to best compromise data being used within the application.

For example, a hostile program can observe critical data being passed in the clear from one component to another. A clear text password typed on a web page can be captured, providing access to the user’s account. Alternatively, application developers have been known to embed the database password into a script, rather than elsewhere in the application. Some developers even use the database system administrator (sa) account and password for casual Web users to make database queries. Any hostile program that captures this password has full access to the production database.

Unfortunately, setting up database access and passing data across the tiers of a distributed application are often among the many detailed decisions individual developers make in implementing an application design. This level of implementation isn’t planned in advance. So unless individual developers are both trained in security concepts and vigilant in their implementation, it’s very possible that critical information in an application is vulnerable.

Identifying these vulnerabilities after the application is coded is a significant challenge, because you have to be able to look inside the application to watch how data moves about. Tools exist that can display data passed in the clear between application components, to assist in identifying these and similar vulnerabilities.

The tip of the iceberg

There are an almost infinite number of code implementations that can result in security weaknesses. Unless you specialise in identifying and repairing security holes in applications, it is unlikely any single developer can know about even a significant number of them. And that means your applications will almost certainly have security holes. Some of them will be serious. Without help, these holes will be in the production application. Having security holes doesn’t necessarily mean that your application will get hacked. You can rely on your good luck at not drawing the attention of hackers for protection, or that your code is simply too complex for a hacker to bother with. But both of those defenses will only get you so far. Why make it easy for a hacker? Automating the identification and diagnosis of security weaknesses will mean you need less luck, and can write better code in the process.

 
     
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