capa/capa/features/extractors/common.py

# Copyright (C) 2021 Mandiant, Inc. All Rights Reserved.
# Licensed under the Apache License, Version 2.0 (the "License");
#  you may not use this file except in compliance with the License.
# You may obtain a copy of the License at: [package root]/LICENSE.txt
# Unless required by applicable law or agreed to in writing, software distributed under the License
#  is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and limitations under the License.
import io
import re
import logging
import binascii
import contextlib
from typing import Tuple, Iterator

import pefile

import capa.features
import capa.features.extractors.elf
import capa.features.extractors.pefile
import capa.features.extractors.strings
from capa.features.common import (
    OS,
    OS_ANY,
    OS_AUTO,
    ARCH_ANY,
    FORMAT_PE,
    FORMAT_ELF,
    OS_WINDOWS,
    FORMAT_FREEZE,
    FORMAT_RESULT,
    Arch,
    Format,
    String,
    Feature,
)
from capa.features.freeze import is_freeze
from capa.features.address import NO_ADDRESS, Address, FileOffsetAddress

logger = logging.getLogger(__name__)

# match strings for formats
MATCH_PE = b"MZ"
MATCH_ELF = b"\x7fELF"
MATCH_RESULT = b'{"meta":'
MATCH_JSON_OBJECT = b'{"'


def extract_file_strings(buf: bytes, **kwargs) -> Iterator[Tuple[String, Address]]:
    """
    extract ASCII and UTF-16 LE strings from file
    """
    for s in capa.features.extractors.strings.extract_ascii_strings(buf):
        yield String(s.s), FileOffsetAddress(s.offset)

    for s in capa.features.extractors.strings.extract_unicode_strings(buf):
        yield String(s.s), FileOffsetAddress(s.offset)


def extract_format(buf: bytes) -> Iterator[Tuple[Feature, Address]]:
    if buf.startswith(MATCH_PE):
        yield Format(FORMAT_PE), NO_ADDRESS
    elif buf.startswith(MATCH_ELF):
        yield Format(FORMAT_ELF), NO_ADDRESS
    elif is_freeze(buf):
        yield Format(FORMAT_FREEZE), NO_ADDRESS
    elif buf.startswith(MATCH_RESULT):
        yield Format(FORMAT_RESULT), NO_ADDRESS
    elif re.sub(rb"\s", b"", buf[:20]).startswith(MATCH_JSON_OBJECT):
        # potential start of JSON object data without whitespace
        # we don't know what it is exactly, but may support it (e.g. a dynamic CAPE sandbox report)
        # skip verdict here and let subsequent code analyze this further
        return
    else:
        # we likely end up here:
        #  1. handling a file format (e.g. macho)
        #
        # for (1), this logic will need to be updated as the format is implemented.
        logger.debug("unknown file format: %s", buf[:4].hex())
        return


def extract_arch(buf) -> Iterator[Tuple[Feature, Address]]:
    if buf.startswith(MATCH_PE):
        yield from capa.features.extractors.pefile.extract_file_arch(pe=pefile.PE(data=buf))

    elif buf.startswith(MATCH_RESULT):
        yield Arch(ARCH_ANY), NO_ADDRESS

    elif buf.startswith(MATCH_ELF):
        with contextlib.closing(io.BytesIO(buf)) as f:
            arch = capa.features.extractors.elf.detect_elf_arch(f)

        if arch not in capa.features.common.VALID_ARCH:
            logger.debug("unsupported arch: %s", arch)
            return

        yield Arch(arch), NO_ADDRESS

    else:
        # we likely end up here:
        #  1. handling shellcode, or
        #  2. handling a new file format (e.g. macho)
        #
        # for (1) we can't do much - its shellcode and all bets are off.
        # we could maybe accept a further CLI argument to specify the arch,
        # but i think this would be rarely used.
        # rules that rely on arch conditions will fail to match on shellcode.
        #
        # for (2), this logic will need to be updated as the format is implemented.
        logger.debug("unsupported file format: %s, will not guess Arch", binascii.hexlify(buf[:4]).decode("ascii"))
        return


def extract_os(buf, os=OS_AUTO) -> Iterator[Tuple[Feature, Address]]:
    if os != OS_AUTO:
        yield OS(os), NO_ADDRESS

    if buf.startswith(MATCH_PE):
        yield OS(OS_WINDOWS), NO_ADDRESS
    elif buf.startswith(MATCH_RESULT):
        yield OS(OS_ANY), NO_ADDRESS
    elif buf.startswith(MATCH_ELF):
        with contextlib.closing(io.BytesIO(buf)) as f:
            os = capa.features.extractors.elf.detect_elf_os(f)

        if os not in capa.features.common.VALID_OS:
            logger.debug("unsupported os: %s", os)
            return

        yield OS(os), NO_ADDRESS

    else:
        # we likely end up here:
        #  1. handling shellcode, or
        #  2. handling a new file format (e.g. macho)
        #
        # for (1) we can't do much - its shellcode and all bets are off.
        # rules that rely on OS conditions will fail to match on shellcode.
        #
        # for (2), this logic will need to be updated as the format is implemented.
        logger.debug("unsupported file format: %s, will not guess OS", binascii.hexlify(buf[:4]).decode("ascii"))
        return