Mobile Application Security : Windows Mobile Security - Development and Security Testing (part 3)

Code Security

The two primary development languages for Windows Mobile are C/C++ and .NET. However, several additional language runtimes have been ported to the platform, and developers can choose to write code targeting Python and others. For these alternative runtimes, application developers must have users install the runtime manually or they must include the runtime with the application.

C/C++ Security

C and C++ are the primary development languages for Windows Mobile. Both of these languages provide access to the entire Windows Mobile API set. Because programmers must manually manage memory in C/C++ and there is no intermediate runtime required for execution, Microsoft refers to code written in these languages as native code. Native code provides no protections against memory corruption vulnerabilities such as buffer overflows, integer overflows, and heap overflows. The onus is placed on the programmer to prevent these vulnerabilities through secure coding practices.

Fortunately, many of the protection technologies introduced first in desktop Windows have been ported to Windows Mobile. Using these technologies, developers can write more secure code that has a lower chance of being successfully exploited. The three main technologies are StrSafe.h, IntSafe.h, and Stack Cookie protection.

StrSafe.h

Many buffer overflows result from mishandling string data during copying, formatting, and concatenation operations. Standard string functions such as strcpy, strncpy, strcat, strncat, and sprintf are difficult to use, do not have a standard interface, and fail to provide robust error information. Microsoft introduced the StrSafe.h string-manipulation library to help developers working with strings by addressing all of these problems. StrSafe.h is included within the Windows Mobile 6 SDK and defines the following functions: StringXXXCat, StringXXXCatN, StringXXXCopy, StringXXXCopyN, StringXXXGets, StringXXXPrintf, and StringXXXLength. In the preceding function definitions, XXX is replaced with either Cch for functions that work with character counts or Cb for functions that require the number of bytes in either the input or output buffer.

StrSafe.h functions always require the size of the destination buffer and always null-terminate the output. Additionally, StrSafe.h returns detailed status through an HRESULT. Using StrSafe.h is as simple as including the StrSafe.h file in the target project. StrSafe.h undefines all of the functions it is designed to replace, thus leading to compile errors. These errors are eliminated by replacing the dangerous functions, such as strcpy, with their StrSafe.h equivalents. For more detail and full guidance on how to use StrSafe.h, review the Microsoft documentation on MSDN (http://msdn.microsoft.com/en-us/library/ms647466.aspx).

IntSafe.h

Integer overflows are another native code issue that often leads to security vulnerabilities. An integer overflow results when two numbers are added or multiplied together and the result exceeds the maximum value that can be represented by the integer type. For example, adding 0x0000FFFF to 0xFFFFFFF3 exceeds the maximum value that can be stored in a DWORD. When this happens, the calculation overflows and the resulting value will be smaller than the initial value. If this overflowed size is used to allocate a buffer, the buffer will be smaller than expected. A subsequent buffer overflow could result from this poorly sized buffer. The solution for integer overflows involves checking every mathematical operation for overflow. Although this seems straightforward, several potential problems can occur due to the complexity of C/C++’s type system.

IntSafe.h provides addition, subtraction, multiplication, and conversion functions for performing integer operations safely. Use these functions when doing any integer operations with user-supplied data. Each function returns an HRESULT value indicating whether the operation succeeded or if an integer overflow occurred. For more detail, review the IntSafe.h documentation on MSDN (http://msdn.microsoft.com/en-us/library/dd361843%28VS.85%29.aspx). The following sample code shows how to use the DWordAdd function properly:

//dwResult holds the output of the calculation.
DWORD dwResult = 0;

//dwUserData is supplied by the user
//0xFFFF is the value to add to dwUserData
if (FAILED(DWordAdd(dwUserData, 0xFFFF, &dwResult))
{
      //An integer overflow or underflow occurred.
      //Exit the program or handle appropriately.
}

Stack Cookie Protection

The final protection for native code is the Stack Cookie protection mechanism, also referred to as “/GS,” which is the compiler parameter used to turn it on. Stack Cookies are used to mitigate buffer overflows that occur when stack-based data is overwritten. Included on the stack are return addresses, and if these addresses are overwritten an attacker can gain control of a program’s execution. To mitigate this risk, the compiler places a “cookie” between user data and the return address. This cookie is a random value generated on application startup. In order to reach the return address, an attacker has to overwrite the cookie. Before using the return address, the application checks to see if the cookie has been modified. If the cookie has changed, the application assumes a buffer overflow has occurred and the program quickly exits. This mechanism has reduced the exploitability of many stack-based buffer overflows and continues to improve with each new version of Microsoft’s compiler.

Unlike StrSafe.h or IntSafe.h, enabling Stack Cookie protection does not require code modifications because the cookie-checking code is automatically inserted at compile time. Additionally, Stack Cookie protection does not actually remove vulnerabilities from code; it simply makes them more difficult to exploit. Non-stack-based buffer overflows, such as heap overflows, are not mitigated by Stack Cookie protection. Mitigating these vulnerabilities by fixing code is still a necessity. The Visual Studio 2005 compiler enables the /GS flag by default, and forces developers to explicitly disable it. Therefore, almost all recently compiled applications have Stack Cookie protection enabled.

.NET Compact Framework Languages

Windows Mobile includes the .NET Compact Framework (.NET CF), a mobile version of Microsoft’s .NET Framework. The .NET CF consists of a runtime, which provides memory management capabilities, and an extensive class library to support application developers. The most current version is 2.0, which is included as part of the Windows Mobile OS. Prior versions of the .NET CF had to be distributed by application developers manually.

.NET CF supports writing code in both Visual Basic .NET (VB.NET) and C# (pronounced C-sharp). This code is referred to as managed code by Microsoft. All managed languages are compiled by the .NET CF to bytecode known as Microsoft Intermediate Language (MSIL). The .NET CF runtime runs MSIL to carry out the program’s instructions. The class library included with the .NET CF is expansive and includes functions for using the majority of the phone’s capabilities. Developers use this class library instead of the Windows Mobile Native API. For cases where the .NET CF does not include a function for using a phone platform, developers can use Platform Invoke (P/Invoke). This is a marshalling method for calling functions contained within native code.

Because the .NET CF runtime manages memory for developers, integer overflows and buffer overflows are very rare in .NET CF code. Generally, memory corruption vulnerabilities only occur when developers misuse P/Invoke functionality. This is because P/Invoke is similar to using the Native API directly, and it is possible to provide incorrect parameters to system calls, thus leading to memory corruption. If developers avoid using P/Invoke, code vulnerabilities should be limited to business logic flaws.

There is a performance impact to using managed code, and developers often choose to write native code for performance-critical applications. As mobile device memory and processing power increase, more developers will write managed applications, thus further reducing the potential for memory management errors.

PythonCE

PythonCE is a port of the popular Python scripting language to Windows Mobile. The runtime is freely available and includes much of the class library and functionality from Python 2.5. Because Python is a scripting language and does not require compilation, it is a useful tool for exploring Windows Mobile. PythonCE is not signed and runs at the Normal privilege level. To call Privileged APIs from PythonCE script, configure the security policy to Unlocked.

To call native platform APIs, use the ctypes interop package. This package can load DLLs, marshal parameters, and call platform methods. Due to a large distribution size and complexity in porting Python to Windows CE, PythonCE development has slowed. The project continues, but updates are slow in coming.

Application Packaging and Distribution

The methods for distributing Windows Mobile applications include CAB files, PC installers, and SMS download. The Cabinet (CAB) file format is used for packaging applications regardless of distribution mechanism. Applications can also be distributed through raw file copy to the device’s file system, but this presents two drawbacks: not having an installer and not having the application registered with the system’s program manager.

CAB Files

The CAB file format was originally developed for distributing desktop Windows installation media and is used in many Microsoft technologies. Each CAB file can contain multiple files and/or directories; optionally, the CAB file can be compressed. Unlike most archive file formats, CAB files are considered executables and are therefore subject to the same security policies. Developers bundle the application and any required resource files within the CAB file; this way, applications can be distributed as one single file. The desktop Windows Explorer supports the CAB file format, so CAB files can be easily opened and extracted on the PC.

Windows Mobile applications packaged in CAB files can also contain custom setup code, application provisioning information, and registry key information. This functionality is implemented not within the CAB format itself, but by including a special provisioning XML document the Windows Mobile application installer looks for. This document must be named _setup.xml and be stored in the root folder of the CAB archive. When the user installs the CAB file, Windows Mobile will open the _setup.xml file and carry out the provisioning instructions within.

The _setup.xml file contains wap_provisioning XML, and it’s capable of modifying much of the device’s configuration. The wap_provisioning format is documented in detail on MSDN and is relatively easy to read after the first couple of times. The registry and file elements are the most interesting when you are security-testing and reverse-engineering an application’s install process. The following XML blob shows the portion of a _setup.xml file used for installing files. Each node includes an XML comment describing the node’s purpose.

<!-- Mark the start of a file operation -->
<characteristic type="FileOperation">
    <!-- Signals a directory node named "\Windows" -->
    <characteristic type="\Windows" translation="install">
        <!-- Instruct the Installer to Create the Directory -->
        <characteristic type="MakeDir" />
        <!-- Signals a file node named "mypro_image.bmp" -->
        <characteristic type="cclark_image.bmp" translation="install">
            <!-- Instruct the installer to expand the file -->
            <characteristic type="Extract">
                <!-- The file "MYPRO~1.001" will be expanded to
                     "cclark_image.bmp" in the "\Windows" directory -->
                <parm name="Source" value="MYPRO~1.001" />
                <parm name="WarnIfSkip" />
            </characteristic>
        </characteristic>
    </characteristic>
</characteristic>

A minor annoyance is that all files stored within the Windows Mobile CAB archive must be named in the 8.3 file format (for example, MYPRO~1.001), a holdover from the format’s use during the days of MS-DOS. Truncated filenames make browsing the CAB file for executables or DLLs difficult. To work around this, either install the application to an emulator and copy the files off, or read _setup.xml to find executable files and their 8.3 sources. Either method involves manual effort, but unfortunately this is the only way.

Windows Mobile files can also contain a CE Setup DLL. This DLL contains native code that is invoked before and after installation. Installation authors use the setup DLL to perform custom installation steps that cannot be expressed using wap_provisioning XML. The DLL will run with the permissions of the CAB file granted by the device’s security policy.

CAB files can be signed with an Authenticode signature. The signature is embedded within the CAB file and maintains the integrity of the CAB file’s metadata and contents. The signature prevents tampering and enables users to make trust decisions based on the publisher of an application. To view the signature, use the Security Configuration Manager tool and select Check File Signature from the File menu. Browse to the desired CAB file and click Open. Security Configuration Manager will display the signature on the CAB file.

To generate CAB files, use the CabWiz.exe tool bundled with Visual Studio. To use this tool properly, an Information File (.INF) must be provided that lists the application’s publisher, files bundled with the application, registry keys and default values, and other information such as the shortcuts to create upon installation. CabWiz.exe consumes the .INF file, generates the appropriate _setup.xml file, renames installation files, and produces the output CAB file. This file can then be signed and deployed to devices.

Manual Deployment

To deploy CAB files manually, copy the CAB file to the device and navigate to the containing directory using the device’s File Explorer. Selecting the CAB file will invoke the installer and process the CAB file. After installation is complete, Windows Mobile displays a status message and adds the program to the device’s Program directory.

PC-based Deployment

Applications can be deployed from a PC when a device is cradled. To package these applications, developers create a Windows Installer package and device CAB package. When the Windows installer runs, it invokes the Mobile Application Manager (CeAppMgr.exe) and registers the application for installation the next time the device is cradled. When the user cradles a device, the Mobile Application Manager is launched and the application is pushed to the device for installation. The user is then able to manage the application through the Mobile Application Manager on their PC. The same signing requirements as manual deployment are enforced.

OTA SMS Deployment

Starting with Pocket PC 2003, applications can be deployed using SMS messages. The SMS messages appear within the user’s Message inbox. When a user reads the message, they can choose whether or not to install the application. If they select to install the application, the CAB file will be downloaded and then executed on the device. Some mobile software providers, such as Handango, distribute purchased applications using this technique.