EDRSandblast - Tool That Weaponize A Vulnerable Signed Driver To Bypass EDR Detections And LSASS Protections

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.

Description

EDR bypass through Kernel callbacks removal

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:

PspCreateProcessNotifyRoutine for process creation PspCreateThreadNotifyRoutine for thread creation PspLoadImageNotifyRoutine for 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.

EDR bypass through deactivation of the ETW Microsoft-Windows-Threat-Intelligence provider

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.

EDR bypass through userland hooking bypass

How userland hooking works

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:

Hooks detection

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 InLoadOrderModuleList located int the PEB (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.

Hook bypass using ... unhooking

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).

Hook bypass using a custom trampoline

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:

Identifying 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)

Finally, 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.

Hook bypass using the own EDR's trampoline

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.

Hook bypass using duplicate DLL

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.

Hook bypass using direct syscalls

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.

RunAsPPL bypass

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 NtQuerySystemInformation to find the opened handle on the current process, and the address of the current process' EPROCESS structure in kernel memory. use the arbitrary read / write vulnerability of the Micro-Star MSI Afterburner driver to overwrite the _PS_PROTECTION field of the current process in kernel memory. The offsets of the _PS_PROTECTION field relative to the EPROCESS structure (defined by the ntoskrnl version in use) are hardcoded in the NtoskrnlOffsets.csv file.

Credential Guard bypass

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).

ntoskrnl and wdigest offsets

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).

Usage

The vulnerable RTCore64.sys driver can be retrieved at:

Quick usage

Options

Build

EDRSandBlast (x64 only) was built on Visual Studio 2019 (Windows SDK Version: 10.0.19041.0 and Plateform Toolset: Visual Studio 2019 (v142)).

ExtractOffsets.py usage

Note that ExtractOffsets.py has only be tested on Windows.

Detection

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.

Driver whitelisting

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).

Kernel-memory integrity checks

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.

User-mode detection

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).

Acknowledgements

Kernel callbacks enumeration and removal: https://github.com/br-sn/CheekyBlinder

Kernel memory Read / Write primitives through the vulnerable Micro-Star MSI Afterburner driver: 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 Wdigest through LSASS memory patching: https://teamhydra.blog/2020/08/25/bypassing-credential-guard/

Authors

Thomas DIOT (Qazeer) Maxime MEIGNAN (themaks)

Licence

CC BY 4.0 licence - https://creativecommons.org/licenses/by/4.0/