MacOS Apps - Inspecting, debugging and Fuzzing
Static Analysis
otool
SuspiciousPackage
****SuspiciousPackage is a tool useful to inspect .pkg files (installers) and see what is inside before installing it.
These installers have preinstall
and postinstall
bash scripts that malware authors usually abuse to persist the malware.
hdiutil
This tool allows to mount Apple disk images (.dmg) files to inspect them before running anything:
It will be mounted in /Volumes
Objective-C
When a function is called in a binary that uses objective-C, the compiled code instead of calling that function, it will call objc_msgSend
. Which will be calling the final function:
The params this function expects are:
The first parameter (self) is "a pointer that points to the instance of the class that is to receive the message". Or more simply put, it’s the object that the method is being invoked upon. If the method is a class method, this will be an instance of the class object (as a whole), whereas for an instance method, self will point to an instantiated instance of the class as an object.
The second parameter, (op), is "the selector of the method that handles the message". Again, more simply put, this is just the name of the method.
The remaining parameters are any values that are required by the method (op).
Packed binaries
Check for high entropy
Check the strings (is there is almost no understandable string, packed)
The UPX packer for MacOS generates a section called "__XHDR"
Dynamic Analysis
Note that in order to debug binaries, SIP needs to be disabled (csrutil disable
or csrutil enable --without debug
) or to copy the binaries to a temporary folder and remove the signature with codesign --remove-signature <binary-path>
or allow the debugging of the binary (you can use this script)
Note that in order to instrument system binaries, (such as cloudconfigurationd
) on macOS, SIP must be disabled (just removing the signature won't work).
dtruss
ktrace
You can use this one even with SIP activated
dtrace
It allows users access to applications at an extremely low level and provides a way for users to trace programs and even change their execution flow. Dtrace uses probes which are placed throughout the kernel and are at locations such as the beginning and end of system calls.
The available probes of dtrace can be obtained with:
The probe name consists of four parts: the provider, module, function, and name (fbt:mach_kernel:ptrace:entry
). If you not specifies some part of the name, Dtrace will apply that part as a wildcard.
A more detailed explanation and more examples can be found in https://illumos.org/books/dtrace/chp-intro.html
Examples
In line
script
ProcessMonitor
****ProcessMonitor is a very useful tool to check the process related actions a process is performing (for example, monitor which new processes a process is creating).
FileMonitor
****FileMonitor allows to monitor file events (such as creation, modifications, and deletions) providing detailed information about such events.
fs_usage
Allows to follow actions performed by processes:
TaskExplorer
****Taskexplorer is useful to see the libraries used by a binary, the files it's using and the network connections. It also checks the binary processes against virustotal and show information about the binary.
lldb
lldb is the de facto tool for macOS binary debugging.
When calling the objc_sendMsg
function, the rsi register holds the name of the method as a null-terminated (“C”) string. To print the name via lldb do:
(lldb) x/s $rsi: 0x1000f1576: "startMiningWithPort:password:coreCount:slowMemory:currency:"
(lldb) print (char*)$rsi:
(char *) $1 = 0x00000001000f1576 "startMiningWithPort:password:coreCount:slowMemory:currency:"
(lldb) reg read $rsi: rsi = 0x00000001000f1576 "startMiningWithPort:password:coreCount:slowMemory:currency:"
Anti-Dynamic Analysis
VM detection
The command
sysctl hw.model
returns "Mac" when the host is a MacOS but something different when it's a VM.Playing with the values of
hw.logicalcpu
andhw.physicalcpu
some malwares try to detect if it's a VM.Some malwares can also detect if the machine is VMware based on the MAC address (00:50:56).
It's also possible to find if a process is being debugged with a simple code such us:
if(P_TRACED == (info.kp_proc.p_flag & P_TRACED)){ //process being debugged }
It can also invoke the
ptrace
system call with thePT_DENY_ATTACH
flag. This prevents a debugger from attaching and tracing.You can check if the
sysctl
orptrace
function is being imported (but the malware could import it dynamically)As noted in this writeup, “Defeating Anti-Debug Techniques: macOS ptrace variants” : “The message Process # exited with status = 45 (0x0000002d) is usually a tell-tale sign that the debug target is using PT_DENY_ATTACH”
Fuzzing
ReportCrash analyzes crashing processes and saves a crash report to disk. A crash report contains information that can help a developer diagnose the cause of a crash.
For applications and other processes running in the per-user launchd context, ReportCrash runs as a LaunchAgent and saves crash reports in the user's ~/Library/Logs/DiagnosticReports/
For daemons, other processes running in the system launchd context and other privileged processes, ReportCrash runs as a LaunchDaemon and saves crash reports in the system's /Library/Logs/DiagnosticReports
If you are worried about crash reports being sent to Apple you can disable them. If not, crash reports can be useful to figure out how a server crashed.
Sleep
While fuzzing in a MacOS it's important to not allow the Mac to sleep:
systemsetup -setsleep Never
pmset, System Preferences
SSH Disconnect
If you are fuzzing via a SSH connection it's important to make sure the session isn't going to day. So change the sshd_config file with:
TCPKeepAlive Yes
ClientAliveInterval 0
ClientAliveCountMax 0
Internal Handlers
Checkout this section **to find out how you can find which app is responsible of handling the specified scheme or protocol**.
Enumerating Network Processes
This interesting to find processes that are managing network data:
Or use netstat
or lsof
More Fuzzing MacOS Info
References
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