# EDR bypass with EDRSandBlast *The present note is a copy of the* [*EDRSandBlast project's README*](https://github.com/wavestone-cdt/EDRSandblast)*.* [`EDRSandBlast`](https://github.com/wavestone-cdt/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](https://docs.microsoft.com/en-us/windows-hardware/drivers/ifs/allocated-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](https://github.com/br-sn/CheekyBlinder). 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](https://github.com/Qazeer/InfoSec-Notes/blob/master/Red_Team/Offsets/README.md). ### 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](https://public.cnotools.studio/bring-your-own-vulnerable-kernel-driver-byovkd/exploits/data-only-attack-neutralizing-etwti-provider), 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 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`, ...), 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 being 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 ``` #### 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`: 1. The list of all loaded DLLs is enumerated thanks to the `InLoadOrderModuleList` located in the `PEB` (to avoid calling any API that could be monitored and suspect). 2. 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. 3. 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. 4. 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: 1. Change the permissions on the page where the hook is located (`RX` -> `RWX` or `RW`). 2. Write the original bytes that are known thanks to the on-disk DLL content. 3. 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 mechanism 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 set to `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: ``` mov r10, rcx mov eax, 50h ``` 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()`: ``` jmp NtProtectVirtualMemory+8 ``` 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 other 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 set to `UNHOOK_WITH_INHOUSE_NTPROTECTVIRTUALMEMORY_TRAMPOLINE`. Please remember that this 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 Windows API except `VirtualQuery`, which is most likely not monitored being an otherwise innocuous function. To find the trampoline in memory, we browse the whole address space using `VirtualQuery` looking for committed and executable memory pages. 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`s. For implementation details, check the `unhook()` function's code path when `unhook_method` is set to `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 do 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 behavior by itself by `EDR` products. This technique is nevertheless implemented in `EDRSandblast`. For implementation details, check the `unhook()` function's code path when `unhook_method` is set to `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 may 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 direct `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 to not only 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 (else relying on hardcoded offset for known Windows versions). For implementation details, check the `unhook()` function's code path when `unhook_method` is set to `UNHOOK_WITH_DIRECT_SYSCALL`. ### RunAsPPL bypass The `Local Security Authority (LSA) Protection` mechanism, firstly 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 process holding the `SeDebugPrivilege` privilege. The protection level of a process is defined in its `EPROCESS` structure, used by the Windows kernel to represent processes in memory. The `EPROCESS` structure includes a `_PS_PROTECTION` field, defining the protection level of a process through its `Type` (`_PS_PROTECTED_TYPE`) and `Signer` (`_PS_PROTECTED_SIGNER`) attributes. If no `EDR` drivers callbacks are detected, the current process is self protected as `PsProtectedSignerWinTcb-Light`. This level of protection is sufficient to dump the `LSASS` process memory, with `RunAsPPL` enabled, as the `PsProtectedSignerWinTcb` signer "dominates" `PsProtectedSignerLsa-Light` (and both process are of `PsProtectedTypeProtectedLight` type). `EDRSandBlast` implements the self protection as follow: * open an handle to the current process * leak all system handles using `NtQuerySystemInformation` to find the opened handle on the current process (which correspond to 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 (with out requiring a reboot of the system). Refer to the [original research blog post](https://teamhydra.blog/2020/08/25/bypassing-credential-guard/) 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](https://winbindex.m417z.com/) can be automatically downloaded (and their offsets extracted). This allow to extract offsets from that files which appear in Windows update packages (to date 350+ `ntoskrnl.exe` and 30+ `wdigest.dll` versions). ## Usage The vulnerable `RTCore64.sys` driver can be retrieved at: ``` http://download-eu2.guru3d.com/afterburner/%5BGuru3D.com%5D-MSIAfterburnerSetup462Beta2.zip ``` ### Quick usage ``` Usage: EDRSandblast.exe [-h | --help] [-v | --verbose] [--usermode [--unhook-method ]] [--kernelmode] [--dont-unload-driver] [--dont-restore-callbacks] [--driver ] [--service ] [--nt-offsets ] [--wdigest-offsets ] [--add-dll ]* [-o | --dump-output ] ``` ### Options ``` -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 with out taking actions. dump Dump the LSASS process, by default as 'lsass' in the current directory or at the specified file using -o | --output . 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-lank actions required. --usermode Perform user-land operations (DLL unhooking). --kernelmode Perform kernel-land operations (Kernel callbacks removal and ETW TI disabling). --unhook-method 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 Path to the Micro-Star MSI Afterburner vulnerable driver file. Default to 'RTCore64.sys' in the current directory. --service Name of the vulnerable service to intall / start. --nt-offsets Path to the CSV file containing the required ntoskrnl.exe's offsets. Default to 'NtoskrnlOffsets.csv' in the current directory. --wdigest-offsets 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 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 Output path to the dump file that will be generated by the 'dump' mode. Default to 'lsass' in the current directory. ``` ### 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. ``` # 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. ``` ## 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 heavily 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](https://docs.microsoft.com/en-us/windows/security/threat-protection/windows-defender-application-control/microsoft-recommended-driver-block-rules): any HVCI (*Hypervisor-protected code integrity*) enabled Windows device embeds a drivers blocklist, and this will be progressively become a default behavior 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)](https://www.microsoft.com/security/blog/2020/07/08/introducing-kernel-data-protection-a-new-platform-security-technology-for-preventing-data-corruption/) 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 is 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: * Kernel memory Read / Write primitives through the vulnerable `Micro-Star MSI Afterburner` driver: * Disabling of the ETW Threat Intelligence provider: * Driver install / uninstall: * Initial list of `EDR` drivers names: * Credential Guard bypass by re-enabling `Wdigest` through `LSASS` memory patching: ## Authors [Thomas DIOT (Qazeer)](https://github.com/Qazeer/) [Maxime MEIGNAN (themaks)](https://github.com/themaks) ## Licence CC BY 4.0 licence -