Full Report
Runtime analysis is an integral part of most application security assessment processes. Many powerful tools have been developed to perform execution/data flow analysis and code debugging for desktop and server operating systems. Although a few dynamic analysis tools such as DroidBox are available for Android, I currently know of no similar public tools for the Windows Phone 7 platform. The main challenge for Windows Phone 7 is the lack of a programable debugging interface in both the Emulator and phone devices. The Visual Studio 2010 debugger for Phone applications does not have an “Attach to process” feature and can only be used to debug applications for which the source code is available. Although the Kernel Independent Transport Layer (KITL) can be enabled on some Windows Phone devices at boot time which could be very useful for Kernel and unmanged code debugging, it can’t be used directly for code tracing of phone applications which are executed by the .NET compact framework.
Analysis Summary
# Research: Runtime analysis of Windows Phone 7 Applications
## Metadata
- Authors: behrang (SensePost Author)
- Institution: SensePost
- Publication: SensePost Blog
- Date: September 14, 2011 (with updates on 9/21/2011)
## Abstract
This analysis addresses the deficit of dynamic runtime analysis tools for the Windows Phone 7 platform, contrasting it with the availability of similar tools for desktop/server OSes and Android (e.g., DroidBox). The core challenge identified is the lack of a programmable debugging interface (like "Attach to process") in the WP7 Emulator and physical devices, which prevents standard debugging or tracing of compiled managed code applications ($\text{.NET}$ Compact Framework). The research proposes a novel, indirect method to capture execution and $\text{data flow}$ traces by instrumenting the application and hooking into the $\text{Emulator}$'s output process.
## Research Objective
To develop a methodology and associated tool capable of performing dynamic runtime analysis (execution/data flow tracing) on Windows Phone 7 applications, circumventing the limitations imposed by the platform's restricted debugging interfaces.
## Methodology
### Approach
The methodology relies on **application-level instrumentation** combined with **host-side process hooking** to capture runtime events without relying on the native debugger interfaces.
1. **Instrumentation:** The target Windows Phone 7 application is modified to print method names and variable values directly to the $\text{Emulator}$ console during execution.
2. **Console Enabling:** The $\text{Emulator}$ console must be enabled, typically via a specific registry key modification.
3. **Data Capture Hooking:** An **$\text{API Hook}$** targeting the `WriteFile` API within the $\text{Emulator}$ process is employed. This hook intercepts the output stream destined for the console.
4. **Trace Generation:** The intercepted console output is redirected and saved into a dedicated `runtrace` file.
### Dataset/Environment
The target environment is the Windows Phone 7 platform, specifically utilizing the **Windows Phone 7 $\text{Emulator}$**. The analysis is focused on applications executed via the $\text{.NET}$ Compact Framework.
### Tools & Technologies
* **Custom Tool:** "$\text{XAP Spy}$" (developed in $\text{C}\#$) to automate the process.
* **Hooking Library:** $\text{EasyHook}$ library (requires $\text{.NET}$ Framework 2.0).
* **Framework Requirements:** $\text{Windows Phone 7 SDK}$, $\text{.NET}$ Framework 4.0 and 2.0.
## Key Findings
### Primary Results
1. A functional, publicly available mechanism ($\text{XAP Spy}$) exists to capture execution traces and variable data flow from $\text{WP}7$ applications running in the $\text{Emulator}$.
2. The development of this tracing capability was necessitated by the inability to use standard debugging features like "Attach to process" provided by Visual Studio 2010 for $\text{WP}7$ applications.
3. The $\text{Kernel Independent Transport Layer}$ ($\text{KITL}$), while useful for kernel or unmanaged code debugging on some devices, cannot be directly used for tracing managed $\text{.NET}$ applications.
### Novel Contributions
* The development and release of "$\text{XAP Spy}$," providing the first known public dynamic analysis tool specifically tailored to $\text{WP}7$ applications by bypassing standard debugging restrictions.
* The technique of using application instrumentation *in conjunction* with a $\text{WriteFile}$ $\text{API}$ hook on the $\text{Emulator}$ process to capture dynamically generated trace logs.
## Technical Details
The solution leverages $\text{API}$ hooking, specifically targeting the `WriteFile` function called by the $\text{Emulator}$ process when writing data to the console stream. This technique redirects the instrumented application's standard output—which contains the dynamic execution log—to a persistent file, bypassing the $\text{Emulator}$'s limited terminal buffer. The $\text{XAP Spy}$ tool relies on the $\text{EasyHook}$ library, indicating the hook injection likely occurs into the $\text{Emulator}$ host process space.
## Practical Implications
### For Security Practitioners
This technique enables security assessments of proprietary $\text{WP}7$ applications where source code access is unavailable, allowing for behavioral analysis, $\text{data flow}$ reconstruction, and identification of sensitive operations, even without traditional dynamic debugging capabilities.
### For Defenders
Defenders within the $\text{WP}7$ ecosystem (though dating from 2011) gain a potential method for tracing potentially malicious or compromised applications if they can gain access to the $\text{Emulator}$ environment or similar control over the execution host.
### For Researchers
This research establishes a baseline methodology for dynamic analysis on restricted mobile platforms lacking formal debug interfaces, suggesting similar evasion techniques may be applicable elsewhere.
## Limitations
* The described trace capture mechanism is heavily dependent on the $\text{Emulator}$ environment and relies on enabling potentially undocumented features (the console registry key).
* The methodology involves instrumenting the application ($\text{XAP}$ modification), meaning it only applies to applications where the payload can be modified or built with instrumentation included.
* The $\text{KITL}$ limitation restricts this tracing method primarily to managed $\text{.NET CF}$ applications rather than lower-level code.
## Comparison to Prior Work
Prior work provided dynamic analysis for platforms like Android ($\text{DroidBox}$). This research fills a significant gap for $\text{WP}7$ by creating a functional equivalent method where existing formal debugging tooling (Visual Studio debugger) was insufficient due to missing features like process attachment.
## Future Work
The author implies the need for source code for the $\text{XAP Spy}$ tool to be available to allow the community to adapt and improve upon the methodology or apply it to emerging $\text{WP}7$ SDK versions (as reflected by the subsequent update providing $\text{SDK}7.1$ binaries). Further work would involve adapting this technique to physical devices if possible, or analyzing other aspects of the managed code execution environment.
## References
* DroidBox (Mentioned as a benchmark for Android dynamic analysis tools).
* Visual Studio 2010 Debugger documentation (Implied context for limitations).
* $\text{EasyHook}$ Library documentation (Source for $\text{API}$ hooking).