Tuesday, September 11, 2018

Comparing Our Micropatch With Microsoft's Official Patch For CVE-2018-8440

by Mitja Kolsek, the 0patch Team

As expected, Windows Update has just brought the official patch for CVE-2018-8440 today, a patch that would replace our "unofficial" micropatch we've issued 13 days ago.

To quickly refresh your memory, 15 days ago security researcher SandboxEscaper published details and proof-of-concept (POC) for a "0day" local privilege escalation vulnerability in Windows Task Scheduler service, allowing a local unprivileged user to change permissions of any file on the system. The next day, we at 0patch have analyzed the vulnerability and prepared a "micropatch candidate" (a micropatch that blocks the POC but needs to be tested for functional side effects). We published the micropatch for Windows 10 version 1803 the day after, and subsequently ported this micropatch to a number of other Windows versions.

Obviously, correcting someone else's code is not to be taken lightly (especially if that code is running on millions of computers), and not having the source code doesn't make it any easier. However, our team of security researchers and the growing community of experts who want to get their vulnerabilities fixed as quickly and efficiently as possible, are doing just that.

In this particular case, our analysis showed that a call for changing permissions of a file, made from Task Scheduler's code, should have been impersonated - but it wasn't. We corrected that by making a call to RpcImpersonateClient right before the call, and another call to RpcRevertToSelf right after it. Pretty basic stuff. And it worked.

Our micropatches are sometimes very similar to official patches (at least functionally speaking), and other times very different; not surprisingly, there is always more than one way to skin a cat - or to fix a vulnerability. So when the official patch from Microsoft came out today, we were naturally curious as to how they fixed it.

The first thing we noticed was that the Windows Update replaced schedsvc.dll with a new version. Promising - this is the exact DLL we had micropatched. Next we ran BinDiff to compare the new and the old version. Even more promising - the only modified function was RpcServer::SetSecurity, which is the very function we had micropatched. And finally, BinDiff showed the exact difference between the old and the new version of this function. Let's take a look at that (note: the diff is for 64bit Windows 7).

As you can see, Microsoft's official patch is functionally identical to our micropatch. We could say that this official patch delivers Microsoft's unofficial confirmation that our approach to patching this vulnerability - in a complex closed-source product - was also optimal from their perspective. That's very positive feedback for us and a great case for "unofficial" 3rd-party micropatches.

Don't get carried away, however! There have been, and will be cases where a 3rd-party micropatch will fix a vulnerability differently and less well than the official vendor fix - or even break a thing or two. While we're doing our best and leverage our advantages (agility, instant patch deployment, instant patch removal), original product developers will always have advantage in their knowledge about the product and its environment, and their code fixes will be preferred. Nevertheless, our mistakes are much easier to remedy than those delivered in a 300MB package that requires a reboot both to install and uninstall. When a micropatch needs to be revoked, it takes just a few minutes for us to do that, and within an hour all online agents stop applying it without interrupting users in the slightest. And an improved version of the micropatch can later be delivered just as easily.

In any case, if you have installed 0patch Agent to micropatch this vulnerability on your computer(s), it is now time to let the official fix take over. To do that, you simply apply today's Windows Updates and as soon as they replace the vulnerable schedsvc.dll, our micropatch will automatically stop applying because the cryptographic hash of the DLL will no longer match that associated with the micropatch. You don't have to do anything else; this is 0patch being a good citizen and stepping aside when it's no longer your best option.



Friday, August 31, 2018

How We Micropatched a Publicly Dropped 0day in Task Scheduler (CVE-2018-8440)

Being Who You Are Can be a Bad Thing if You're a System Service

by Mitja Kolsek, the 0patch Team

[Update 9/11/2018] Official fix for this vulnerability is now available and users are advised to apply Windows Updates, which will automatically result in our micropatch not getting applied any more. You're welcome to read about our comparison between our micropatch and Microsoft's official fix. 

Earlier this week security researcher SandboxEscaper published details and proof-of-concept (POC) for a "0day" local privilege escalation vulnerability in Windows Task Scheduler service, which allows a local unprivileged user to change permissions of any file on the system - and thus subsequently replace or modify that file.

As the researcher's POC demonstrates, one can use this vulnerability to replace a system executable file and wait for a privileged process to execute it. In particular, it was shown that a printing-related DLL could be replaced and then executed by triggering the Print Spooler Service to load it. (The latter being a legitimate system operation, only used for demonstrating how replacing a system executable leads to elevated privileges. One could alternatively replace one of a large number of other system executables, or perhaps even a configuration file that gets loaded by a privileged process.)

SandboxEscaper's documentation properly identifies the problem being in Task Scheduler's SchRpcSetSecurity method, which is externally accessible via Advanced Local Procedure Call (ALPC) facility. This method, which can be called by any local process, sets a desired security descriptor (sddl) on a task or folder, i.e., on a provided file path (path).

HRESULT SchRpcSetSecurity(
   [in, string] const wchar_t* path,
   [in, string] const wchar_t* sddl,
   [in] DWORD flags

SandboxEscaper noticed that this method fails to impersonate the requesting client when setting the security descriptor, which results in Task Scheduler changing the access control list of the chosen file or folder as Local System user even if the user calling this method is a low-privileged user. Impersonation is a feature where, to put it simply, a process running as user A gets a request for some action from user B and performs this action disguised as user B, borrowing user B's permissions for that. Task Scheduler is such a process running as user Local System, and when some other user calls its SchRpcSetSecurity method, it should impersonate the caller to perform the file operation using their identity - but apparently it doesn't, and uses its own powerful permissions to do so.

What the POC does to demonstrate this issue is:

  1. create an UpdateTask.job file in folder %SystemRoot%\Tasks where any user is allowed to create files (this is needed in the process of creating a new scheduled task, and non-admin users are allowed to do that); however, this file is not an ordinary file but rather a hard link pointing to a system file PrintConfig.dll. (which non-system user can't modify or replace);
  2. call Task Scheduler's SchRpcSetSecurity method to change permissions on UpdateTask.job such that everyone will be able to modify it; this actually changes permissions of the linked-to PrintConfig.dll file, which thus becomes user-modifiable;
  3. replace PrintConfig.dll with a "malicious" DLL that simply launched Notepad;
  4. trigger the Print Spooler service to load and execute the modified PrintConfig.dll using its own Local System identity.

Vulnerability Analysis 

The problem is clearly in step #2, which allows a non-admin user to change permissions on a system executable, and one can quickly assess the root cause of the problem to be a combination of two facts:

  1. Task Scheduler doesn't impersonate the caller in SchRpcSetSecurity method when performing the SetSecurityFile file system operation, and 
  2. Task Scheduler being willing to perform SchRpcSetSecurity on a hard link.

After running the POC, we took a look at operations performed on UpdateTask.job with Process Monitor, and found the one that changes permissions:

So we took a look at its call stack to see who invoked this action:

Okay, there's schedsvc.dll (Task Scheduler's executable) making a call to taskcomp.dll (Task Scheduler's helper library), which ends up with a call to kernel's NtSetSecurityObject. So we disassembled schedsvc.dll and taskcomp.dll to see what's going on in there at the identified locations. What we found was interesting.

The call from schedsvc.dll to taskcomp.dll occurs in function RpcServer::SetSecurity (in the orange block):

We were expecting to see code without any impersonation here, but actually found impersonation being used - just that the call that sets file permissions is done before the impersonation (in the lowest code block) begins.

The plan was clear: let's begin impersonation before the offending call to make sure that said call will be impersonated. So we created a micropatch with a single patchlet containing a call to RpcImpersonateCient and placed it at the beginning of the block preceding the orange block. How about reverting the impersonation? It turns out that wasn't needed because all code execution paths were leading directly to another impersonation call without making any other kernel calls that might be affected by our impersonation.

We tried this micropatch, but the exploit still worked !?! What was going on?

It turned out that there is another permissions-setting call in function RpcServer::SetSecurity, possibly a fallback mechanism in case the first one failed. So we made the first one fail, and the second one came to the rescue - again without impersonation (the middle orange block).

In this case, we can see a call to RpcRevertToSelf right before the offending call, which means that previous impersonation was reverted too soon to include the said call.

What we did here was remove the premature RpcRevertToSelf call and insert a replacement RpcRevertToSelf call to the code block following the offending call. While this block has many other branches leading to it, we checked that these are not impersonated which means our inserted call won't erroneously prematurely revert some other impersonation.

So finally, our micropatch worked and Process Monitor showed this instead:

You can see the "Impersonating" line, proving that we have successfully forced Task Scheduler to impersonate the calling user when trying to set permissions on UpdateTask.job. Now, since this file was a hard link to another file which our user had insufficient permissions to modify ACL for, the result was ACCESS DENIED, as it should be.

This is the source code of our micropatch, with all of its 4 instructions in three patchlets:

; Patch for VULN-4051 in schedsvc.dll version 10.0.17134.1 64bit
MODULE_PATH "..\AffectedModules\schedsvc.dll_10.0.17134.1_64bit\schedsvc.dll"
VULN_ID 4051

 PIT rpcrt4.dll!RpcImpersonateClient
  xor ecx, ecx ; Impersonating the client that made the request
  call PIT_RpcImpersonateClient

 JUMPOVERBYTES 6 ; We eliminate the 6-byte call to RevertToSelf

 PIT rpcrt4.dll!RpcRevertToSelf
  call PIT_RpcRevertToSelf

Micropatch In Action

This video shows our micropatch in action.

Frequently Asked Questions

Q: Which Windows versions does this micropatch apply to?

Currently we have instances of this micropatch applying to:
  1. fully updated 64bit Windows 7 [added on 9/6]
  2. fully updated 64bit Windows Server 2008 [added on 9/6]
  3. fully updated 64bit Windows 10 version 1607 [added on 9/4]
  4. fully updated 64bit Windows 10 version 1709 [added on 9/5]
  5. fully updated 64bit Windows 10 version 1803
  6. fully updated 64bit Windows Server 2016 
  7. fully updated 64bit Windows Server 1803 [added on 9/6]
[Update 9/6/2018] Big thanks to Will Dormann for confirming the vulnerability as well as effectiveness of our micropatch on Windows Server 1803 in a real-time Twitter DM session!

We can quickly port the micropatch to other affected versions but we'll only do that on request (support@0patch.com). As far as we know at this point, the vulnerability was confirmed to also be present and exploitable on 32bit Windows 10 and 32bit Windows 7, so it's safe to assume that at least all Windows versions from Windows 7 and Windows Server 2008 are likely affected.

Q: Will modifying the exploit allow attackers to bypass this micropatch?

No, and that's one of the significant advantages of changing the code compared to signature- or behavior- based exploit prevention systems. For instance, while most antivirus products will detect the original POC by now, Will Dormann modified the POC and showed that it went undetected. Such modifications always allow for bypassing detection-based systems, while fixing the code actually removes the vulnerability. There is simply nothing there to bypass. There is no more efficient and reliable way to address a vulnerability than to actually remove it. (Although the entire industry built around vulnerabilities will try to convince you otherwise.)   

Q: How do we apply this micropatch?

If you have 0patch Agent already installed, this micropatch is already downloaded and applied so you don't have to do anything. Otherwise, download and launch the 0patch Agent installer, create a free 0patch account and register the agent to that account. You will immediately receive all micropatches including this one, and it will automatically get applied to Task Scheduler.

Q: Is this patch functionally identical to how Microsoft will fix it?

Obviously we can't know that. As we always claim, the original vendor - with their internal knowledge of the product - is best-positioned to correct their own code. Nobody else knows all the possible side effects of a code change as well as they do (granted, with large products even they often don't see everything) and in an ideal world software vendors would be issuing micropatches like this to quickly and painlessly fix vulnerabilities. That said, Microsoft may do the same as we did, but they may also prevent Task Scheduler from changing permissions on hard links. Or they may find that they need to support hard links and not impersonating the user is essential for some other operation that Task Scheduler performs - and will make a substantial code change. We often create micropatches after the vendor has issued the official update, which allows us to see what they did and ideally replicate their logic in a micropatch. With a 0day, this is obviously not possible.

You should therefore consider our micropatch a temporary solution while waiting for the official fix.  

Q: What will happen on Patch Tuesday?

When Microsoft makes their official fix available, you simply apply it as you would if you had never heard of 0patch. Applying it will automatically obsolete this micropatch on your computer as the update will replace a vulnerable executable with a fixed one, thereby changing its cryptographic hash. Since our micropatches are associated with specific hashes, this will make the micropatch inapplicable without intervention on either your end or ours. 

Q: Can we keep using this micropatch instead of applying Microsoft's update?

We strongly recommend against that. Microsoft's update will not only fix this issue in a more informed way, but will also bring fixes for other vulnerabilities that we don't have micropatches for. Yes, we hate losing hours of our lives to updating our systems too, but wouldn't dream of outright replacing official updates with our micropatches ;)

Q: How can you provide a micropatch so quickly compared to original vendors?

While having a micropatch candidate ready 24 hours after a 0day was dropped is quick relative to today's standards of software patching, a couple of things must be considered:
  1. Software vendors know their products much better than we do, and are likely to create a more comprehensive code fix than we can without their knowledge and source code. That takes more time than writing a micropatch.
  2. Software vendors bundle numerous fixes together in a "fat update" that replaces entire executables, which requires a lot more testing across the board. We test our micropatches with focused tests targeting only the patched code.
  3. "Fat updates" are a huge problem for users and vendors when something goes wrong, which is why software vendors are even more wary of issuing a defective update. Of course a micropatch also can be flawed, but it can be revoked remotely and instantly replaced with a corrected version without users ever noticing anything. That said, we will always have "fat updates" for substantial functional changes, we're just arguing that we may not need them this frequently because most vulnerabilities could be patched with micropatches. 
  4. Software vendors must issue patches for all supported versions, and extensively test all of them. We currently only have this micropatch for two three four six seven affected Windows versions. Nevertheless, porting to other versions, basic testing and deployment would take us about two hours of effort for each additional version, so that could still be done in one day.    

All that said, comparing our speed with software vendors' must also account for the difference in our deliverables. A micropatch can be quickly created, deployed to all computers in a hour's time and applied without even the slightest disturbance to users. But it must be considered a temporary security measure until the official patch can be applied.

Q: What should we do if we encounter problems with Task Scheduler after applying this micropatch?

Obviously we can't guarantee that our micropatch won't cause some unwanted side effects, e.g., with non-admin users editing existing scheduled tasks under certain circumstances. (Then again, software vendors can't guarantee that either.) The rule of thumb for using 0patch (or any other 3rd party behavior-changing product like antivirus or malware blockers) should be to first disable the agent and see if the problem persists, before contacting the original software vendor for the affected product. If the problem persists, the culprit is unlikely to be the micropatch. If the problem goes away, it's probably us and we'd like to hear from you at support@0patch.com.

Fortunately, in contrast to standard "fat update" patching that software products employ today, 0patch allows you to instantly revert a patch with a click of a button.



Wednesday, May 30, 2018

0patching Foxit Reader Buffer... Oops... Integer Overflow (CVE-2017-17557)

by Luka Treiber, 0patch Team


In April, Steven Seeley of Source Incite published a report  of a vulnerability in Foxit Reader and PhantomPDF versions up to 9.0.1 that could allow for remote code execution on a target system. Public release of this report was coordinated with an official vendor fix included in the April's Foxit Reader and PhantomPDF 9.1. release.

According to our analysis the PoC attached to the report triggers a heap-based buffer overflow in a Bitmap image data copy operation inside ConvertToPDF_x86.dll module using an overlong biWidth attribute.

When dropping SRC-2018-0009.bmp into Foxit Reader we immediately got a crash and inspected it by hooking WinDbg with Page Heap enabled.

(3250.3480): Access violation - code c0000005 (first chance)
First chance exceptions are reported before any exception handling.
This exception may be expected and handled.
*** ERROR: Symbol file could not be found.  Defaulted to export symbols for C:\Program Files (x86)\Foxit Software\Foxit Reader\Plugins\Creator\x86\ConvertToPDF_x86.dll -
eax=00000004 ebx=00000000 ecx=00000008 edx=15db5ef8 esi=15db9f38 edi=15dc2000
eip=5f5f9d17 esp=1866f904 ebp=1866f920 iopl=0         nv up ei pl nz na po cy
cs=0023  ss=002b  ds=002b  es=002b  fs=0053  gs=002b             efl=00010203
5f5f9d17 8807            mov     byte ptr [edi],al          ds:002b:15dc2000=??

It first looked like a typical buffer overflow, where a missing boundary check allows data to be written over the edge of destination buffer addressed by edi. (Note: the crash offset is marked in red.)

But the loop of the copy operation is constrained by checking biWidth (at esi+54h) which is read from Bitmap image header. So why is there an access violation despite this check?

When inspecting that buffer's properties something stuck out: an unusually small buffer size was reported by !heap to have been allocated, specifically just 4 bytes (UserSize in the WinDbg output below).

0:012> !heap -p -a edi
    address 15dc2000 found in
    _DPH_HEAP_ROOT @ 157e1000
    in busy allocation ( DPH_HEAP_BLOCK:  UserAddr  UserSize -  VirtAddr  VirtSize)
                               15ce264c:  15dc1ff8         4 -  15dc1000      2000
    65688e89 verifier!AVrfDebugPageHeapAllocate+0x00000229
    7748103e ntdll!RtlDebugAllocateHeap+0x00000030
    7743abe2 ntdll!RtlpAllocateHeap+0x000000c4
    773e34a1 ntdll!RtlAllocateHeap+0x0000023a
    5f564b50 ConvertToPDF_x86!CreateFXPDFConvertor+0x0015f120
    5f6de073 ConvertToPDF_x86!CreateFXPDFConvertor+0x002d8643
    5f6ddead ConvertToPDF_x86!CreateFXPDFConvertor+0x002d847d
    5f6de124 ConvertToPDF_x86!CreateFXPDFConvertor+0x002d86f4
   *5f5f9975 ConvertToPDF_x86!CreateFXPDFConvertor+0x001f3f45
    5f5d8598 ConvertToPDF_x86!CreateFXPDFConvertor+0x001d2b68

Inspecting SRC-2018-0009.bmp in a hex editor revealed that biWidth is set to a huge value of 40000001h - a remotely controllable attribute in case the image used in conversion to PDF was obtained from the attacker. So if biWidth was used in calculation of destination buffer size before buffer allocation, that calculation was probably prone to integer overflow in order to result in so much lower a value (4).

Besides heap block properties, !heap also printed out the buffer allocation call stack. We inspected that and indeed we found an overflow-prone buffer size calculation right before the orange-marked return address in the above call stack. The code graph below shows only a part of the vulnerable execution path, but the omitted code is very similar so it suffices for our explanation. There are three code blocks; edi represents biWidth read from image data in the first block, in the third block eax is the destination buffer size to be allocated, ecx represents biBitCount (number of bits per pixel).

There are 3 instructions that can overflow in the last code block:

  1. imul eax, ecx - in case of SRC-2018-0009.bmp, eax=40000001h and ecx=4 so this is the operation that overflows (result is 00000001`00000004h but eax can only hold the lower DWORD - 00000004h)
  2. add eax,ebx - addition of 1Fh to a potentially huge number held by eax could overflow in case previous multiplication didn't overflow
  3. add eax,edx - addition of up to 1Fh (edx is and-ed to 1Fh beforehand) to a potentially huge number held by eax could overflow

As already said, this is not the only vulnerable code block before the destination buffer allocation. Depending on biBitCount value that can hold at most 20h two other similar buffer size calculations can occur. In order to fix all of them, many checks would have to be inserted so we decided for a more compact solution. Given that all constraints to the buffer size calculation are known - buffer size can not theoretically exceed 0xffffffff, biBitCount can be at most 20h and two potentially added values are at most 1Fh -, the maximum valid biWidth could be calculated beforehand as follows:

(0xffffffff-0x1f-0x1f)/0x20 = 0x07fffffe

However, one of the vulnerable blocks does not properly handle signed values so this also needs to be taken into account by halving the maximum buffer size to 0xffffffff/2 = 0x7fffffff. Once we do that, the add eax,edx instruction can't overflow because edx is the sign extension (mind the cdq instruction) and will always be 0. So the final constraint calculation goes like this:

(0x7fffffff-0x1f)/0x20 = 0x03ffffff

Knowing this, a single check can be placed right before biWidth is first used - at the first block of the code graph above - that makes sure only biWidth values lower than 0x03ffffff can pass. If this condition is not met, we can set biWidth to 0 so the subsequent jle would raise a handled exception and abort further processing of a malformed image. Here is the patch code that does this:

; CVE-2017-17557 patch for Foxit Reader 9.0.1 
MODULE_PATH "C:\0patch\Patches\CVE-2017-17557\ConvertToPDF_x86.dll"
PATCH_ID 1000031
VULN_ID 3556


    mov edi, [esi+54h]      ; original instruction replaced by jump to this
    cmp edi, 03ffffffh      ; is edi<(7fffffffh-1fh)/20h?
    jb skip                 ; if so, everything's okay 
    call PIT_ExploitBlocked ; otherwise, we had an integer overflow - 
                              show the popup
    xor edi,edi             ; and annulate biWidth so the subsequent logic 
                              will throw a handled exception
    ;test    edi, edi       ; original instruction displaced to the trampoline

When this patch is applied to Foxit Reader's memory, the first two instructions from the top code block above (mov edi, [esi+54h] and test edi, edi, making up exactly 5 bytes) are replaced by a 5-byte jump to the patch code, forcing us to repeat the first instruction (mov edi, [esi+54h]) at the beginning of our patch code if we want to inject our code between the two instructions. The second instruction (test edi, edi) is automatically placed after the patch code by 0patch Loader because JUMPOVERBYTES 3 directs it to only omit the first instruction (3 bytes) while keeping the remaining 2-byte instruction.

This video shows our micropatch in action.

This micropatch has already been published and distributed to all installed 0patch Agents. If you're using Foxit Reader or Foxit PhantomPDF version, you can download our free 0patch Agent, create a free 0patch account and register the agent to that account to immediately receive this micropatch and have it applied to your Foxit software.

If you're using some other version of Foxit Reader and would like to have micropatches for it, please contact us at support@0patch.com.

Tuesday, May 15, 2018

Windows Updates Broke Your Networking? Free Micropatches To The Rescue (CVE-2018-8174)

A Single-Instruction Micropatch For a Critical Remote Execution Issue

by Mitja Kolsek, 0patch Team

Last week, Microsoft issued an update resolving (among others) a critical remote code execution issue in VBScript Engine named CVE-2018-8174, exploit for which has previously been detected in the wild.

Unfortunately, Microsoft's update breaks networking on some computers (details below), prompting users to avoid their application. Windows updates are "all or nothing" these days, so users can't just remove a defective KB and enjoy the protection provided by other KBs issued on the same Patch Tuesday. Fortunately, micropatching provided by 0patch is the exact opposite of that, addressing each vulnerability individually. Let's start with the exploit and the underlying vulnerability.   

It appears the exploit has been caught by at least two security firms, Kaspersky and 360, and they both issued detailed analyses of the infection chain and, more importantly for us, the vulnerability itself [Kaspersky's analysis, 360's analysis]

The vulnerability is triggered by this simple proof-of-concept provided by Kaspersky:

Dim ArrA(1)
Dim ArrB(1)

Class ClassVuln
    Private Sub Class_Terminate()
        Set ArrB(0) = ArrA(0)
        ArrA(0) = 31337
    End Sub
End Class

Sub TriggerVuln
    Set ArrA(0) = New ClassVuln
    Erase ArrA
    Erase ArrB
End Sub


By simply saving the above code to poc.vbs and double-clicking it on a Windows computer without May 2018 updates installed, we trigger the vulnerability and get wscript.exe to crash in oleaut32.dll's VariantClear function .

Access Violation in OLEAUT32.DLL's VariantClear due to accessing an unallocated memory address


Let's see what goes on here. We create two fixed-size arrays ArrA and ArrB with one element each.
A ClassVuln object is created and assigned to the first element of the ArrA array, then the ArrA array is erased. There is just one element in the array, namely the ClassVuln object, and Erase sets ArrA(0) to Nothing, which removes all references to the object and triggers its destruction.

Our object has a weird method called Class_Terminate. This is a class event that's not officially supported in VBScript anymore but apparently still works and gets called upon object's destruction. (If you're wondering why unsupported functionality still works it's probably because removing it would break an unknown number of production scripts that are using it, and cause much more headache than keeping it and removing it from documentation.)

In our Class_Terminate, the first element of ArrB is set to ArrA(0) - but wait, ArrA(0) is the very object being destroyed, and you can imagine where this is heading. This assignment increases the reference ("lock") count for the object, but the following instruction (ArrA(0) = 31337) decreases the same reference count, which will lead to the object being actually destroyed.

However, after being destroyed, there will still be a reference to this object in ArrB(0), which is called a "dangling pointer," i.e., a pointer to a memory block that has already been freed. In this case the object was allocated on the heap, so now ArrB(0) has a reference to some location on the heap, and those versed in exploiting "use after free" issues have a nice starting point to make use of this reference.

We're not interested in exploiting this issue, however, but in patching it. To do that, we first need to understand the vulnerability.

Kaspersky researchers pointed their fingers at the VBScriptClass::Release function, which only checks the object reference count before calling VBScriptClass::TerminateClass (this executes our Class_Terminate code), and doesn't account for the case where Class_Terminate would increase the object reference count. So even though the reference count was increased (by Set ArrB(0) = ArrA(0)), the object still gets destroyed.

We're not sure that this is the (entire) root cause, although we would probably try to fix the issue using this avenue if we didn't have Microsoft's official patch that contains information on how they fixed it.

After applying the May 2018 Windows updates, executing the PoC has a different result:

Patched Windows show a runtime error

The error states: "Object required: 'ArrA(...)'" in line 6, indicating that the assignment of ArrA(0) is not possible as ArrA is not an object (anymore).

First we decided to compare code execution between the patched and the unpatched version using WinDbg's wt (trace and watch data) command. Fortunately symbols are available for oleaut32.dll so the traces looked fairly identical up to the point where the patched version "derailed" from the unpatched version's path towards crashing. It turned out the fork happened in function vbscript!AssignVar, executed as a result of assigning ArrA(0) to ArrB(0). In this function (which hasn't been modified by the patch), a test is being made on the vt (variant type) member of a VARIANT structure, which in our case denotes the ArrA(0) element. And it turns out on the unpatched computer, vt is 9 (VT_DISPATCH) while on the patched computer, it is 0 (VT_EMPTY).

This means someone must have changed the value of vt from 9 to 0 along the way. In order to find out who did that, we placed an access breakpoint to the address of vt sometime earlier in the execution where it was still 9 on the patched computer.

Bingo! We found a "mov word [rdi], bp" instruction in our old friend, the VariantClear function, which sets our vt to 0. (rdi points to the VARIANT structure, and ebp is 0 throughout the function.)

It was time for BinDiff. We compared the patched and the last unpatched version of oleaut32.dll (which is where the crash occurs), and looked at the modified functions. Unsurprisingly, VariantClear was slightly modified in several places (orange blocks on the image below). Three of these blocks set the vt (variant type) member of a VARIANT structure to 0, which means VT_EMPTY, or an empty object.

Left: unpatched code; Right: patched code

Our PoC executes the middle orange block, and that block contains - in its patched version - the instruction that sets vt to 0.

So we created a micropatch that injects a logically identical instruction to the vulnerable code. Since the vulnerable code uses rbx as the pointer to the VARIANT, this is what we needed to inject:

mov word [rbx], 0

And this is the source code for our micropatch:

; Patch for CVE-2018-8174 in oleaut32.dll version 6.1.7601.23775 64bit
MODULE_PATH "c:\windows\system32\oleaut32.dll"
VULN_ID 3568



  mov word [rbx], 0 ; Set VARIANT's vt to 0 (VT_EMPTY)



Sure enough, applying this micropatch stopped the wscript.exe from crashing and made its behavior identical to the officially patched version:

The old version with our micropatch has an identical reaction to our PoC as the new version.

Our micropatches for this vulnerability have been labeled ZP-320 and ZP-321 for 32-bit and 64-bit version of oleaut32.dll respectively, and are applicable on Windows 7 and Windows 2008 Server updated up to April 2018 Windows updates. Why only these versions? Well, updates KB4103718 and KB4103712 are reportedly causing networking problems on some computers, which prompts users to delay their application - remaining vulnerable to the issue we have micropatched here. We have reports of Windows 7 and Windows 2008 Servers being affected, but if you or someone you know is experiencing this issue on other Windows versions, just ping us and we'll quickly port these micropatches.

As always, if you have 0patch Agent installed, the above-mentioned micropatches should already be present and applied on your system. If not, you can download a free copy of 0patch Agent to protect your system from CVE-2018-8174 at least until Microsoft resolves the functional problems with their updates.



Friday, March 30, 2018

Security Patching is Hard - Survey Results 2017


by Stanka Salamun, 0patch Team

Are you tired of constant inflow of software updates that require your attention and time? Well, you are not alone. While timely security patching plays an important role in providing a secure enterprise IT environment, dealing with patches as they are released at an unmanageable rate, seems to be an overwhelming task. 

We are presenting our first Security Patching is Hard - Survey Results 2017, revealing struggles and obstacles companies and individuals deal with when they try to be up to date with security patching. The survey was inspired by security experts that opened a debate over their patch fatigue after the Equifax leak.

For the first impression on survey results check out the infographic of the key takeaways.

Security Patching is Hard - Survey Results 2017

There is one obvious message from our research: the process of software patching should be simplified. 

For a dive into the facts and numbers behind the security update gap, download the report.  It reveals the main reasons for the patching fatigue, the impact of legacy software and how the relationship with software vendors influences the pace of applying security updates.


Monday, February 19, 2018

Two Interesting Micropatches For 7-Zip (CVE-2017-17969 and CVE-2018-5996)

by Luka Treiber, 0patch Team

Based on the vulnerability report from Dave we developed micropatches for heap buffer overflow CVE-2017-17969 and insufficient exception handling CVE-2018-5996 in 7-Zip version 16.04.


By diffing source code of the patched version 18.00 and the vulnerable version 16.04 we found the following changes related to CVE-2017-17969 (highlighted lines mark patched code).

001: // ShrinkDecoder.cpp

163:     lastSym = sym;
164:     unsigned cur = sym;
165:     unsigned i = 0;
167:     while (cur >= 256)
168:     {
169:       _stack[i++] = _suffixes[cur];
170:       cur = _parents[cur];
171:       // don't change that code:
172:       // Orphan Check and self-linked Orphan check (_stack overflow check);
173:       if (cur == kEmpty || i >= kNumItems)
174:         break;
175:     }
177:     if (cur == kEmpty || i >= kNumItems)
178:       break;

We disassembled 7z.dll version 16.04 and found the above changes affect two offsets - 0x000a5ab4 and 0x000a5abb (both marked in red):

We then created micropatch code resembling source code lines 173 - 174 and 177 - 178 by using two patchlets, the first one extending (piggybacking on) an existing conditional statement (the end of the while loop - jnb loc_100A5A9C) and the second one introducing an additional conditional statement:

; CVE-2017-17969 patch for 7z.dll 16.04 
MODULE_PATH "C:\0patch\Patches\7zip\16.4\7z.dll"
VULN_ID 3295


;piggybacking jnb 
;3D 00 01 00 00                       cmp     eax, 100h
;73 CE                                jnb     short loc_100AAF28
;if (cur == kEmpty || i >= kNumItems)
;    break;
    cmp eax, 100h
    jne skip1  ;cur != kEmpty?
    STC             ;set piggyback condition for jnb to NOT jump
    call PIT_ExploitBlocked
    jmp end
    cmp esi, 2000h
    jb skip2  ;i < kNumItems?
    STC             ;set piggyback condition for jnb to NOT jump
    call PIT_ExploitBlocked
    jmp end
    cmp eax, 100h ;original code

PIT 7z.dll!0xa5bc4

    cmp eax, 100h
    je block        ; cur == kEmpty?
    cmp esi, 2000h
    jge block       ; i >= kNumItems?
    jmp skip
    call PIT_ExploitBlocked
    jmp PIT_0xa5bc4 ; break


The source code patch for CVE-2018-5996 introduced the following changes (highlighted lines mark patched code):

001: // Rar3Decoder.h
194:   bool m_IsSolid;
195:   bool _errorMode;

001: // Rar3Decoder.cpp

089: CDecoder::CDecoder():
090:   _window(0),
091:   _winPos(0),
092:   _wrPtr(0),
093:   _lzSize(0),
094:   _writtenFileSize(0),
095:   _vmData(0),
096:   _vmCode(0),
097:   m_IsSolid(false),
098:   _errorMode(false)
099: {
100:   Ppmd7_Construct(&_ppmd);
101: }

827: HRESULT CDecoder::CodeReal(ICompressProgressInfo *progress)
828: {
829:   _writtenFileSize = 0;
830:   _unsupportedFilter = false;
832:   if (!m_IsSolid)
833:   {
834:     _lzSize = 0;
835:     _winPos = 0;
836:     _wrPtr = 0;
837:     for (int i = 0; i < kNumReps; i++)
838:       _reps[i] = 0;
839:     _lastLength = 0;
840:     memset(m_LastLevels, 0, kTablesSizesSum);
841:     TablesRead = false;
842:     PpmEscChar = 2;
843:     PpmError = true;
844:     InitFilters();
845:     _errorMode = false;
846:   }
848:   if (_errorMode)
849:     return S_FALSE;
851:   if (!m_IsSolid || !TablesRead)
852:   {
853:     bool keepDecompressing;
854:     RINOK(ReadTables(keepDecompressing));
855:     if (!keepDecompressing)
856:       return S_OK;
857:   }

890:   return S_OK;
891: }

929:   catch(const CInBufferException &e) { _errorMode = true; return e.ErrorCode;}
930:   catch(...) { _errorMode = true; return S_FALSE; }

When developing a micropatch for this vulnerability, we had to patch the CDecoder class that had to be extended by a new member - the _errorMode variable. This was the first time we attempted something like that with 0patch. So we either had to make room for the new variable (increase the allocated object memory block) or find existing unused space within the object's memory layout. By searching for cross references to CDecoder member variables (in offset range around the m_IsSolid variable 1c6d - 1c7c) we found offsets 1C6Fh, 1C79h, 1C7Ah, 1C7Bh to be unused. Based on this we selected and assigned the first one of the available offsets - 1C6Fh - to the new _errorMode variable.

Our micropatch contains five patchlets that correspond to five code offsets in 7z.dll that had to be patched:

  • Patchlet 1: We micropatched offset 0x000a0388 (the CDecoder::CDecoder constructor) with code resembling line 195 of RarDecoder.h and line 98 of Rar3Decoder.cpp.

  • Patchlets 2 and 3: We micropatched offsets 0x000a25bf and 0x000a25de with  code resembling lines 845 and 848-849.

  • Patchlets 4 and 5: We micropatched  offsets 0x000a22ee and 0x000a22f6 with code resembling changes to lines 929-930.

This is the 0pp file implementing these patchlets:

; CVE-2018-5996 patch for 7z.dll 16.04 
MODULE_PATH "C:\0patch\Patches\7zip\16.4\7z.dll"
VULN_ID 3296

; added variables:
;  _errorMode (this+1C6Dh)

  mov [esi+1C6Fh], bl ; _errorMode=0 //ebx set to 0 at offset a030a


  mov  [esi+1C6Fh], bl ; _errorMode=0 //ebx set to 0 at offset a2278

PIT 7z.dll!0x000A2452

  cmp [esi+1C6Fh],bl ; _errorMode=0 //ebx set to 0 at offset a2278
  jz skip
  call PIT_ExploitBlocked
  jmp PIT_0x000A2452

  mov  eax, [ebp+08h]      ; catch(const CInBufferException &e) {
  mov byte [eax+1C6Fh], 1  ;  _errorMode = true;
                           ;  return e.ErrorCode;}


  mov  eax, [ebp+08h]      ; catch(...) { 
  mov byte [eax+1C6Fh], 1  ;  _errorMode = true; 
                             ;  return S_FALSE; }

These two micropatches have already been published and distributed to all installed 0patch Agents. If you're using 32-bit 7-Zip version 16.04, you can download our free 0patch Agent, create a free 0patch account and register the agent to that account to immediately receive these micropatches and have them applied to your 7-Zip executable.

If you're using some other version of 7-Zip and would like to have micropatches for it, please contact us at support@0patch.com.