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Microsoft didn’t sandbox Windows Defender


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Microsoft exposed their users to a lot of risks when they released Windows Defender without a sandbox. This surprised me. Sandboxing is one of the most effective security-hardening techniques. Why did Microsoft sandbox other high-value attack surfaces such as the JIT code in Microsoft Edge, but leave Windows Defender undefended?


As a proof of concept, I sandboxed Windows Defender for them and, am now open sourcing my code as the Flying Sandbox Monster. The core of Flying Sandbox Monster is AppJailLauncher-rs, a Rust-based framework to contain untrustworthy apps in AppContainers.


It also allows you to wrap the I/O of an application behind a TCP server, allowing the sandboxed application to run on a completely different machine, for an additional layer of isolation.


In this blog post, I describe the process and results of creating this tool, as well as thoughts about Rust on Windows.



Flying Sandbox Monster running Defender in a sandbox to scan a WannaCry binary.

The Plan

Windows Defender’s unencumbered access to its host machine and wide-scale acceptance of hazardous file formats make it an ideal target for malicious hackers. The core Windows Defender process, MsMpEng, runs as a service with SYSTEM privileges. The scanning component, MpEngine, supports parsing an astronomical number of file formats. It also bundles full-system emulators for various architectures and interpreters for various languages. All of this, performed with the highest level of privilege on a Windows system. Yikes.


This got me thinking. How difficult would it be to sandbox MpEngine with the same set of tools that I had used to sandbox challenges for the CTF community two years ago?


The first step towards a sandboxed Windows Defender is the ability to launch AppContainers. I wanted to re-use AppJailLauncher, but there was a problem. The original AppJailLauncher was written as a proof-of-concept example. If I had any sense back then, I would’ve written it in C++ Core rather than deal with the pains of memory management. Over the past two years, I’ve attempted rewriting it in C++ but ended up with false starts (why are dependencies always such a pain?).


But then inspiration struck. Why not rewrite the AppContainer launching code in Rust?

Building The Sandbox

A few months later, after crash coursing through Rust tutorials and writing a novel of example Rust code, I had the three pillars of support for launching AppContainers in Rust: SimpleDacl, Profile, and WinFFI.

  • SimpleDacl is a generalized class that handles adding and removing simple discretionary access control entries (ACE) on Windows. While SimpleDacl can target both files and directories, it has a few setbacks. First, it completely overwrites the existing ACL with a new ACL and converts inherited ACEs to “normal” ACEs. Also, it disregards any ACEs that it cannot parse (i.e. anything other than AccessAllowedAce and AccessDeniedAce. Note: we don’t support mandatory and audit access control entries.).
  • Profile implements creation of AppContainer profiles and processes. From the profile, we can obtain a SID that can be used to create ACE on resources the AppContainer needs to access.
  • WinFFI contains the brunt of the functions and structures winapi-rs didn’t implement as well as useful utility classes/functions. I made a strong effort to wrap every raw HANDLE and pointer in Rust objects to manage their lifetimes.

Next, I needed to understand how to interface with the scanning component of Windows Defender. Tavis Ormandy’s loadlibrary repository already offered an example C implementation and instructions for starting an MsMpEng scan.


Porting the structures and function prototypes to Rust was a simple affair to automate, though I initially forgot about array fields and function pointers, which caused all sorts of issues; however, with Rust’s built-in testing functionality, I quickly resolved all my porting errors and had a minimum test case that would scan an EICAR test file.



The basic architecture of Flying Sandbox Monster.

Our proof-of-concept, Flying Sandbox Monster, consists of a sandbox wrapper and the Malware Protection Engine (MpEngine). The single executable has two modes: parent process and child process. The mode is determined by the presence of an environment variable that contains the HANDLEs for the file to be scanned and child/parent communication. The parent process populates these two HANDLE values prior to creating an AppContainer’d child process. The now-sandboxed child process loads the malware protection engine library and scans the input file for malicious software.


This was not enough to get the proof-of-concept working. The Malware Protection Engine refused to initialize inside an AppContainer. Initially, I thought this was an access control issue. After extensive differential debugging in ProcMon (comparing AppContainer vs non-AppContainer execution), I realized the issue might actually be with the detected Windows version.


Tavis’s code always self-reported the Windows version as Windows XP. My code was reporting the real underlying operating system; Windows 10 in my case. Verification via WinDbg proved that this was indeed the one and only issue causing the initialization failures. I needed to lie to MpEngine about the underlying Windows version. When using C/C++, I would whip up a bit of function hooking code with Detours.


Unfortunately, there was no equivalent function hooking library for Rust on Windows (the few hooking libraries available seemed a lot more “heavyweight” than what I needed). Naturally, I implemented a simple IAT hooking library in Rust (32-bit Windows PE only).

Introducing AppJailLauncher-rs

Since I had already implemented the core components of AppJailLauncher in Rust, why not just finish the job and wrap it all in a Rust TCP server? I did, and now I’m happy to announce “version 2” of AppJailLauncher, AppJailLauncher-rs.


AppJailLauncher was a TCP server that listened on a specified port and launched an AppContainer process for every accepted TCP connection. I tried not to reinvent the wheel, but mio, the lightweight IO library for Rust, just didn’t work out. First, mio’s TcpClient did not provide access to raw “socket HANDLEs” on Windows. Second, these raw “socket HANDLEs” were not inheritable by the child AppContainer process. Because of these issues, I had to introduce another “pillar” to support appjaillauncher-rs: TcpServer.


TcpServer is responsible for instantiating an asynchronous TCP server with a client socket that is compatible with STDIN/STDOUT/STDERR redirection. Sockets created by the socket call cannot redirect a process’s standard input/output streams. Properly working standard input/output redirection requires “native” sockets (as constructed via WSASocket). To allow the redirection, TcpServer creates these “native” sockets and does not explicitly disable inheritance on them.

My Experience with Rust

My overall experience with Rust was very positive, despite the minor setbacks. Let me describe some key features that really stood out during AppJailLauncher’s development.


Cargo. Dependency management with C++ on Windows is tedious and complex, especially when linking against third-party libraries. Rust neatly solves dependency management with the cargo package management system. Cargo has a wide breadth of packages that solve many common-place problems such as argument parsing (clap-rs), Windows FFI (winapi-rs et. al.), and handling wide strings (widestring).


Built-in Testing. Unit tests for C++ applications require a third-party library and laborious, manual effort. That’s why unit test are rarely written for smaller projects, like the original AppJailLauncher. In Rust, unit test capability is built into the cargo system and unit tests co-exist with core functionality.


The Macro System. Rust’s macro system works at the abstract syntax tree (AST) level, unlike the simple text substitution engine in C/C++. While there is a bit of a learning curve, Rust macros completely eliminate annoyances of C/C++ macros like naming and scope collisions.


Debugging. Debugging Rust on Windows just works. Rust generates WinDbg compatible debugging symbols (PDB files) that provide seamless source-level debugging.


Foreign Function Interface. The Windows API is written in, and meant to be called from, C/C++ code. Other languages, like Rust, must use a foreign function interface (FFI) to invoke Windows APIs. Rust’s FFI to Windows (the winapi-rs crate) is mostly complete. It has the core APIs, but it is missing some lesser used subsystems like access control list modification APIs.


Attributes. Setting attributes is very cumbersome because they only apply to the next line. Squashing specific code format warnings necessitates a sprinkling of attributes throughout the program code.


The Borrow Checker. The concept of ownership is how Rust achieves memory safety. Understanding how the borrow checker works was fraught with cryptic, unique errors and took hours of reading documentation and tutorials. In the end it was worth it: once it “clicked,” my Rust programming dramatically improved.


Vectors. In C++, std::vector can expose its backing buffer to other code. The original vector is still valid, even if the backing buffer is modified. This is not the case for Rust’s Vec. Rust’s Vec requires the formation of a new Vec object from the “raw parts” of the old Vec.


Option and Result types. Native option and result types should make error checking easier, but instead error checking just seems more verbose. It’s possible to pretend errors will never exist and just call unwrap, but that will lead to runtime failure when an Error (or None) is inevitably returned.


Owned Types and Slices. Owned types and their complementary slices (e.g. String/str, PathBuf/Path) took a bit of getting used to. They come in pairs, have similar names, but behave differently. In Rust, an owned type represents a growable, mutable object (typically a string). A slice is a view of an immutable character buffer (also typically a string).

The Future

The Rust ecosystem for Windows is still maturing. There is plenty of room for new Rust libraries to simplify development of secure software on Windows. I’ve implemented initial versions of a few Rust libraries for Windows sandboxing, PE parsing, and IAT hooking. It is my hope that these are useful to the nascent Rust on Windows community.


I used Rust and AppJailLauncher to sandbox Windows Defender, Microsoft’s flagship anti-virus product. My accomplishment is both great and a bit shameful: it’s great that Windows’ robust sandboxing mechanism is exposed to third-party software. It’s shameful that Microsoft hasn’t sandboxed Defender on its own accord.


Microsoft bought what eventually became Windows Defender in 2004. Back in 2004 these bugs and design decisions would be unacceptable, but understandable. During the past 13 years Microsoft has developed a great security engineering organization, advanced fuzzing and program testing, and sandboxed critical parts of Internet Explorer.


Somehow Windows Defender got stuck back in 2004. Rather than taking Project Zero’s approach to the problem by continually pointing out the symptoms of this inherent flaw, let’s bring Windows Defender back to the future.


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