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#!/usr/bin/env python3 | |
''' | |
A simplified FLOSS implementation that only supports stackstrings. | |
requirements: | |
- yara-python | |
- unicorn | |
author: Willi Ballenthin | |
email: [email protected] | |
''' | |
import os | |
import re | |
import sys | |
import zipfile | |
import os.path | |
import logging | |
import collections | |
import yara | |
import unicorn | |
import argparse | |
logger = logging.getLogger(__name__) | |
# via: https://gist.github.com/williballenthin/8e3913358a7996eab9b96bd57fc59df2 | |
ASCII_BYTE = b' !\"#\$%&\'\(\)\*\+,-\./0123456789:;<=>\?@ABCDEFGHIJKLMNOPQRSTUVWXYZ\[\]\^_`abcdefghijklmnopqrstuvwxyz\{\|\}\\\~\t' | |
def ascii_strings(buf, n=4): | |
''' | |
extract ASCII encoded strings from the given bytes. | |
Args: | |
buf (bytes): the raw bytes to search. | |
n (int): the minimum string length, default: 4. | |
Yields: | |
str: the extracted string. | |
''' | |
reg = b'([%s]{%d,})' % (ASCII_BYTE, n) | |
ascii_re = re.compile(reg) | |
for match in ascii_re.finditer(buf): | |
yield match.group().decode('ascii') | |
def unicode_strings(buf, n=4): | |
''' | |
extract UTF-16LE encoded strings from the given bytes. | |
Args: | |
buf (bytes): the raw bytes to search. | |
n (int): the minimum string length, default: 4. | |
Yields: | |
str: the extracted string. | |
''' | |
reg = b'((?:[%s]\x00){%d,})' % (ASCII_BYTE, n) | |
uni_re = re.compile(reg) | |
for match in uni_re.finditer(buf): | |
try: | |
yield match.group().decode('utf-16') | |
except UnicodeDecodeError: | |
pass | |
def align(value, alignment): | |
''' | |
align the given value. | |
result will be greater than or equal to the given value. | |
Args: | |
value (int): the base value. | |
alignment (int): the alignment increment. | |
Returns: | |
int: the aligned value. | |
''' | |
if value % alignment == 0: | |
return value | |
return value + (alignment - (value % alignment)) | |
CODE_ADDR = 0x00401000 | |
STACK_ADDR = 0x69690000 | |
def emulate_stackstrings(insns): | |
''' | |
emulate the given instructions and return the stack memory region. | |
its assumed that the instructions simply manipulate the stack memory. | |
the state of general purpose registers, aside from RSP and RBP, are undefined. | |
the active stack frame region between RBP and RSP is 0x1000 bytes in size. | |
Args: | |
insns (bytes): the raw stack string creation instructions. must be less than 0x1000 bytes. | |
Returns: | |
bytes: the 0x3000 bytes of stack memory after emulation. | |
''' | |
emu = unicorn.Uc(unicorn.UC_ARCH_X86, unicorn.UC_MODE_64) | |
logging.debug('mapping instructions') | |
emu.mem_map(CODE_ADDR, align(len(insns), 0x1000)) | |
emu.mem_write(CODE_ADDR, insns) | |
# stack layout: | |
# | |
# min-addr -> STACK_ADDR | |
# rsp ------> STACK_ADDR + 0x1000 | |
# rbp ------> STACK_ADDR + 0x2000 | |
# max-addr -> STACK_ADDR + 0x3000 | |
logging.debug('mapping stack') | |
emu.mem_map(STACK_ADDR, 0x3000) | |
emu.reg_write(unicorn.x86_const.UC_X86_REG_RSP, STACK_ADDR + 0x1000) | |
emu.reg_write(unicorn.x86_const.UC_X86_REG_RBP, STACK_ADDR + 0x2000) | |
try: | |
logging.debug('emulating instructions') | |
emu.emu_start(CODE_ADDR, CODE_ADDR+len(insns)) | |
except unicorn.UcError as e: | |
logging.debug('emulation error: %s', str(e)) | |
logging.debug('fetching stack region') | |
stack = emu.mem_read(STACK_ADDR, 0x3000) | |
return stack | |
def match_yara(rules, buf): | |
''' | |
match the given YARA rules against the given buffer. | |
ignore overlapping hits for the same rule; that is, greedily take the first hit. | |
Args: | |
rules (yara.Rules): the compiled YARA rules. | |
buf (bytes): the bytes to search. | |
Yields: | |
tuple[str, str, int, bytes]: the matched rule name, string name, hit offset, and hit bytes. | |
''' | |
seen = collections.defaultdict(lambda: set([])) | |
for match in rules.match(data=buf): | |
for (offset, sname, s) in match.strings: | |
if offset in seen[match.rule]: | |
continue | |
yield match.rule, sname, offset, s | |
for addr in range(offset, offset+len(s)): | |
seen[match.rule].add(addr) | |
def extract_stackstrings(rules, buf): | |
''' | |
extract stack strings via using YARA and emulation. | |
match the given YARA rules given buffer to identify potential stack string creation instructions. | |
then emulate the instructions and run strings on the modified stack region. | |
Args: | |
rules (yara.Rules): the compiled YARA rules. | |
buf (bytes): the bytes to search. | |
Yields: | |
tuple[int, str]: the instructions start offset and decoded string. | |
''' | |
logger.debug('matching YARA rules...') | |
for rname, sname, offset, insns in match_yara(rules, buf): | |
stack = emulate_stackstrings(insns) | |
for string in ascii_strings(stack): | |
yield offset, string | |
for string in unicode_strings(stack): | |
yield offset, string | |
FileEntry = collections.namedtuple('FileEntry', ['path', 'f']) | |
def enumerate_files(path, password): | |
''' | |
generate the paths to files found under the given directory. | |
Args: | |
path (str): file system path to a directory. | |
Yields: | |
str: file system path to a file. | |
''' | |
# the dup here is annoying... | |
if os.path.isfile(path): | |
if path.lower().endswith('.zip'): | |
with zipfile.ZipFile(path, 'r') as z: | |
z.setpassword(password.encode('ascii')) | |
for name in z.namelist(): | |
with z.open(name, 'r') as f: | |
yield FileEntry(path='%s:%s' % (path, name), f=f) | |
for root, dirs, files in os.walk(path): | |
for file in files: | |
path = os.path.join(root, file) | |
if file.lower().endswith('.zip'): | |
with zipfile.ZipFile(path, 'r') as z: | |
z.setpassword(password.encode('ascii')) | |
for name in z.namelist(): | |
with z.open(name, 'r') as f: | |
yield FileEntry(path='%s:%s' % (path, name), f=f) | |
else: | |
with open(path, 'rb') as f: | |
yield FileEntry(path=path, f=f) | |
def main(argv=None): | |
if argv is None: | |
argv = sys.argv[1:] | |
cd = os.path.dirname(__file__) | |
parser = argparse.ArgumentParser(description='Extract stack strings from raw x86 blobs.') | |
parser.add_argument("input", type=str, help="Path to input file") | |
parser.add_argument("--yara", type=str, help="Path to file containing YARA rules", | |
default=os.path.join(cd, "stackstrings.yara")) | |
parser.add_argument("-r", "--recursive", action="store_true", | |
help="Enable recursive scanning of files") | |
parser.add_argument("-v", "--verbose", action="store_true", | |
help="Enable debug logging") | |
parser.add_argument("-q", "--quiet", action="store_true", | |
help="Disable all output but errors") | |
args = parser.parse_args(args=argv) | |
if args.verbose: | |
logging.basicConfig(level=logging.DEBUG) | |
logging.getLogger().setLevel(logging.DEBUG) | |
elif args.quiet: | |
logging.basicConfig(level=logging.ERROR) | |
logging.getLogger().setLevel(logging.ERROR) | |
else: | |
logging.basicConfig(level=logging.INFO) | |
logging.getLogger().setLevel(logging.INFO) | |
logger.debug('compiling YARA rules...') | |
rules = yara.compile(args.yara) | |
if args.recursive: | |
logger.debug('collecting files to scan...') | |
files = enumerate_files(args.input, password='infected') | |
else: | |
# TODO: leak | |
f = open(args.input) | |
files = [FileEntry(path=args.input, f=f)] | |
for entry in files: | |
logger.debug('scanning file: %s', entry.path) | |
buf = entry.f.read() | |
for offset, string in extract_stackstrings(rules, buf): | |
print('%s:0x%x: %s' % (entry.path, offset, string)) | |
return 0 | |
if __name__ == "__main__": | |
sys.exit(main()) |
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rule stack_strings | |
{ | |
meta: | |
author = "William Ballenthin" | |
email = "[email protected]" | |
license = "Apache 2.0" | |
copyright = "FireEye, Inc" | |
description = "Match x86 that appears to be stack string creation." | |
strings: | |
// stack string near the frame pointer. | |
// the compiler may choose to use a single byte offset from $bp. | |
// like: mov [ebp-10h], 25h | |
// | |
// regex explanation: | |
// 4 times: | |
// byte C6 (mov) | |
// byte 45 ($bp-relative, one-byte offset) | |
// any byte (the offset from $bp) | |
// printable ascii (the immediate constant) | |
// 1 times: | |
// byte C6 (mov) | |
// byte 45 ($bp-relative, one-byte offset) | |
// any byte (the offset from $bp) | |
// byte 00 (the immediate constant, null terminator) | |
$ss_small_bp = /(\xC6\x45.[a-zA-Z0-9 -~]){4,}\xC6\x45.\x00/ | |
// stack strings further away from the frame pointer. | |
// the compiler may choose to use a four-byte offset from $bp. | |
// like: mov byte ptr [ebp-D80h], 5Ch | |
// we restrict the offset to be within 0xFFF (4095) of the frame pointer. | |
// | |
// regex explanation: | |
// 4 times: | |
// byte C6 (mov) | |
// byte 85 ($bp-relative, four-byte offset) | |
// any byte (LSB of the offset from $bp) | |
// byte 0xF0-0xFF (second LSB of the offset from $bp) | |
// byte FF (second MSB) | |
// byte FF (MSB of the offset from $bp) | |
// printable ascii (the immediate constant) | |
// 1 times: | |
// byte C6 (mov) | |
// byte 85 ($bp-relative, four-byte offset) | |
// any byte (LSB of the offset from $bp) | |
// byte 0xF0-0xFF (second LSB of the offset from $bp) | |
// byte FF (second MSB) | |
// byte FF (MSB of the offset from $bp) | |
// byte 00 (the immediate constant, null terminator) | |
$ss_big_bp = /(\xC6\x85.[\xF0-\xFF]\xFF\xFF[a-zA-Z0-9 -~]){4,}\xC6\x85.[\xF0-\xFF]\xFF\xFF\x00/ | |
// stack string near the stack pointer. | |
// the compiler may choose to use a single byte offset from $sp. | |
// like: mov byte ptr [esp+0Bh], 24h | |
// | |
// regex explanation: | |
// 4 times: | |
// byte C6 (mov) | |
// byte 44 ($sp-relative, one-byte offset) | |
// byte 24 ($sp-relative, one-byte offset) | |
// any byte (the offset from $sp) | |
// printable ascii (the immediate constant) | |
// 1 times: | |
// byte C6 (mov) | |
// byte 44 ($sp-relative, one-byte offset) | |
// byte 24 ($sp-relative, one-byte offset) | |
// any byte (the offset from $sp) | |
// byte 00 (the immediate constant, null terminator) | |
$ss_small_sp = /(\xC6\x44\x24.[a-zA-Z0-9 -~]){4,}\xC6\x44\x24.\x00/ | |
// stack strings further away from the stack pointer. | |
// the compiler may choose to use a four-byte offset from $sp. | |
// like: byte ptr [esp+0DDh], 49h | |
// we restrict the offset to be within 0xFFF (4095) of the stack pointer. | |
// | |
// regex explanation: | |
// 4 times: | |
// byte C6 (mov) | |
// byte 84 ($sp-relative, four-byte offset) | |
// byte 24 ($sp-relative, four-byte offset) | |
// any byte (LSB of the offset from $sp) | |
// byte 0x00-0x0F (second LSB of the offset from $sp) | |
// byte 00 (second MSB) | |
// byte 00 (MSB of the offset from $sp) | |
// printable ascii (the immediate constant) | |
// 1 times: | |
// byte C6 (mov) | |
// byte 84 ($sp-relative, four-byte offset) | |
// byte 24 ($sp-relative, four-byte offset) | |
// any byte (LSB of the offset from $sp) | |
// byte 0x00-0x0F (second LSB of the offset from $sp) | |
// byte 00 (second MSB) | |
// byte 00 (MSB of the offset from $sp) | |
// byte 00 (the immediate constant, null terminator) | |
$ss_big_sp = /(\xC6\x84\x24.[\x00-\x0F]\x00\x00[a-zA-Z0-9 -~]){4,}\xC6\x84\x24.[\x00-\x0F]\x00\x00\x00/ | |
// like: | |
// .text:10001A34 C7 06 57 61 69 74 mov dword ptr [esi], 74696157h | |
// .text:10001A3A C7 46 04 46 6F 72 53 mov dword ptr [esi+4], 53726F46h | |
// .text:10001A41 C7 46 08 69 6E 67 6C mov dword ptr [esi+8], 6C676E69h | |
// .text:10001A48 C7 46 0C 65 4F 62 6A mov dword ptr [esi+0Ch], 6A624F65h | |
// .text:10001A4F C7 46 10 65 63 74 00 mov dword ptr [esi+10h], 74 | |
$ss_small_reg_dword_ascii = /(\xC7.?.[a-zA-Z0-9 -~][a-zA-Z0-9 -~][a-zA-Z0-9 -~][a-zA-Z0-9 -~]){2,}/ | |
// like: | |
// unicode string | |
// like: | |
// .text:0000000180008993 C7 44 24 64 6D 00 6C 00 mov dword ptr [rsp+64h], 6C006Dh | |
// .text:000000018000899B C7 44 24 68 2E 00 64 00 mov dword ptr [rsp+68h], 64002Eh | |
// .text:00000001800089A3 C7 44 24 6C 6C 00 6C 00 mov dword ptr [rsp+6Ch], 6 | |
// like: | |
// .text:0000000180008916 C7 45 08 65 00 73 00 mov dword ptr [rbp+8], 730065h | |
// .text:000000018000891D C7 45 0C 2E 00 64 00 mov dword ptr [rbp+0Ch], 64002Eh | |
// .text:0000000180008924 C7 45 10 6C 00 6C 00 mov dword ptr [rbp+10h], 6 | |
$ss_small_reg_dword_uni = /(\xC7.?.?.[a-zA-Z0-9 -~][a-zA-Z0-9 -~][a-zA-Z0-9 -~][a-zA-Z0-9 -~]){2,}/ | |
condition: | |
$ss_small_bp or $ss_big_bp or $ss_small_sp or $ss_big_sp or $ss_small_reg_dword_ascii or $ss_small_reg_dword_uni | |
} |
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