EDRSandBlast is a tool written in C that weaponize a vulnerable signed
driver to bypass EDR detections (Kernel callbacks and ETW TI provider) and
LSASS protections. Multiple userland unhooking techniques are also
implemented to evade userland monitoring.
As of release, combination of userland (--usermode) and Kernel-land
(--kernelmode) techniques were used to dump LSASS memory under EDR
scrutiny, without being blocked nor generating "OS Credential Dumping"-related
events in the product (cloud) console. The tests were performed on 3 distinct
EDR products and were successful in each case.
EDR products use Kernel callbacks on Windows to be notified by the kernel of
system activity, such as process and thread creation and loading of images
(exe / DLL).
The Kernel callbacks are defined from user-land using a number of documented
APIs (nt!PsSetCreateProcessNotifyRoutine, nt!PsSetCreateThreadNotifyRoutine,
etc.). The user-land APIs add driver-supplied callback routines to undocumented
arrays of routines in Kernel-space:
PspCreateProcessNotifyRoutinefor process creationPspCreateThreadNotifyRoutinefor thread creationPspLoadImageNotifyRoutinefor image loading
EDRSandBlast enumerates the routines defined in those arrays and remove any
callback routine linked to a predefined list of EDR drivers (more than 1000
thousands drivers of security products from the
allocated filter altitudes).
The enumeration and removal are made possible through the exploitation of an
arbitrary Kernel memory read / write vulnerability of the
Micro-Star MSI Afterburner driver (CVE-2019-16098). The enumeration and
removal code is largely inspired from
br-sn's CheekyBlinder project.
The offsets of the aforementioned arrays are hardcoded in the
NtoskrnlOffsets.csv file for more than 350 versions of the Windows Kernel
ntoskrnl.exe. The choice of going with hardcoded offsets instead of pattern
searches is justified by the fact that the undocumented APIs responsible for
Kernel callbacks addition / removal are subject to change and that any attempt
to write Kernel memory at the wrong address may (and often will) result in a
Bug Check (Blue Screen of Death). For more information on how the offsets
were gathered, refer to Offsets section.
The ETW Microsoft-Windows-Threat-Intelligence provider log data about the
usages of some Windows API commonly used maliciously. This include the
nt!MiReadWriteVirtualMemory API, called by nt!NtReadVirtualMemory (which is
used to dump LSASS memory) and monitored by the nt!EtwTiLogReadWriteVm
function.
EDR products can consume the logs produced by the ETW TI provider through
services or processes running as, respectively,
SERVICE_LAUNCH_PROTECTED_ANTIMALWARE_LIGHT or
PS_PROTECTED_ANTIMALWARE_LIGHT, and associated with an Early Launch Anti Malware (ELAM) driver.
As published by
slaeryan in a CNO Development Labs blog post,
the ETW TI provider can be disabled altogether by patching, in kernel memory,
its ProviderEnableInfo attribute to 0x0. Refer to the great aforementioned
blog post for more information on the technique.
Similarly to the Kernel callbacks removal, the necessary ntoskrnl.exe offsets
(nt!EtwThreatIntProvRegHandleOffset, _ETW_REG_ENTRY's GuidEntry, and
_ETW_GUID_ENTRY's ProviderEnableInfo) are hardcoded in the
NtoskrnlOffsets.csv file for a number of the Windows Kernel versions.
In order to easily monitor actions that are performed by processes, EDR products often deploy a mechanism called userland hooking. First, EDR products register a kernel callback (usually image loading or process creation callbacks, see above) that allows them to be notified upon each process start.
When a process is loaded by Windows, and before it actually starts, the EDR is able to inject some custom DLL into the process address space, which contains its monitoring logic. While loading, this DLL injects "hooks" at the start of every function that is to be monitored by the EDR. At runtime, when the monitored functions are called by the process under surveillance, these hooks redirect the control flow to some supervision code present in the EDR's DLL, which allows it to inspect arguments and return values of these calls.
Most of the time, monitored functions are system calls (such as NtReadVirtualMemory,
NtOpenProcess, etc.), whose implementations reside in ntdll.dll. Intercepting calls to
Nt* functions allows products to be as close as possible to the userland / kernel-land
boundary (while remaining in userland), but functions from some higher-level DLLs may also
be monitored as well.
Bellow are examples of the same function, before and after beeing hooked by the EDR product:
NtProtectVirtualMemory proc near
mov r10, rcx
mov eax, 50h
test byte ptr ds:7FFE0308h, 1
jnz short loc_18009D1E5
syscall
retn
loc_18009D1E5:
int 2Eh
retn
NtProtectVirtualMemory endp NtProtectVirtualMemory proc near
jmp sub_7FFC74490298 ; --> "hook", jump to EDR analysis function
int 3 ; overwritten instructions
int 3 ; overwritten instructions
int 3 ; overwritten instructions
test byte_7FFE0308, 1 ; <-- execution resumes here after analysis
jnz short loc_7FFCB44AD1E5
syscall
retn
loc_7FFCB44AD1E5:
int 2Eh
retn
NtProtectVirtualMemory endp Userland hooks have the "weakness" to be located in userland memory, which means they are directly observable and modifiable by the process under scrutiny. To automatically detect hooks in the process address space, the main idea is to compare the differences between the original DLL on disk and the library residing in memory, that has been potentially altered by an EDR. To perform this comparison, the following steps are followed by EDRSandblast:
- The list of all loaded DLLs is enumerated thanks to the
InLoadOrderModuleListlocated int thePEB(to avoid calling any API that could be monitored and suspicious) - For each loaded DLL, its content on disk is read and its headers parsed. The corresponding library, residing in memory, is also parsed to identify sections, exports, etc.
- Relocations of the DLL are parsed and applied, by taking the base address of the corresponding loaded library into account. This allows the content of both the in-memory library and DLL originating from disk to have the exact same content (on sections where relocations are applied), and thus making the comparison reliable.
- Exported functions are enumerated and the first bytes of the "in-memory" and "on-disk" versions are compared. Any difference indicates an alteration that has been made after the DLL was loaded, and thus is very probably an EDR hook.
Note: The process can be generalized to find differences anywhere in non-writable sections
and not only at the start of exported functions, for example if EDR products start to
apply hooks in the middle of function :) Thus not used by the tool, this has been
implemented in findDiffsInNonWritableSections.
In order to bypass the monitoring performed by these hooks, multiples techniques are possible, and each has benefits and drawbacks.
The most intuitive method to bypass the hook-based monitoring is to remove the hooks. Since the hooks are present in memory that is reachable by the process itself, to remove a hook, the process can simply:
- Change the permissions on the page where the hook is located (RX -> RWX or RW)
- Write the original bytes that are known thanks to the on-disk DLL content
- Change back the permissions to RX
This approach is fairly simple, and can be used to remove every detected hook all at once. Performed by an offensive tool at its beginning, this allows the rest of the code to be completely unaware of the hooking mechnanism and perform normally without being monitored.
However, it has two main drawbacks. The EDR is probably monitoring the use of
NtProtectVirtualMemory, so using it to change the permissions of the page where the
hooks have been installed is (at least conceptually) a bad idea. Also, if a thread is
executed by the EDR and periodically check the integrity of the hooks, this could also
trigger some detection.
For implementation details, check the unhook() function's code path when unhook_method is
UNHOOK_WITH_NTPROTECTVIRTUALMEMORY.
Important note: for simplicity, this technique is implemented in EDRSandblast as the
base technique used to showcase the other bypass techniques; each of them demonstrates
how to obtain an unmonitored version of NtProtectVirtualMemory, but performs the same
operation afterward (unhooking a specific hook).
To bypass a specific hook, it is possible to simply "jump over" and execute the rest of the function as is. First, the original bytes of the monitored function, that have been overwritten by the EDR to install the hook, must be recovered from the DLL file. In our previous code example, this would be the bytes corresponding to the following instructions:
mov r10, rcx
mov eax, 50hIdentifying these bytes is a simple task since we are able to perform a clean diff of
both the memory and disk versions of the library, as previously described. Then, we
assemble a jump instruction that is built to redirect the control flow to the code
following immediately the hook, at address NtProtectVirtualMemory + sizeof(overwritten_instructions)
jmp NtProtectVirtualMemory+8Finally, we concatenate these opcodes, store them in (newly) executable memory and keep a
pointer to them. This object is called a "trampoline" and can then be used as a function
pointer, strictly equivalent to the original NtProtectVirtualMemory function.
The main benefit of this technique as for every techniques bellow, is that the hook is never erased, so any integrity check performed on the hooks by the EDR should pass. However, it requires to allocate writable then executable memory, which is typical of a shellcode allocation, thus attracting the EDR's scrutiny.
For implementation details, check the unhook() function's code path when unhook_method is
UNHOOK_WITH_INHOUSE_NTPROTECTVIRTUALMEMORY_TRAMPOLINE. Please remember the technique is
only showcased in our implementation and is, in the end, used to remove hooks from
memory, as every technique bellow.
The EDR product, in order for its hook to work, must save somewhere in memory the opcodes that it has removed. Worst (or "better", from the attacker point of view), to effectively use the original instructions the EDR has probably allocated itself a trampoline somewhere to execute the original function after having intercepted the call.
This trampoline can be searched for and used as a replacement for the hooked function,
without the need to allocate executable memory, or call any API except VirtualQuery,
which is most likely not monitored being an innocuous function.
To find the trampoline in memory, we browse the whole address space using VirtualQuery
looking for commited and executable memory. For each such region of memory, we scan it to
look for a jump instruction that targets the address following the overwritten
instructions (NtProtectVirtualMemory+8 in our previous example). The trampoline can then
be used to call the hooked function without triggering the hook.
This technique works surprisingly well as it recovers nearly all trampolines on tested
EDR. For implementation details, check the unhook() function's code path when
unhook_method is UNHOOK_WITH_EDR_NTPROTECTVIRTUALMEMORY_TRAMPOLINE.
Another simple method to get access to an unmonitored version of NtProtectVirtualMemory
function is to load a duplicate version of the ntdll.dll library into the process address
space. Since two identical DLLs can be loaded in the same process, provided they have
different names, we can simply copy the legitimate ntdll.dll file into another location,
load it using LoadLibrary (or reimplement the loading process), and access the function
using GetProcAddress for example.
This technique is very simple to understand and implement, and have a decent chance of success, since most of EDR products does not re-install hooks on newly loaded DLLs once the process is running. However, the major drawback is that copying Microsoft signed binaries under a different name is often considered as suspicious by EDR products as itself.
This technique is nevertheless implemented in EDRSandblast. For implementation details, check
the unhook() function's code path when unhook_method is
UNHOOK_WITH_DUPLICATE_NTPROTECTVIRTUALMEMORY.
In order to use system calls related functions, one program can reimplement syscalls (in
assembly) in order to call the corresponding OS features without actually touching the
code in ntdll.dll, which might be monitored by the EDR. This completely bypasses any
userland hooking done on syscall functions in ntdll.dll.
This nevertheless has some drawbacks. First, this implies being able to know the list of
syscall numbers of functions the program needs, which changes for each version of
Windows. Also, functions that are not technically syscalls
(e.g. LoadLibraryX/LdrLoadDLL) could be monitored as well, and cannot simply be
reimplemented using a syscall.
This technique is implemented in EDRSandblast. As previously stated, it is only used to
execute NtProtectVirtualMemory safely, and remove all detected hooks. However, in order
not to rely on hardcoded offsets, a small heuristic is implemented to search for mov eax, imm32 instruction at the start of the NtProtectVirtualMemory function and recover the
syscall number from it if found (otherwise relying on hardcoded offset for known Windows
versions).
For implementation details, check the unhook() function's code path when unhook_method is
UNHOOK_WITH_DIRECT_SYSCALL.
The Local Security Authority (LSA) Protection mechanism, first introduced
in Windows 8.1 and Windows Server 2012 R2, leverage the Protected Process Light (PPL) technology to restrict access to the LSASS process. The PPL
protection regulates and restricts operations, such as memory injection or
memory dumping of protected processes, even from a process holding the
SeDebugPrivilege privilege. Under the process protection model, only
processes running with higher protection levels can perform operations on
protected processes.
The _EPROCESS structure, used by the Windows kernel to represent a process
in kernel memory, includes a _PS_PROTECTION field defining the protection level
of a process through its Type (_PS_PROTECTED_TYPE) and Signer (_PS_PROTECTED_SIGNER)
attributes.
By writing in kernel memory, the EDRSandblast process is able to upgrade its own
protection level to PsProtectedSignerWinTcb-Light. This level is sufficient to
dump the LSASS process memory, since it "dominates" to PsProtectedSignerLsa-Light,
the protection level of the LSASS process running with the RunAsPPL mechanism.
EDRSandBlast implements the self protection as follow:
- open a handle to the current process
- leak all system handles using
NtQuerySystemInformationto find the opened handle on the current process, and the address of the current process'EPROCESSstructure in kernel memory. - use the arbitrary read / write vulnerability of the
Micro-Star MSI Afterburnerdriver to overwrite the_PS_PROTECTIONfield of the current process in kernel memory. The offsets of the_PS_PROTECTIONfield relative to theEPROCESSstructure (defined by thentoskrnlversion in use) are hardcoded in theNtoskrnlOffsets.csvfile.
Microsoft Credential Guard is a virtualization-based isolation technology,
introduced in Microsoft's Windows 10 (Enterprise edition) which prevents
direct access to the credentials stored in the LSASS process.
When Credentials Guard is activated, an LSAIso (LSA Isolated) process is
created in Virtual Secure Mode, a feature that leverages the virtualization
extensions of the CPU to provide added security of data in memory. Access to
the LSAIso process are restricted even for an access with the
NT AUTHORITY\SYSTEM security context. When processing a hash, the LSA
process perform a RPC call to the LSAIso process, and waits for the
LSAIso result to continue. Thus, the LSASS process won't contain any
secrets and in place will store LSA Isolated Data.
As stated in original research conducted by N4kedTurtle: "Wdigest can be
enabled on a system with Credential Guard by patching the values of
g_fParameter_useLogonCredential and g_IsCredGuardEnabled in memory".
The activation of Wdigest will result in cleartext credentials being stored
in LSASS memory for any new interactive logons (without requiring a reboot of
the system). Refer to the
original research blog post
for more details on this technique.
EDRSandBlast simply make the original PoC a little more opsec friendly and
provide support for a number of wdigest.dll versions (through hardcoded
offsets for g_fParameter_useLogonCredential and g_IsCredGuardEnabled).
The required ntoskrnl.exe and wdigest.dll offsets (mentioned above) are
extracted using r2pipe, as implemented in the ExtractOffsets.py Python
script. In order to support more Windows versions, the ntoskrnl.exe and
wdigest.dll referenced by Winbindex can be
automatically downloaded (and their offsets extracted). This allows to extract
offsets from nearly all files that were ever published in Windows update packages
(to date 350+ ntoskrnl.exe and 30+ wdigest.dll versions).
The vulnerable RTCore64.sys driver can be retrieved at:
http://download-eu2.guru3d.com/afterburner/%5BGuru3D.com%5D-MSIAfterburnerSetup462Beta2.zip
Usage: EDRSandblast.exe [-h | --help] [-v | --verbose] <audit | dump | cmd | credguard> [--usermode [--unhook-method <N>]] [--kernelmode] [--dont-unload-driver] [--dont-restore-callbacks] [--driver <RTCore64.sys>] [--service <SERVICE_NAME>] [--nt-offsets <NtoskrnlOffsets.csv>] [--wdigest-offsets <WdigestOffsets.csv>] [--add-dll <dll name or path>]* [-o | --dump-output <DUMP_FILE>]
-h | --help Show this help message and exit.
-v | --verbose Enable a more verbose output.
Actions mode:
audit Display the user-land hooks and / or Kernel callbacks without taking actions.
dump Dump the LSASS process, by default as 'lsass' in the current directory or at the
specified file using -o | --output <DUMP_FILE>.
cmd Open a cmd.exe prompt.
credguard Patch the LSASS process' memory to enable Wdigest cleartext passwords caching even if
Credential Guard is enabled on the host. No kernel-land actions required.
--usermode Perform user-land operations (DLL unhooking).
--kernelmode Perform kernel-land operations (Kernel callbacks removal and ETW TI disabling).
--unhook-method <N>
Choose the userland un-hooking technique, from the following:
1 (Default) Uses the (probably monitored) NtProtectVirtualMemory function in ntdll to remove all
present userland hooks.
2 Constructs a 'unhooked' (i.e. unmonitored) version of NtProtectVirtualMemory, by
allocating an executable trampoline jumping over the hook, and remove all present
userland hooks.
3 Searches for an existing trampoline allocated by the EDR itself, to get an 'unhooked'
(i.e. unmonitored) version of NtProtectVirtualMemory, and remove all present userland
hooks.
4 Loads an additional version of ntdll library into memory, and use the (hopefully
unmonitored) version of NtProtectVirtualMemory present in this library to remove all
present userland hooks.
5 Allocates a shellcode that uses a direct syscall to call NtProtectVirtualMemory,
and uses it to remove all detected hooks
Other options:
--dont-unload-driver Keep the Micro-Star MSI Afterburner vulnerable driver installed on the host
Default to automatically unsinstall the driver.
--dont-restore-callbacks Do not restore the EDR drivers' Kernel Callbacks that were removed.
Default to restore the callbacks.
--driver <RTCore64.sys> Path to the Micro-Star MSI Afterburner vulnerable driver file.
Default to 'RTCore64.sys' in the current directory.
--service <SERVICE_NAME> Name of the vulnerable service to intall / start.
--nt-offsets <NtoskrnlOffsets.csv> Path to the CSV file containing the required ntoskrnl.exe's offsets.
Default to 'NtoskrnlOffsets.csv' in the current directory.
--wdigest-offsets <WdigestOffsets.csv> Path to the CSV file containing the required wdigest.dll's offsets
(only for the 'credguard' mode).
Default to 'WdigestOffsets.csv' in the current directory.
--add-dll <dll name or path> Loads arbitrary libraries into the process' address space, before starting
anything. This can be useful to audit userland hooking for DLL that are not
loaded by default by this program. Use this option multiple times to load
multiple DLLs all at once.
Example of interesting DLLs to look at: user32.dll, ole32.dll, crypt32.dll,
samcli.dll, winhttp.dll, urlmon.dll, secur32.dll, shell32.dll...
-o | --output <DUMP_FILE> Output path to the dump file that will be generated by the 'dump' mode.
Default to 'lsass' in the current directory.
EDRSandBlast (x64 only) was built on Visual Studio 2019 (Windows SDK
Version: 10.0.19041.0 and Plateform Toolset: Visual Studio 2019 (v142)).
Note that ExtractOffsets.py has only be tested on Windows.
# Installation of Python dependencies
pip.exe install -m .\requirements.txt
# Script usage
ExtractOffsets.py [-h] -i INPUT [-o OUTPUT] [-d] mode
positional arguments:
mode ntoskrnl or wdigest. Mode to download and extract offsets for either ntoskrnl or wdigest
optional arguments:
-h, --help show this help message and exit
-i INPUT, --input INPUT
Single file or directory containing ntoskrnl.exe / wdigest.dll to extract offsets from.
If in dowload mode, the PE downloaded from MS symbols servers will be placed in this folder.
-o OUTPUT, --output OUTPUT
CSV file to write offsets to. If the specified file already exists, only new ntoskrnl versions will be
downloaded / analyzed.
Defaults to NtoskrnlOffsets.csv / WdigestOffsets.csv in the current folder.
-d, --dowload Flag to download the PE from Microsoft servers using list of versions from winbindex.m417z.com.
From the defender (EDR vendor, Microsoft, SOC analysts looking at EDR's telemetry, ...) point of view, multiple indicators can be used to detect or prevent this kind of techniques.
Since every action performed by the tool in kernel-mode memory relies on a vulnerable driver to read/write arbitrary content, driver loading events should be heaviliy scrutinized by EDR product (or SOC analysts), and raise an alert at any uncommon driver loading, or even block known vulnerable drivers. This latter approach is even recommended by Microsoft themselves: any HVCI (Hypervisor-protected code integrity) enabled Windows device embeds a drivers blocklist, and this will be progressively become a default behaviour on Windows (it already is on Windows 11).
Since an attacker could still use an unknown vulnerable driver to perform the same actions in memory, the EDR driver could periodically check that its kernel callbacks are still registered, directly by inspecting kernel memory (like this tool does), or simply by triggering events (process creation, thread creation, image loading, etc.) and checking the callback functions are indeed called by the executive kernel.
As a side note, this type of data structure could be protected via the recent Kernel Data Protection (KDP) mechanism, which relies on Virtual Based Security, in order to make the kernel callbacks array non-writable without calling the right APIs.
The same logic could apply to sensitive ETW variables such as the ProviderEnableInfo, abused by this tool to disable the ETW Threat Intelligence events generation.
The first indicator that a process is actively trying to evade user-land hooking is the file accesses to each DLL corresponding to loaded modules; in a normal execution, a userland process rarely needs to read DLL files outside of a LoadLibrary call, especially ntdll.dll.
In order to protect API hooking from being bypassed, EDR products could periodically check that hooks are not altered in memory, inside each monitored process.
Finally, to detect hooking bypass (abusing a trampoline, using direct syscalls, etc.) that does not imply the hooks removal, EDR products could potentially rely on kernel callbacks associated to the abused syscalls (ex. PsCreateProcessNotifyRoutine for NtCreateProcess syscall, ObRegisterCallbacks for NtOpenProcess syscall, etc.), and perform user-mode call-stack analysis in order to determine if the syscall was triggered from a normal path (kernel32.dll -> ntdll.dll -> syscall) or an abnormal one (ex. program.exe -> direct syscall).
-
Kernel callbacks enumeration and removal: https://github.com/br-sn/CheekyBlinder
-
Kernel memory Read / Write primitives through the vulnerable
Micro-Star MSI Afterburnerdriver: https://github.com/Barakat/CVE-2019-16098/ -
Disabling of the ETW Threat Intelligence provider: https://public.cnotools.studio/bring-your-own-vulnerable-kernel-driver-byovkd/exploits/data-only-attack-neutralizing-etwti-provider
-
Driver install / uninstall: https://github.com/gentilkiwi/mimikatz
-
Initial list of EDR drivers names: https://github.com/SadProcessor/SomeStuff/blob/master/Invoke-EDRCheck.ps1
-
Credential Guard bypass by re-enabling
WdigestthroughLSASSmemory patching: https://teamhydra.blog/2020/08/25/bypassing-credential-guard/
Thomas DIOT (Qazeer) Maxime MEIGNAN (themaks)
CC BY 4.0 licence - https://creativecommons.org/licenses/by/4.0/