Apr 12, 2022
Understanding and Defending Against Reflective Code Loading on macOS
Introduction
This blogpost will describe the concept of loading executables in-memory on macOS. This technique is mapped to MITRE ATT&CK under Reflective Code Loading (T1620).
Over the many years I’ve used Cobalt Strike for red teaming, I’ve always appreciated the ability to load third-party software such as .NET assemblies and PowerShell scripts, through execute-assembly and powershell/psinject/powerpick, respectively. The ability to load third-party tooling on the fly greatly increases the flexibility of an agent. I decided to explore this concept on macOS to eventually incorporate into Hermes.
After introducing the concept of reflective code loading on macOS, I present my Swift implementation. We will explore how this technique differs on Big Sur/Monterey and I will explain my methodology behind digging into these differences.
Lastly, we will look into how reflective code loading can be detected, on Monterey, by monitoring for temporary files created during the “in-memory” loading process.
Executing Mach-Os In-Memory
Note: Skip this section if you’re familiar with reflective code loading on macOS and want to dive into differences on how it works on Big Sur vs Monterey.
On macOS we can use the dynamic linker APIs to load two file types in-memory:
- Mach-O Binaries
- Loadable Bundles (.bundle)
The following macOS API calls can be used to execute and clean-up Mach-Os in-memory: NSCreateObjectFileImageFromMemory(), NSLinkModule(), NSLookupSymbolInModule(), NSAddressOfSymbol(), NSUnLinkModule(), NSDestroyObjectFileImage().
Calling a Function
Before we can perform in-memory execution, we need to first load the binary or bundle into a region of memory. This can be done in a variety of ways such as loading the binary from disc, downloading the binary into memory, etc.
If the file being loaded is not a bundle, swap the filetype within the Mach-O header from MH_EXECUTE to MH_BUNDLE. This is necessary because NSCreateObjectFileImageFromMemory() checks that the file being loaded is a bundle. If the file being loaded is a bundle, no changes to the header are necessary.
NSObjectFileImageReturnCode NSCreateObjectFileImageFromMemory(
const void* address,
size_t size,
NSObjectFileImage *objectFileImage
);NSCreateObjectFileImageFromMemory() takes the address of your binary in memory and will create an NSObjectFileImage structure.
NSModule NSLinkModule(
NSObjectFileImage objectFileImage,
const char* moduleName,
uint32_t options
);NSLinkModule() will link the NSObjectFileImage into the program and return a module handle for it. This will add all shared libraries referenced by the module to the list of libraries searched.
NSSymbol NSLookupSymbolInModule(
NSModule module,
const char* symbolName
);NSLookupSymbolInModule() takes the module handle returned from NSLinkModule() and searches for a symbol. We pass the function name we want to execute within the binary to symbolName.
void* NSAddressOfSymbol(
NSSymbol symbol
);NSAddressOfSymbol() takes the NSSymbol returned by NSLookupSymbolInModule() and returns the memory address of the symbol. Finally, we execute the address as a function pointer and pass any arguments to achieve code execution!
Executing the Entry Point of a Binary
Instead of searching for a particular function within a binary using NSLookupSymbolInModule() and NSAddressOfSymbol(), we can also search for and execute the binary’s entry point. This technique was well documented by Stephanie Archibald here.
To find the binary’s entry point, we can do the following:
- Locate the base address of our Mach-O in memory
- Find the entry point offset and add this to the Mach-O base address
To locate the Mach-O base address, we can use NSLookupSymbolInModule() + NSAddressOfSymbol() to search for __mh_execute_header, the address of the mach header in a Mach-O executable file type. This technique was used by Dwight Hohnstein here.
To locate the entry point offset, we walk the load commands and search for the LC_MAIN command.
We execute the address, obtained by adding the entry point offset to the Mach-O base address, as a function pointer to achieve code execution! Additionally, we can pass in command line arguments to our binary using argc and argv.
Cleaning Up
After executing our binary, we want to clean it up from memory using NSUnLinkModule() and NSDestroyObjectFileImage().
bool NSUnLinkModule(
NSModule module,
uint32_t options
);NSUnLinkModule() unlinks the specified module handle from the program and unmaps the image, but does not release the NSObjectFileImage.
bool NSDestroyObjectFileImage(
NSObjectFileImage objectFileImage
);NSDestroyObjectFileImage() releases the NSObjectFileImage and frees the memory.
Further Operationalizing
Handling Universal Binaries
On macOS, there are two types of object files:
- Mach-O files (thin)
- Universal Binaries (fat)
Mach-O files hold object code for a single architecture while Universal Binaries can contain object code for multiple architectures (i386, x86_64, arm, arm64, etc.).
My initial proof-of-concept (POC) worked great for Mach-O files but not for Universal Binaries. John Baek did some awesome work adding Universal Binary header parsing to Stephanie’s original POC.
Using John’s implementation, we could now load system binaries that already exist on disk such as /sbin/ping!
Retrieving stdout/stderr Using Inter-Process Communication (IPC)
Additionally, I wanted to retrieve output without direct access to the program’s stdout/stederr, such as through C2. I looked into multiple methods but ultimately decided on named pipes for IPC for ease of use.
I start a named pipe server with a random name in /tmp, redirect stdout/stderr using freopen(), retrieve the output of the Mach-O loaded in-memory, and clean up the named pipe after.
Preventing In-Memory Mach-O From Exiting
Normally, when a Mach-O binary finishes execution, the program exits and returns back to the caller (like your terminal); however, this exit call, when called from your current process, will exit your loading process (https://github.com/djhohnstein/macos_shell_memory)
To prevent the in-memory Mach-O from exiting, I copied Dwight’s atexit() routine implementation.
- Just before calling the program’s entry point or function, we save a
jump_bufwithsetjmp(). This will allow us to restore the call stack later - We then create an
atexit()routine that willlongjmp()back to our savedjump_buf. This will hook any exit functions and jump back to our saved call stack, preventing the in-memory Mach-O from exiting. We also set a global Boolean to prevent execution of the entry point or function after restoring the call stack - Once the call stack is returned, the global Boolean is checked, the in-memory execution is skipped, and normal program execution continues
SwiftInMemoryLoading
Here’s my Swift implementation for in-memory loading on macOS, it works on Big Sur as well as Monterey.
Reflective Code Loading: Big Sur vs Monterey
Segmentation Faults on Monterey
On Monterey, various red teams found that dereferencing the pointer returned by NSLinkModule() was causing a segfault. Timo Schmid and Carl Svensson discovered that this was due to a change in the API in more recent versions of dyld. They found that NSLinkModule() eventually calls handleFromLoader() which adds a flag at the end and performs a left shift of one byte on the return value.
In SwiftInMemoryLoading, this is handled by checking the operation system version with ProcessInfo.processInfo.isOperatingSystemAtLeast() and counteracting the left shift with a one byte right shift.
“In-Memory”?
Attackers generally favor in-memory execution techniques because it can be harder to detect vs executing malicious programs on disc. I had heard that this technique was no longer file-less from multiple sources and wanted to confirm this. Let’s observe the difference on macOS Big Sur (11.6.5) and macOS Monterey (12.3.1).
On Big Sur, we can use strings to determine the version of dyld. We see that we are running dyld-852.2.
Within the source code for dyld-852, particularly under dyld3 we see that temporary file creation was introduced inside NSLinkModule(). Let’s see if we can intercept that file creation.
We executed SwiftInMemoryLoading and observe any potential file events with FileMonitor. We notice file events related to opening up the targeted binary and creation of the named pipe in /private/tmp, but nothing related to a temp file named NSCreateObjectFileImageFromMemory-***** like the source code implies.
Running the program with a debugger, lldb, we see that there are two breakpoints for NSLinkModule(). Allowing the program to execute, we hit a breakpoint for libdyld.dylib`NSLinkModule.
Since NSLinkModule() lives within /usr/lib/dyld, we can search for it with Hopper Disassembler. Comparing the recovered psuedocode, this matches the source code for dyld.
It appears that our binaries are using NSLinkModule() from the dyld namespace vs the dyld3 namespace. In my testing, no temporary file creation occurred on Big Sur.
Let’s repeat this process on Monterey!
On Monterey, we can use strings to determine the version of dyld. We see that we are running dyld-955.
Again, we executed SwiftInMemoryLoading with lldb and set a breakpoint for NSLinkModule().
slyd0g-dev@slyd0g-devs-Mac Downloads % lldb SwiftInMemoryLoading -- /sbin/ping -c3 google.com(lldb) target create "SwiftInMemoryLoading"Current executable set to '/Users/slyd0g-dev/Downloads/SwiftInMemoryLoading' (x86_64).(lldb) settings set -- target.run-args "/sbin/ping" "-c3" "google.com"(lldb) b NSLinkModuleBreakpoint 1: where = libdyld.dylib`NSLinkModule, address = 0x00007ff80032cd87(lldb) rProcess 631 launched: '/Users/slyd0g-dev/Downloads/SwiftInMemoryLoading' (x86_64)[+] Named pipe created successfully at /private/tmp/1E09F7A1-DE07-43F7-AA0D-E2F47E2099FE[+] Waiting for a client to write ...Process 631 stopped* thread #1, queue = 'com.apple.main-thread', stop reason = breakpoint 1.1frame #0: 0x00007ff805c74d87 libdyld.dylib`NSLinkModulelibdyld.dylib`NSLinkModule:-> 0x7ff805c74d87 <+0>: pushq %rbp0x7ff805c74d88 <+1>: movq %rsp, %rbp0x7ff805c74d8b <+4>: movl %edx, %ecx0x7ff805c74d8d <+6>: movq %rsi, %rdxTarget 0: (SwiftInMemoryLoading) stopped....
...
...(lldb) sProcess 631 stopped* thread #1, queue = 'com.apple.main-thread', stop reason = instruction step intoframe #0: 0x000000010004b7d6 dyld`dyld4::APIs::NSLinkModule(__NSObjectFileImage*, char const*, unsigned int)...
...(lldb) ddyld`dyld4::APIs::NSLinkModule:...
...0x10004b83c <+102>: movq (%rdi), %rax0x10004b83f <+105>: leaq 0x3a5c3(%rip), %rsi ; "TMPDIR"...
...0x10004b8b7 <+225>: leaq 0x3a558(%rip), %rsi ; "NSCreateObjectFileImageFromMemory-XXXXXXXX"...
...0x10004b8f6 <+288>: callq 0x10006a63c ; pwrite
Upon hitting the breakpoint, we step through a couple instructions and see that dyld4::NSLinkModule() is called! We also see hints of temporary file creation that we also saw in dyld3::NSLinkModule() source code.
Let’s step through the program with lldb and see if we can spot the file creation with File Monitor.
Since we know the prefix to the temp file, we can grep for that specifically.
slyd0g-dev@slyd0g-devs-Mac Downloads % sudo /Applications/FileMonitor.app/Contents/MacOS/FileMonitor | grep SwiftInMemory | grep NSCreateObject{"event":"ES_EVENT_TYPE_NOTIFY_CREATE","timestamp":"2022-04-11 22:11:51 +0000","file":{"destination":"/private/var/folders/z3/2kpksmzs38x9yjgr7yndd57h0000gn/T/NSCreateObjectFileImageFromMemory-zvSxCzD5","process":{"pid":1084,"name":"SwiftInMemoryLoading","path":"/Users/slyd0g-dev/Downloads/SwiftInMemoryLoading","uid":501,"architecture":"Intel","arguments":["/Users/slyd0g-dev/Downloads/SwiftInMemoryLoading","/sbin/ping","-c3","google.com"],"ppid":1085,"rpid":1084,"ancestors":[1085,387,1],"signing info (reported)":{"csFlags":838860839,"platformBinary":0,"signingID":"SwiftInMemoryLoading-55554944e33587e81304398cac8e385f4c6c1b03","teamID":"","cdHash":"8D55D6EB5E3A38BF3A67142E5DB1D0C155B21C08"},"signing info (computed)":{"signatureStatus":0,"signatureSigner":"AdHoc","signatureID":"SwiftInMemoryLoading-55554944e33587e81304398cac8e385f4c6c1b03"}}}}slyd0g-dev@slyd0g-devs-Mac Downloads % shasum /private/var/folders/z3/2kpksmzs38x9yjgr7yndd57h0000gn/T/NSCreateObjectFileImageFromMemory-zvSxCzD57d031b92a86d815bac7bb2c1b298e8000c470593 /private/var/folders/z3/2kpksmzs38x9yjgr7yndd57h0000gn/T/NSCreateObjectFileImageFromMemory-zvSxCzD5slyd0g-dev@slyd0g-devs-Mac Downloads % shasum /sbin/ping7bbd3cd1d03386823b2bcf37c25431e08cea4733 /sbin/ping
After a bit of stepping, we see the temp file get created. We confirm that the temp file created has the same hash as the path being passed to SwiftInMemoryLoading. A similar analysis has been done by @roguesys here.
Unfortunately, the technique presented above is no longer truly file-less on macOS Monterey. The file loaded into memory is written to disc during NSLinkModule() and then dlopen() is used to load it back into memory.
This can also be seen in the latest release of dyld-940 here.
Detection
On Monterey, we can detect attempts at in-memory Mach-O loading by alerting on file creations with the predictable temporary file name (NSCreateObjectFileImageFromMemory-*******). These files will be written either to the value within the environment variable $TMPDIR or /tmp.
Conclusion
To recap, I learned about the necessary API calls to perform in-memory Mach-O loading on macOS and presented my Swift implementation.
I then explored this technique on Big Sur and Monterey and documented the differences. On Monterey, the file loaded in-memory is temporarily written to disc and then loaded with dlopen(). This presents us with a detection vector because the temporary file name and location is predictable:
/private/tmp/NSCreateObjectFileImageFromMemory-*******$TMPDIR/NSCreateObjectFileImageFromMemory-*******
Thanks for taking the time to read this post, I hope you learned a little about macOS API, reflective code loading, and debugging on macOS!
If you see any errors please don’t hesitate to let me know by e-mail (jbui006@ucr.edu) or Twitter (@slyd0g).
Credits
I realize this is not a new technique and wanted to explore it for my own knowledge, I am standing on the shoulders of giants and want to give credit where credit is due so big thanks to the following people/resources. Please see their prior work!
- Timo Schmid and Carl Svensson for discovering the changes in NSLinkModule’s return value on Monterey
- John Baek for their work on Universal Binary header parsing
- Dwight Hohnstein for their
atexit()routine concept and alternative method to bruteforcing addresses withchmodto find Mach-O headers in memory - https://objective-see.com/blog/blog_0x51.html
- File Monitor
