서문
Elastic Defend provides over 550 rules (and counting) to detect and stop malicious behavior in real time on endpoints. We recently added kernel call stack enrichments to provide additional context to events and alerts. Call stacks are a win-win-win for behavioral protections, simultaneously improving false positives, false negatives, and alert explainability. In this article, we'll show you how we achieve all three of these, and how you can leverage call stacks to better understand any alerts you encounter in your environment.
What is a call stack?
When a thread running function A calls function B, the CPU automatically saves the current instruction’s address (within A) to a thread-specific region of memory called the stack. This saved pointer is known as the return address - it's where execution will resume once the B has finished its job. If B were to call a third function C, then a return address within B will also be saved to the stack. These return addresses can be retrieved through a process known as a stack walk, which reconstructs the sequence of function calls that led to the current thread state. Stack walks list return addresses in reverse-chronological order, so the most recent function is always at the top.
In Windows, when we double-click on notepad.exe, for example, the following series of functions are called:
- The green section is related to base thread initialization performed by the operating system and is usually identical across all operations (file, registry, process, library, etc.)
- The red section is the user code; it is often composed of multiple modules and provides approximate details of how the process creation operation was reached
- The blue section is the Win32 and Native API layer; this is operation-specific, including the last 2 to 3 intermediary Windows modules before forwarding the operation details for effective execution in kernel mode
The following screenshot depicts the call stack for this execution chain:
Here is an example of file creation using notepad.exe where we can see a similar pattern:
- The blue part lists the last user mode intermediary Windows APIs before forwarding the create file operation to kernel mode drivers for effective execution
- The red section includes functions from user32.dll and notepad.exe, which indicate that this file operation was likely initiated via GUI
- The green part represents the initial thread initialization
Events Explainability
Apart from using call stacks for finding known bad, like unbacked memory regions with RWX permissions that may be the remnants of prior code injection. Call stacks provide very low-level visibility that often reveals greater insights than logs can otherwise provide.
As an example, while hunting for suspicious process executions started by WmiPrvSe.exe via WMI, you find this instance of notepad.exe:
Reviewing the standard event log fields, you may expect that it was started using the Win32_Process class using the wmic.exe process call create notepad.exe syntax. However, the event details describe a series of modules and functions:
The blue section depicts the standard intermediary CreateProcess Windows APIs, while the red section highlights better information in that we can see that the DLL before the first call to CreateProcessW is wbemcons.dll and when inspecting its properties we can see that it’s related to WMI Event Consumers. We can conclude that this notepad.exe instance is likely related to a WMI Event Subscription. This will require specific incident response steps to mitigate the WMI persistence mechanism.
Another great example is Windows scheduled tasks. When executed, they are spawned as children of the Schedule service, which runs within a svchost.exe host process. Modern Windows 11 machines may have 50 or more svchost.exe processes running. Fortunately, the Schedule service has a specific process argument -s Schedule which differentiates it:
In older Windows versions, the Scheduled Tasks service is a member of the Network Service group and executed as a component of the netsvcs shared svchost.exe instance. Not all children of this process are necessarily scheduled tasks in these older versions:
Inspecting the call stack on both versions, we can see the module that is adjacent to the CreateProcess call is the same ubpm.dll (Unified Background Process Manager DLL) executing the exported function ubpm.dll!UbpmOpenTriggerConsumer:
Using the following KQL query, we can hunt for task executions on both versions:
event.action :"start" and
process.parent.name :"svchost.exe" and process.parent.args : netsvcs and
process.parent.thread.Ext.call_stack_summary : *ubpm.dll*Another interesting example occurs when a user double-clicks a script file from a ZIP archive that was opened using Windows Explorer. Looking at the process tree, you will see that explorer.exe is the parent and the child is a script interpreter process like wscript.exe or cmd.exe.
This process tree can be confused with a user double-clicking a script file from any location on the file system, which is not very suspicious. But if we inspect the call stack we can see that the parent stack is pointing to zipfld.dll (Zipped Folders Shell Extension):
Detection Examples
Now that we have a better idea of how to use the call stack to better interpret events, let’s explore some advanced detection examples per event type.
Process
Suspicious Process Creation via Reflection
Dirty Vanity is a recent code-injection technique that abuses process forking to execute shellcode within a copy of an existing process. When a process is forked, the OS makes a copy of an existing process, including its address space and any inheritable handles therein.
When executed, Dirty Vanity will fork an instance of a targeted process (already running or a sacrificial one) and then inject into it. Using process creation notification callbacks won’t log forked processes because the forked process initial thread isn’t executed. But in the case of this injection technique, the forked process will be injected and a thread will be started, which triggers the process start event log with the following call stack:
We can see the call to RtlCreateProcessReflection and RtlCloneUserProcess to fork the process. Now we know that this is a forked process, and the next question is “Is this common in normal conditions?” While diagnostically this behavior appears to be common and alone, it is not a strong signal of something malicious. Checking further to see if the forked processes perform any network connections, loads DLLs, or spawns child processes revealed to be less common and made for good detections:
// EQL detecting a forked process spawning a child process - very suspicious
process where event.action == "start" and
descendant of
[process where event.action == "start" and
_arraysearch(process.parent.thread.Ext.call_stack, $entry,
$entry.symbol_info:
("*ntdll.dll!RtlCreateProcessReflection*",
"*ntdll.dll!RtlCloneUserProcess*"))] and
not (process.executable :
("?:\\WINDOWS\\SysWOW64\\WerFault.exe",
"?:\\WINDOWS\\system32\\WerFault.exe") and
process.parent.thread.Ext.call_stack_summary :
"*faultrep.dll|wersvc.dl*")// EQL detecting a forked process loading a network DLL
// or performs a network connection - very suspicious
sequence by process.entity_id with maxspan=1m
[process where event.action == "start" and
_arraysearch(process.parent.thread.Ext.call_stack,
$entry, $entry.symbol_info:
("*ntdll.dll!RtlCreateProcessReflection*",
"*ntdll.dll!RtlCloneUserProcess*"))]
[any where
(
event.category : ("network", "dns") or
(event.category == "library" and
dll.name : ("ws2_32.dll", "winhttp.dll", "wininet.dll"))
)]Here’s an example of forking explore.exe and executing shellcode that spawns cmd.exe from the forked explorer.exe instance:
Direct Syscall via Assembly Bytes
The second and final example for process events is process creation via direct syscall. This directly uses the syscall instruction instead of calling the NtCreateProcess API. Adversaries may use this method to avoid security products that are reliant on usermode API hooking (which Elastic Defend is not):
process where event.action : "start" and
// EQL detecting a call stack not ending with ntdll.dll
not process.parent.thread.Ext.call_stack_summary : "ntdll.dll*" and
/* last call in the call stack contains bytes that execute a syscall
manually using assembly <mov r10,rcx, mov eax,ssn, syscall> */
_arraysearch(process.parent.thread.Ext.call_stack, $entry,
($entry.callsite_leading_bytes : ("*4c8bd1b8??????000f05",
"*4989cab8??????000f05", "*4c8bd10f05", "*4989ca0f05")))This example matches when the final memory region in the call stack is unbacked and contains assembly bytes that end with the syscall instruction (0F05):
File
Suspicious Microsoft Office Embedded Object
The following rule logic identifies suspicious file extensions written by a Microsoft Office process from an embedded OLE stream, frequently used by malicious documents to drop payloads for initial access.
// EQL detecting file creation event with call stack indicating
// OleSaveToStream call to save or load the embedded OLE object
file where event.action != "deletion" and
process.name : ("winword.exe", "excel.exe", "powerpnt.exe") and
_arraysearch(process.thread.Ext.call_stack, $entry, $entry.symbol_info:
("*!OleSaveToStream*", "*!OleLoad*")) and
(
file.extension : ("exe", "dll", "js", "vbs", "vbe", "jse", "url",
"chm", "bat", "mht", "hta", "htm", "search-ms") or
/* PE & HelpFile */
file.Ext.header_bytes : ("4d5a*", "49545346*")
)Example of matches :
Suspicious File Rename from Unbacked Memory
Certain ransomware may inject into signed processes before starting their encryption routine. File rename and modification events will appear to originate from a trusted process, potentially bypassing some heuristics that exclude signed processes as presumed false positives. The following KQL query looks for file rename of documents, from a signed binary and with a suspicious call stack:
file where event.action : "rename" and
process.code_signature.status : "trusted" and file.extension != null and
file.Ext.original.name : ("*.jpg", "*.bmp", "*.png", "*.pdf", "*.doc",
"*.docx", "*.xls", "*.xlsx", "*.ppt", "*.pptx") and
not file.extension : ("tmp", "~tmp", "diff", "gz", "download", "bak",
"bck", "lnk", "part", "save", "url", "jpg", "bmp", "png", "pdf", "doc",
"docx", "xls", "xlsx", "ppt", "pptx") and
process.thread.Ext.call_stack_summary :
("ntdll.dll|kernelbase.dll|Unbacked",
"ntdll.dll|kernelbase.dll|kernel32.dll|Unbacked",
"ntdll.dll|kernelbase.dll|Unknown|kernel32.dll|ntdll.dll",
"ntdll.dll|kernelbase.dll|Unknown|kernel32.dll|ntdll.dll",
"ntdll.dll|kernelbase.dll|kernel32.dll|Unknown|kernel32.dll|ntdll.dll",
"ntdll.dll|kernelbase.dll|kernel32.dll|mscorlib.ni.dll|Unbacked",
"ntdll.dll|wow64.dll|wow64cpu.dll|wow64.dll|ntdll.dll|kernelbase.dll|
Unbacked", "ntdll.dll|wow64.dll|wow64cpu.dll|wow64.dll|ntdll.dll|
kernelbase.dll|Unbacked|kernel32.dll|ntdll.dll",
"ntdll.dll|Unbacked", "Unbacked", "Unknown")Here are some examples of matches where explorer.exe (Windows Explorer) is injected by the KNIGHT/CYCLOPS ransomware:
Executable File Dropped by an Unsigned Service DLL
Certain types of malware maintain their presence by disguising themselves as Windows service DLLs. To be recognized and managed by the Service Control Manager, a service DLL must export a function named ServiceMain. The KQL query below helps identify instances where an executable file is created, and the call stack includes the ServiceMain function.
event.category : file and
file.Ext.header_bytes :4d5a* and process.name : svchost.exe and
process.thread.Ext.call_stack.symbol_info :*!ServiceMain*라이브러리
Unsigned Print Monitor Driver Loaded
The following EQL query identifies the loading of an unsigned library by the print spooler service where the call stack indicates the load is coming from SplAddMonitor. Adversaries may use port monitors to run an adversary-supplied DLL during system boot for persistence or privilege escalation.
library where
process.executable : ("?:\\Windows\\System32\\spoolsv.exe",
"?:\\Windows\\SysWOW64\\spoolsv.exe") and not dll.code_signature.status :
"trusted" and _arraysearch(process.thread.Ext.call_stack, $entry,
$entry.symbol_info: "*localspl.dll!SplAddMonitor*")Example of match:
Potential Library Load via ROP Gadgets
This EQL rule identifies the loading of a library from unusual win32u or ntdll offsets. This may indicate an attempt to bypass API monitoring using Return Oriented Programming (ROP) assembly gadgets to execute a syscall instruction from a trusted module.
library where
// adversaries try to use ROP gadgets from ntdll.dll or win32u.dll
// to construct a normal-looking call stack
process.thread.Ext.call_stack_summary : ("ntdll.dll|*", "win32u.dll|*") and
// excluding normal Library Load APIs - LdrLoadDll and NtMapViewOfSection
not _arraysearch(process.thread.Ext.call_stack, $entry,
$entry.symbol_info: ("*ntdll.dll!Ldr*",
"*KernelBase.dll!LoadLibrary*", "*ntdll.dll!*MapViewOfSection*"))This example matches when AtomLdr loads a DLL using ROP gadgets from win32u.dll instead of using ntdll’s load library APIs (LdrLoadDll and NtMapViewOfSection).
Evasion via LdrpKernel32 Overwrite
The [LdrpKernel32(https://github.com/rbmm/LdrpKernel32DllName) evasion is an interesting technique to hijack the early execution of a process during the bootstrap phase by overwriting the bootstrap DLL name referenced in ntdll.dll memory– forcing the process to load a malicious DLL.
library where
// BaseThreadInitThunk must be exported by the rogue bootstrap DLL
_arraysearch(process.thread.Ext.call_stack, $entry, $entry.symbol_info :
"*!BaseThreadInitThunk*") and
// excluding kernel32 that exports normally exports BasethreadInitThunk
not _arraysearch(process.thread.Ext.call_stack, $entry, $entry.symbol_info
("?:\\Windows\\System32\\kernel32.dll!BaseThreadInitThunk*",
"?:\\Windows\\SysWOW64\\kernel32.dll!BaseThreadInitThunk*",
"?:\\Windows\\WinSxS\\*\\kernel32.dll!BaseThreadInitThunk*",
"?:\\Windows\\WinSxS\\Temp\\PendingDeletes\\*!BaseThreadInitThunk*",
"\\Device\\*\\Windows\\*\\kernel32.dll!BaseThreadInitThunk*"))Example of match:
Suspicious Remote Registry Modification
Similar to the scheduled task example, the remote registry service is hosted in svchost.exe. We can use the call stack to detect registry modification by monitoring when the Remote Registry service points to an executable or script file. This may indicate an attempt to move laterally via remote configuration changes.
registry where
event.action == "modification" and
user.id : ("S-1-5-21*", "S-1-12-*") and
process.name : "svchost.exe" and
// The regsvc.dll in call stack indicate that this is indeed the
// svchost.exe instance hosting the Remote registry service
process.thread.Ext.call_stack_summary : "*regsvc.dll|rpcrt4.dll*" and
(
// suspicious registry values
registry.data.strings : ("*:\\*\\*", "*.exe*", "*.dll*", "*rundll32*",
"*powershell*", "*http*", "* /c *", "*COMSPEC*", "\\\\*.*") or
// suspicious keys like Services, Run key and COM
registry.path :
("HKLM\\SYSTEM\\ControlSet*\\Services\\*\\ServiceDLL",
"HKLM\\SYSTEM\\ControlSet*\\Services\\*\\ImagePath",
"HKEY_USERS\\*Classes\\*\\InprocServer32\\",
"HKEY_USERS\\*Classes\\*\\LocalServer32\\",
"H*\\Software\\Microsoft\\Windows\\CurrentVersion\\Run\\*") or
// potential attempt to remotely disable a service
(registry.value : "Start" and registry.data.strings : "4")
)This example matches when the Run key registry value is modified remotely via the Remote Registry service:
결론
As we’ve demonstrated, call stacks are not only useful for finding known bad patterns, but also for reducing ambiguity in standard EDR events, and easing behavior interpretation. The examples we've provided here represent just a minor portion of the potential detection possibilities achievable by applying enhanced enrichment to the same dataset.