Having already completed the SDK documentation of our core modules, we have started documenting our file format modules and just finished the first two.
Namely, we have documented the API for parsing Microsoft Cabinet files.
And we have documented our comprehensive API for parsing certificate files in both DER and PEM encodings.
We’ll continue documenting our file format modules in the upcoming months.
You can find the original analysis for this malware at malwarebytes. As a bonus, in the video we show how to improve the static analysis of the final payload by resolving API calls.
This script converts the decrypted blob hashes into a call index → api name dictionary.
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from Pro.Core import *
from Pro.PE import *
from Pro.ccast import *
ror = lambda val, r_bits, max_bits: \
((val & (2**max_bits-1)) >> r_bits%max_bits) | \
(val << (max_bits-(r_bits%max_bits)) & (2**max_bits-1))
def getAPIs(dllpath):
apis = {}
c = createContainerFromFile(dllpath)
dll = PEObject()
if not dll.Load(c):
print("error: couldn't load dll")
return apis
ordbase = dll.ExportDirectory().Num("Base")
functions = dll.ExportDirectoryFunctions()
names = dll.ExportDirectoryNames()
nameords = dll.ExportDirectoryNameOrdinals()
n = functions.Count()
it = functions.iterator()
for x in range(n):
func = it.next()
ep = func.Num(0)
if ep == 0:
continue
apiord = str(ordbase + x)
n2 = nameords.Count()
it2 = nameords.iterator()
name_found = False
for y in range(n2):
no = it2.next()
if no.Num(0) == x:
name = names.At(y)
offs = dll.RvaToOffset(name.Num(0))
name, ret = dll.ReadUInt8String(offs, 500)
apiname = name.decode("ascii")
apis[apiname] = apiord
apis[apiord] = apiname
name_found = True
break
if not name_found:
apis[apiord] = apiord
return apis
def hash(name):
x = 0
for c in name:
x = ror(x, 0xD, 32)
x += ord(c)
x = ror(x, 0xD, 32)
return x
def hashAPIs(apis):
hapis = {}
for i, name in apis.items():
hapis[hash(name)] = name
return hapis
def walkBlob():
i = 0
idxs = {}
pos = 0
while i < len(blob):
e = blob.find(b"\x00", i)
if i == e:
break
s = blob[i:e].decode("ascii")
i = e + 1
if "." in s:
dllname = s
apis = getAPIs("C:\\Windows\\System32\\" + dllname)
apis = hashAPIs(apis)
else:
x = dword(int(s))
apiname = apis[x]
idxs[pos] = apiname
pos += 0x10
print(apiname)
return idxs
idxs = walkBlob()
print(idxs) This script uses the index dictionary from the previous script to comment register-based call instructions in the disassembly with the resolved API name.
idxs = {0: 'VirtualAlloc', 16: 'Sleep', 32: 'CreateThread', 48: 'CloseHandle', 64: 'ReadFile', 80: 'CreateFileA', 96: 'WriteFile', 112: 'GetFileSize', 128: 'lstrlenA', 144: 'lstrlenW', 160: 'lstrcpyA', 176: 'lstrcpyW', 192: 'lstrcatA', 208: 'lstrcatW', 224: 'lstrcmpA', 240: 'lstrcmpW', 256: 'VirtualFree', 272: 'WaitForSingleObject', 288: 'TerminateThread', 304: 'GetTickCount', 320: 'FormatMessageA', 336: 'GetLastError', 352: 'EnterCriticalSection', 368: 'LeaveCriticalSection', 384: 'InitializeCriticalSection', 400: 'DeleteCriticalSection', 416: 'LocalFree', 432: 'MultiByteToWideChar', 448: 'WideCharToMultiByte', 464: 'GetComputerNameW', 480: 'GetModuleFileNameW', 496: 'GetCurrentProcessId', 512: 'GetLocalTime', 528: 'QueryPerformanceFrequency', 544: 'QueryPerformanceCounter', 560: 'IsWow64Process', 576: 'GetCurrentProcess', 592: 'GetVersionExA', 608: 'GlobalFree', 624: 'VirtualFreeEx', 640: 'DuplicateHandle', 656: 'DebugBreak', 672: 'CreateEventW', 688: 'DeviceIoControl', 704: 'DeleteFileA', 720: 'GetTempPathA', 736: 'GetTempFileNameA', 752: 'SetErrorMode', 768: 'FreeLibrary', 784: 'RtlGetNtVersionNumbers', 800: 'RtlNtStatusToDosError', 816: 'RtlDecompressBuffer', 832: 'RtlCompressBuffer', 848: 'RtlGetCompressionWorkSpaceSize', 864: 'NtQuerySystemInformation', 880: 'NtQueryObject', 896: 'PeekMessageW', 912: 'GetMessageW', 928: 'PostThreadMessageW', 944: 'send', 960: 'recv', 976: 'closesocket', 992: 'WSAStartup', 1008: 'socket', 1024: 'bind', 1040: 'listen', 1056: 'accept', 1072: 'connect', 1088: 'WSACleanup', 1104: 'inet_addr', 1120: 'inet_ntoa', 1136: 'htons', 1152: 'getaddrinfo', 1168: 'freeaddrinfo', 1184: 'WSAAddressToStringA', 1200: 'setsockopt', 1216: 'getsockopt', 1232: 'recvfrom', 1248: 'sendto', 1264: 'shutdown', 1280: 'WSAGetLastError', 1296: 'select', 1312: 'getpeername', 1328: 'CryptAcquireContextW', 1344: 'CryptDestroyHash', 1360: 'CryptCreateHash', 1376: 'CryptHashData', 1392: 'CryptGetHashParam', 1408: 'CryptDeriveKey', 1424: 'CryptEncrypt', 1440: 'CryptDecrypt', 1456: 'GetUserNameW', 1472: 'UuidCreate', 1488: 'WinHttpGetIEProxyConfigForCurrentUser', 1504: 'WinHttpOpen', 1520: 'WinHttpGetProxyForUrl', 1536: 'WinHttpCloseHandle', 1552: 'WinHttpConnect', 1568: 'WinHttpOpenRequest', 1584: 'WinHttpAddRequestHeaders', 1600: 'WinHttpSendRequest', 1616: 'WinHttpWriteData', 1632: 'WinHttpQueryDataAvailable', 1648: 'WinHttpQueryOption', 1664: 'WinHttpReceiveResponse', 1680: 'WinHttpReadData', 1696: 'WinHttpSetOption', 1712: 'WinHttpSetCredentials', 1728: 'WinHttpQueryAuthSchemes', 1744: 'GetAdaptersInfo', 1760: 'CryptBinaryToStringA', 1776: 'CryptStringToBinaryA', 1792: 'StrStrIA', 1808: 'URLDownloadToFileA', 1824: 'memset', 1840: 'memmove', 1856: 'memcpy', 1872: 'memcmp', 1888: '_wcsicmp', 1904: 'time', 1920: 'strstr', 1936: 'atoi', 1952: '_itow', 1968: 'srand', 1984: 'rand', 2000: '_wcsnicmp', 2016: 'sprintf', 2032: 'printf'}
from Pro.UI import proContext
from Pro.Carbon import *
from Pro.capstone import *
import re
def commentAPIs():
md = Cs(CS_ARCH_X86, CS_MODE_64)
v = proContext().getCurrentView()
ca = v.getCarbon()
db = ca.getDB()
e = caASEntry()
e.end = 0
while db.getNextASEntry(e.end, e):
if e.type_id != CarbonType_I_x64:
continue
buf = ca.read(e.start, e.end - e.start)
insns = md.disasm(buf, 0)
i = next(insns, None)
if i.mnemonic != "call":
continue
print(i.mnemonic, i.op_str)
if i.op_str.find("rip") != -1:
continue
j = i.op_str.find("+ 0x")
if j == -1:
continue
idx = int(i.op_str[j+2:-1], 16)
if idx > 0x1000:
continue
apiname = idxs.get(idx, None)
if not apiname:
continue
c = caComment()
if db.getComment(e.start, c) and c.text:
continue
c.address = e.start
c.text = apiname
db.setComment(c)
print(" ", apiname)
# update the view
v.update()
commentAPIs()Even though CAB file support is still under development, the CFBF parser already lets us inspect Windows Installer packages and patches.
Having such feature comes in handy when you want to analyse their contents, and eliminates the need for external tools.
In the screenshot above, I’ve selected one of the DLLs that link to MSI custom actions, i.e. code that is potentially executed as soon as Windows Installer opens the package.