CAB & Certificates SDK Documentation

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.

Malicious Windows Link with Embedded Microsoft Cabinet

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()

MSI support

Even though CAB file support is still under development, the CFBF parser already lets us inspect Windows Installer packages and patches.

MSI streams

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.