From e66a3e929b3c1253255f762db5677efd6c0510ec Mon Sep 17 00:00:00 2001 From: Andrew Case Date: Thu, 12 Sep 2024 18:51:47 -0500 Subject: [PATCH] Add detection of direct and indirect system calls --- .../plugins/windows/direct_system_calls.py | 450 ++++++++++++++++++ .../plugins/windows/indirect_system_calls.py | 124 +++++ 2 files changed, 574 insertions(+) create mode 100644 volatility3/framework/plugins/windows/direct_system_calls.py create mode 100644 volatility3/framework/plugins/windows/indirect_system_calls.py diff --git a/volatility3/framework/plugins/windows/direct_system_calls.py b/volatility3/framework/plugins/windows/direct_system_calls.py new file mode 100644 index 000000000..409f24750 --- /dev/null +++ b/volatility3/framework/plugins/windows/direct_system_calls.py @@ -0,0 +1,450 @@ +# This file is Copyright 2019 Volatility Foundation and licensed under the Volatility Software License 1.0 +# which is available at https://www.volatilityfoundation.org/license/vsl-v1.0 +# + +import logging +import capstone +from collections import namedtuple +from typing import List, Tuple, Optional, Generator, Callable + +from volatility3.framework.objects import utility +from volatility3.framework import interfaces, renderers, symbols, exceptions +from volatility3.framework.configuration import requirements +from volatility3.plugins import yarascan +from volatility3.framework.renderers import format_hints +from volatility3.plugins.windows import pslist + +vollog = logging.getLogger(__name__) + + +# Full details on the techniques used in these plugins to detect EDR-evading malware +# can be found in our 20 page whitepaper submitted to DEFCON along with the presentation +# https://www.volexity.com/wp-content/uploads/2024/08/Defcon24_EDR_Evasion_Detection_White-Paper_Andrew-Case.pdf + +syscall_finder_type = namedtuple( + "syscall_finder_type", + [ + "get_syscall_target_address", + "wants_syscall_inst", + "rule_str", + "invalid_ops", + "termination_ops", + ], +) + +syscall_finder_type.__doc__ = """ +This type to used to specify how malicious system call invocations should be found. + +`get_syscall_target_address` is optionally used to extract the address containing the malicious 'syscall' instruction +`wants_syscall_inst` whether or not this method expects the 'syscall' instrunction directly within the malicious code block +`rule` the opcode string to search for the malicious syscall instructions +`invalid_ops` instructions that only appear in invalid code blocks. Stops processing of the code block when encountered. +`termination_ops` instructions that are expected to be present in the code block and that stop processing +""" + + +class DirectSystemCalls(interfaces.plugins.PluginInterface): + """Detects the Direct System Call technique used to bypass EDRs""" + + _required_framework_version = (2, 4, 0) + _version = (1, 0, 0) + + valid_syscall_handlers = ("ntdll.dll", "win32u.dll") + + def __init__(self, *args, **kwargs): + super().__init__(*args, **kwargs) + + self.syscall_finder = syscall_finder_type( + # for direct system calls, we find the `syscall` instruction directly, so we already know the address + None, + # yes, we want the syscall instruction present as it is what this technique looks for + True, + # regex to find "\x0f\x05" (syscall) followed later by "\xc3" (ret) + # we allow spacing in between to break naive anti-analysis forms (e.g., TarTarus Gate) + # Standard techniques, such as HellsGate, look like: + # mov r10, rcx + # mov eax, + # syscall + # ret + "/\\x0f\\x05[^\\xc3]{,24}\\xc3/", + # any of these will not be in a workable, malicious direct system call block + ["jmp", "call", "leave", "int3"], + # the expected form is to end with a "ret" back to the calling code + ["ret"], + ) + + @classmethod + def get_requirements(cls) -> List[interfaces.configuration.RequirementInterface]: + # create a list of requirements for vadyarascan + vadyarascan_requirements = [ + requirements.ModuleRequirement( + name="kernel", + description="Windows kernel", + architectures=["Intel32", "Intel64"], + ), + requirements.PluginRequirement( + name="pslist", plugin=pslist.PsList, version=(2, 0, 0) + ), + requirements.VersionRequirement( + name="yarascanner", component=yarascan.YaraScanner, version=(2, 1, 0) + ), + requirements.PluginRequirement( + name="yarascan", plugin=yarascan.YaraScan, version=(2, 0, 0) + ), + ] + + # get base yarascan requirements for command line options + yarascan_requirements = yarascan.YaraScan.get_yarascan_option_requirements() + + # return the combined requirements + return yarascan_requirements + vadyarascan_requirements + + @staticmethod + def _is_syscall_block( + disasm_func: Callable, + syscall_finder: syscall_finder_type, + data: bytes, + address: int, + ) -> Optional[Tuple[str, capstone._cs_insn]]: + """ + Determines if the bytes starting at `data` represent a valid syscall instrunction invocation block + + To maliciously invoke the system call instruction, malware must do each of the following: + + 1) update RAX to the system call number + 2) update R10 to the first parameter + 3) hit the 'termination' instrunction set in `syscall_finder_type` + + We also track whether the 'syscall' instruction was encountered while parsing + + This function is reusable for every technique we found and studied during the DEFCON research timeframe + + Args: + disasm_func: capstone disassembly function gathered from `get_disasm_function` + syscall_finder: the method and constraints on the malicious system call blocks that the calling plugin knows how to find + data: the bytes from memory to search for malicious syscall invocations + address: the address from where `data` came from in the particular process + Returns: + Optional[Tuple[str, capstone._cs_insn]]: For valid blocks, the disassembled bytes in string from and the last (termination) instruction + """ + found_movr10 = False + found_movreax = False + found_syscall = False + found_end = False + end_inst = None + + disasm_bytes = "" + + for inst in disasm_func(data, address): + disasm_bytes += f"{inst.address:#x}: {inst.mnemonic} {inst.op_str}; " + + # an instruction of all 0x00 opcodes + if inst.opcode.count(0) == len(inst.opcode): + break + + op = inst.mnemonic + + # invalid op, bail + if op in syscall_finder.invalid_ops: + break + + # found the end instruction wanted by the caller + elif op in syscall_finder.termination_ops: + found_end = True + end_inst = inst + break + + # track this no matter what to make code more re-usable + elif op == "syscall": + found_syscall = True + + # if we hit a 'syscall' but RAX or R10 haven't been touched + # then we are in an invalid path, so bail + if not syscall_finder.wants_syscall_inst or ( + not (found_movr10 and found_movreax) + ): + break + + else: + # attempt to see if any other instruction type wrote to registers + try: + _, regs_written = inst.regs_access() + except capstone.CsError: + continue + + if regs_written: + for r in regs_written: + # track writes to eax/rax or R10 + reg = inst.reg_name(r) + if reg in ["eax", "rax"]: + found_movreax = True + + elif reg == "r10": + found_movr10 = True + + # if any of these are missing, the block is invalid regardless of + # the technique we are trying to detect now or in the future + if not (found_movr10 and found_movreax and found_end): + return None + + # if the finder requires a 'syscall' instruction then bail now if we didn't find one + if syscall_finder.wants_syscall_inst and not found_syscall: + return None + + return disasm_bytes, end_inst + + @staticmethod + def get_disasm_function(architecture: str) -> Callable: + """ + Returns the disassembly handler for the given architecture + .detail is used to get full instruction information + + Args: + architecture: the name of the architecture for the process being disassembled + Returns: + The disasm function from capstone for the given architecture + """ + disasm_types = { + "intel": capstone.Cs(capstone.CS_ARCH_X86, capstone.CS_MODE_32), + "intel64": capstone.Cs(capstone.CS_ARCH_X86, capstone.CS_MODE_64), + } + + disasm_type = disasm_types[architecture] + disasm_type.detail = True + return disasm_type.disasm + + @classmethod + def _is_valid_syscall( + cls, + syscall_finder: syscall_finder_type, + proc_layer: interfaces.layers.DataLayerInterface, + architecture: str, + vads: List[Tuple[int, int, str]], + address: int, + ) -> Optional[Tuple[int, str]]: + """ + Args: + syscall_finder: + proc_layer: the memory layer of the process being scanned + architecture: the name of the architecture for the process being disassembled + vads: the ranges of this process under 10MB + address: the starting address to check for malicious syscall code blocks + + Returns: + Optional[Tuple[int, str]]: For valid code blocks, the starting address of the block and the disassembly string + """ + # the number bytes behind the yara rule hit to scan + behind = 32 + + address = address - behind + + try: + data = proc_layer.read(address, behind * 2) + except exceptions.InvalidAddressException: + return None + + disasm_func = cls.get_disasm_function(architecture) + + # since Intel does not have fixed-size instructions, we have to scan + # each byte offset and re-disassemble the remaining block + for offset in range(behind): + # if this looks like a system call back (r10, rax, ret/jmp) + syscall_info = cls._is_syscall_block( + disasm_func, syscall_finder, data[offset:], address + offset + ) + if syscall_info: + disasm_bytes, end_inst = syscall_info + + # if we can recover (and require) a target address for this malware technique + if syscall_finder.get_syscall_target_address: + target_address = syscall_finder.get_syscall_target_address( + proc_layer, end_inst + ) + + # could not determine the address -> invalid basic block + if not target_address: + continue + + # we only care about calls to system call DLLs + path = cls._get_range_path(vads, target_address) + if not isinstance(path, str) or not path.lower().endswith( + cls.valid_syscall_handlers + ): + continue + + # return the address and disassembly string if all checks pass + return address + offset, disasm_bytes + + return None + + @staticmethod + def _get_vad_maps( + task: interfaces.objects.ObjectInterface, + ) -> List[Tuple[int, int, str]]: + """Creates a map of start/end addresses within a virtual address + descriptor tree. + + Args: + task: The EPROCESS object of which to traverse the vad tree + + Returns: + An iterable of tuples containing start and end addresses for each descriptor + """ + vads: List[Tuple[int, int, str]] = [] + + # scan regions under 10MB + scan_max = 10 * 1000 * 1000 + + vad_root = task.get_vad_root() + + for vad in vad_root.traverse(): + if vad.get_size() < scan_max: + vads.append((vad.get_start(), vad.get_size(), vad.get_file_name())) + + return vads + + @staticmethod + def _get_range_path(ranges: List[Tuple[int, int, str]], address: int) -> Optional[str]: + """ + Returns the path for the range holding `address`, if found + + Args: + ranges: VADs collected from `_get_vad_maps` + address: the address to find + Returns: + The path holding the address, if any + """ + for start, size, path in ranges: + if start <= address < start + size: + return path + + return None + + @classmethod + def _get_tasks_to_scan( + cls, + context: interfaces.context.ContextInterface, + layer_name: str, + symbol_table_name: str, + ) -> Generator[ + Tuple[interfaces.objects.ObjectInterface, str, str, str], None, None + ]: + """ + Gathers active processes with the extra information needed + to detect malicious syscall instructions + + Returns: + Generator of the process object, name, memory layer, and architecture + """ + + # gather active processes + filter_func = pslist.PsList.create_active_process_filter() + + is_32bit_arch = not symbols.symbol_table_is_64bit(context, symbol_table_name) + + for proc in pslist.PsList.list_processes( + context=context, + layer_name=layer_name, + symbol_table=symbol_table_name, + filter_func=filter_func, + ): + proc_name = utility.array_to_string(proc.ImageFileName) + + # skip Defender + if proc_name in ["MsMpEng.exe"]: + continue + + try: + proc_layer_name = proc.add_process_layer() + except exceptions.InvalidAddressException: + continue + + if is_32bit_arch or proc.get_is_wow64(): + architecture = "intel" + else: + architecture = "intel64" + + yield proc, proc_name, proc_layer_name, architecture + + @classmethod + def _get_rule_hits( + cls, + context: interfaces.objects.ObjectInterface, + proc_layer: interfaces.layers.DataLayerInterface, + vads: List[Tuple[int, int, str]], + pattern: str, + ) -> Generator[Tuple[int, Optional[str]], None, None]: + """ + Runs the given opcode rule through Yara and returns the address and file path of hits + + Args: + context: + proc_layer: the layer to scan + vads: the ranges inside of the process being scanned + pattern: the opcodes rule from the plugin to detect a particular EDR-bypass technique + + Returns: + Generator of the address and file path of hits + """ + sections = [(vad[0], vad[1]) for vad in vads] + + rule = yarascan.YaraScanner.get_rule(pattern) + + for hit in proc_layer.scan( + context=context, + scanner=yarascan.YaraScanner(rules=rule), + sections=sections, + ): + address = hit[0] + + path = cls._get_range_path(vads, address) + + # ignore hits in the system call DLLs + if isinstance(path, str) and path.lower().endswith( + cls.valid_syscall_handlers + ): + continue + + yield address, path + + def _generator(self) -> Generator[Tuple[int, Tuple[str, int, Optional[str], int, str]], None, None]: + kernel = self.context.modules[self.config["kernel"]] + + for proc, proc_name, proc_layer_name, architecture in self._get_tasks_to_scan( + self.context, kernel.layer_name, kernel.symbol_table_name + ): + proc_layer = self.context.layers[proc_layer_name] + + vads = self._get_vad_maps(proc) + + # for each valid process, look for malicious syscall invocations + for address, vad_path in self._get_rule_hits( + self.context, proc_layer, vads, self.syscall_finder.rule_str + ): + syscall_info = self._is_valid_syscall( + self.syscall_finder, proc_layer, architecture, vads, address + ) + if not syscall_info: + continue + + address, disasm_bytes = syscall_info + + yield 0, ( + proc_name, + proc.UniqueProcessId, + vad_path, + format_hints.Hex(address), + disasm_bytes, + ) + + def run(self) -> renderers.TreeGrid: + return renderers.TreeGrid( + [ + ("Process", str), + ("PID", int), + ("Range", str), + ("Address", format_hints.Hex), + ("Disasm", str), + ], + self._generator(), + ) diff --git a/volatility3/framework/plugins/windows/indirect_system_calls.py b/volatility3/framework/plugins/windows/indirect_system_calls.py new file mode 100644 index 000000000..417c1b46e --- /dev/null +++ b/volatility3/framework/plugins/windows/indirect_system_calls.py @@ -0,0 +1,124 @@ +# This file is Copyright 2024 Volatility Foundation and licensed under the Volatility Software License 1.0 +# which is available at https://www.volatilityfoundation.org/license/vsl-v1.0 +# + +import struct +import logging +from typing import List, Optional + +import capstone + +from volatility3.framework import interfaces, exceptions +from volatility3.framework.configuration import requirements +from volatility3.plugins import yarascan +from volatility3.plugins.windows import pslist, direct_system_calls + +vollog = logging.getLogger(__name__) + + +class IndirectSystemCalls(direct_system_calls.DirectSystemCalls): + _required_framework_version = (2, 4, 0) + _version = (1, 0, 0) + + def __init__(self, *args, **kwargs): + super().__init__(*args, **kwargs) + + self.syscall_finder = direct_system_calls.syscall_finder_type( + # gets the target address of a indirect jmp + self._indirect_syscall_block_target, + # we are looking for indirect system calls, so we don't want 'syscall' instructions in our code block + False, + # jmp [address]; ret + "/\\xff\\x25[^\\xc3]{,24}\\xc3/", + # any of these mean we aren't in a malicious indirect call + ["call", "leave", "int3", "ret"], + # stop at jmp, this should reference the system call instruction + ["jmp"], + ) + + @classmethod + def get_requirements(cls) -> List[interfaces.configuration.RequirementInterface]: + # create a list of requirements for vadyarascan + vadyarascan_requirements = [ + requirements.ModuleRequirement( + name="kernel", + description="Windows kernel", + architectures=["Intel32", "Intel64"], + ), + requirements.PluginRequirement( + name="pslist", plugin=pslist.PsList, version=(2, 0, 0) + ), + requirements.VersionRequirement( + name="yarascanner", component=yarascan.YaraScanner, version=(2, 1, 0) + ), + requirements.PluginRequirement( + name="yarascan", plugin=yarascan.YaraScan, version=(2, 0, 0) + ), + requirements.PluginRequirement( + name="direct_system_calls", + plugin=direct_system_calls.DirectSystemCalls, + version=(1, 0, 0), + ), + ] + + # get base yarascan requirements for command line options + yarascan_requirements = yarascan.YaraScan.get_yarascan_option_requirements() + + # return the combined requirements + return yarascan_requirements + vadyarascan_requirements + + @staticmethod + def _indirect_syscall_block_target( + proc_layer: interfaces.layers.DataLayerInterface, inst: capstone._cs_insn + ) -> Optional[int]: + """ + This function determines the address of a jmp in the following form: + + jmp [address] + + To determine this, we must: + 1) Pull the 4 byte relative offset of 'address' inside the instruction + 2) Compute the full address of this relative offset + 3) Read from the address as it is being dereferenced + 4) Ensure the target address points to a 'syscall' instruction + + Args: + proc_layer: the layer of the potential syscall block + inst: the terminating instruction of the syscall block check + Returns: + The target address of the jump if it can be computed + """ + + try: + jmp_address_str = proc_layer.read(inst.address, 6) + except exceptions.InvalidAddressException: + return None + + # Should be an jmp... + if jmp_address_str[0:2] != b"\xff\x25": + return None + + # get the address of the 'jmp [address]' instrunction + relative_offset = struct.unpack("