Thursday, February 21, 2019

No Source Code For a 14-Year Old Vulnerable DLL? No Problem. (CVE-2018-20250)

Why It Makes Sense To Micropatch a Vulnerability When Official Fix Is Available

by Mitja Kolsek, the 0patch Team



 
 
Yesterday, Checkpoint published a detailed vulnerability report about a serious vulnerability in WinRAR (reportedly used by 500+ million users) which allowed a malicious archive, when extracted, to create an arbitrary file in a chosen location on user's computer. This includes,for instance, user's Startup folder where said file would get executed when the user logged in to Windows again.
Nadav Grossman, the researcher who found the vulnerability and wrote the report, nicely described    
the flow of his analysis, including the problems he stumbled into, and tools he used to analyze the vulnerability. Anyone interested in analyzing vulnerabilities, or in details of this particular one, is advised to read his report.

The vulnerability (CVE-2018-20250) was present in WinRAR's extracting of ACE archives, whereby said extracting was actually done using a dynamic library UNACEV2.DLL written by e-merge GmbH, maintainer of WinACE software. This DLL, interestingly, hasn't changed since 2005 - which actually attracted Nadav's attention.

At the time the report was published, WinRAR producer RARLAB was already offering a fixed version of  WinRAR (5.70 beta 1) on their download page, and their fix was very simple: being unable to fix the vulnerability, they dropped support for ACE format. (You might remember Microsoft doing something similar with the 17-year old Equation Editor last year.)


To Micropatch Or Not To Micropatch?



For us at 0patch, a vulnerability like this is inherently interesting: it allows for a remote code execution, it potentially affects millions of users, and the product doesn't auto-update so it is likely to be present for a long time. And while we've micropatched a bunch of 0days lately, 0patch is primarily meant to bridge the security update gap by fixing vulnerabilities that may already have an official fix.

In deciding whether to micropatch this one, we came up with the following supporting arguments:

  1. This vulnerability is a perfect candidate for malicious phishing campaigns. "Please review the attached purchase order PO.rar."
  2. WinRAR is a really widely used product. Maybe it doesn't actually have 500 million users but still.
  3. No auto-update means that most users will never update it. In fact, unless one has actually read about this vulnerability (surely most users haven't) and feels threatened by it, why would one take the time to update?
  4. Some users may actually need the ACE format support. Granted, there is still WinACE, but in contrast to WinRAR, its maintainer seems long gone.
  5. From defensive perspective, blocking exploits of this vulnerability is not as simple as rejecting files with .ace extension, as a malicious file can also be renamed to .rar or .zip (or likely any other supported extension) as long as it's extracted with WinRAR. The user can't even tell that a ZIP file is actually an ACE file.
  6. Some of 0patch users are probably also users of WinRAR. By creating a micropatch, we will protect them without them even being aware of this WinRAR vulnerability, or the fact that we micropatched it. (Letting users not worry about patches and not disturbing them with patching is a core concept of 0patch.)
  7. Admins with multiple 0patch Agents on users' computers would like the fact that whichever version of WinRAR, even if old and vulnerable, their users use or "temporarily" install, they will be protected against this issue.
  8. Other products than WinRAR may be using this particular version of UNACEV2.DLL, and our micropatch would protected from a similar attack too, without us even knowing what these products are.

As you suspected, we decided to go ahead.



Patching



Fortunately, Nadav's analysis provided most of the details we needed (big thanks, Nadav!) and a potential solution was fairly obvious. If you look at Figure 15 in his report and the accompanying text, you'll see that the exploit causes the execution through a branch that allows the malicious file path to remain a relative path by prepending it with... well, an empty string. So the idea was to always force the execution through the other branch, which prepends the malicious path with the target directory.

This way, instead of allowing malicious paths like:

C:\some_folder\some_file.ext

or

C:../some_folder\some_file.exe

to pass unmodified, they would become (when extracted to C:\temp):

C:\tempC:\some_folder\some_file.ext

or

C:\tempC:../some_folder\some_file.exe

These are both invalid paths and an application can't create a file using them.


One question remained open, however: Do any legitimate use cases need the code branch we were planning to "amputate"? Such use case would include intentionally extracting a file to a location specified with full path on user's computer; which would be considered a vulnerability of equal severity as the issue we're fixing here. So we concluded that removing this branch is an acceptable approach, and wrote this single-instruction micropatch:



MODULE_PATH "..\AffectedModules\UNACEV2.DLL_2.6.0.0_32bit\UNACEV2.DLL"
PATCH_ID 353
PATCH_FORMAT_VER 2
VULN_ID 4852
PLATFORM win32

patchlet_start

 PATCHLET_ID 1
 PATCHLET_TYPE 2
 PATCHLET_OFFSET 0x000CC13
 JUMPOVERBYTES 10 ; We remove mov and call
 N_ORIGINALBYTES 1

 code_start

  mov eax, 0 ; making sure to proceed via the "absolute path" branch,
             ; not the "empty string" branch

 code_end

patchlet_end
 


When the micropatch is applied, extracting a malicious ACE archive results in a number of warnings as shows below, and files aren't extracted.










 
 
Here's a video showing our micropatch in action:





Conclusion

By micropatching this vulnerability in a 14-year old closed-source DLL we demonstrated how suitable in-memory micropatching is for quickly fixing a critical vulnerability, even when fixing it in source code is literally impossible.

One nice side-effect of micropatching a third-party DLL is that while our micropatch was targeted at WinRAR, it actually protects all products using this particular version of UNACEV2.DLL from a similar attack. The DLL will be patched no matter who loads it. If you come across one, do let us know!

As always, if you have our Agent installed and registered, this micropatch is already on your computer - and applied to WinRAR whenever it loads UNACEV2.DLL (enable popups via 0patch Console to see when that happens). Otherwise, you can register a free 0patch account and install 0patch Agent to get this micropatch applied to your WinRAR.

Nevertheless - if you don't need support for ACE format in WinRAR, we recommend you update WinRAR to a fixed version (currently 5.70 beta 1). There may be other vulnerabilities there in UNACEV2.DLL which are more likely to be found now that it's been put in the spotlight.


Cheers!

@mkolsek
@0patch



Monday, February 11, 2019

Sorry, Adobe Reader, We're Not Letting You Phone Home Without User's Consent (CVE-2019-7089)

by Mitja Kolsek, the 0patch Team


[Update 2/12/2019: Just one day after we had issued this micropatch, Adobe released an update of Adobe Reader DC that fixed the vulnerability. We strongly recommend all users of Adobe Reader to apply Adobe's update, at which point our micropatch will automatically no longer get applied.]

Today we'll look at a fairly simple vulnerability in Adobe Reader DC that allows a PDF document automatically send an SMB request to attacker's server as soon as the document is opened. The vulnerability was published by Alex Inf├╝hr along with a proof-of-concept in a detailed report on Alex's blog and hasn't been patched at the time of this writing. [Update 2/12/2019: the vulnerability was subsequently assigned CVE ID CVE-2019-7089.]

This vulnerability, similar to CVE-2018-4993, the so-called Bad-PDF reported by CheckPoint in April last year, allows a remote attacker to steal user's NTLM hash included in the SMB request. It also allows a document to "phone home", i.e., to let the sender know that the user has viewed the document. Obviously, neither of these is desirable.

At the time of this writing, the latest version of Adobe Reader DC, 2019.010.20069, is affected but older versions are likely in the same boat too.


Analysis


To understand the problem, let's start with the above-mentioned Bad-PDF (CVE-2018-4993). The malicious PDF included a certain element that triggered automatic loading of another PDF from a remote share. Until Adobe Reader got patched, this happened without the user noticing anything, or being able to prevent it.

Adobe patched this issue by adding a security warning that was shown to the user before making the request to a remote share:


Security warning added to fix the Bad-PDF issue.

This warning allowed the user to decide whether to allow the potentially malicious document to "phone home" or not.

Now on to the issue at hand: it is functionally identical (for attacker), just in a different place. While Bad-PDF used an /F entry to load a remote file, this issue exploits loading a remote XML style sheet via SMB. Interestingly, if the document tries to do so via HTTP, there is a security warning there:


Attempting to load a remote style sheet via HTTP triggers a warning.


However, when using a UNC path (the type of path that denotes a resource in a shared folder), the loading occurs without a warning


The Micropatch


The Reader already implies the correct behavior by showing a security warning on loading a remote style sheet via HTTP, so we decided to add our own security warning for loading style sheets via UNC as well.

We started by locating the point where the HTTP-loading and UNC-loading execution paths diverge, and patch-in a warning for UNC paths. Finding said divergence point was relatively easy using Process Monitor: we caught CreateFile events on both HTTP and UNC loads, then compared their call stacks to see where the two execution paths said good-bye to each other. This turned out to be in function sub_208B3C53, the relevant code block of which is shown here:




Both HTTP- and UNC-based style sheet loads reach the above code block, whereby an HTTP-based load never returns from the first call marked in red (it triggers an exception that Reader handles somewhere else), and a UNC load is implemented inside the second call marked in red.

As you can see from the comments in the code, we found where the file path resides before the second call (this required a little digging on the stack at that point). We verified it by changing said path on-the-fly in debugger to make sure the path-loading code actually uses that, and then we were ready to inject our patch.

There exists a handy Windows function called PathIsUNCA, which is already being used by Reader elsewhere to check whether a path is a UNC path (the "A" at the end denotes an ASCII path in contrast to a wide-char path). So we decided to first call this function to see if we have a UNC path, and then if we do, issue a prompt to the user. To do the latter, we used another well-known Windows function MessageBoxA. Based on user's response, we would then allow the UNC path to be loaded or not. The only thing to decide at this point was how to implement the "not", i.e., how to prevent the UNC path from loading if the user decided not to trust the document.

In such cases, we always want to keep things simple and minimize possible surprises. We therefore decided to simply empty the path string by putting a zero at its beginning, resulting in Reader trying to load an empty path. It does complain that it can't load it though but the exploit is blocked and, frankly, we're fairly confident that loading style sheets form UNC paths doesn't work at all because as much as we tried, Reader always encountered an error. So maybe UNC style sheets are not a supported use case at all but they did turn out to be a vulnerability.

Here is the source code of our micropatch:



MODULE_PATH "..\AffectedModules\AcroForm.api_19.10.20064.48846_32bit\AcroForm.api"
PATCH_ID 349
PATCH_FORMAT_VER 2
VULN_ID 4694
PLATFORM win32

patchlet_start

 PATCHLET_ID 1
 PATCHLET_TYPE 2
 PATCHLET_OFFSET 0x000b3f6b ; Injecting before the call towards loading
                            ; a non-HTTP URL
 PIT Shlwapi.dll!PathIsUNCA,User32.dll!MessageBoxA

 code_start

  push ecx ; save ecx so we don't corrupt it

  mov eax, dword [eax+10h] ; at this point, the address of path
                           ; is at [eax+10h]+4
  add eax, 4
  push eax
  push eax ; we push the address of path twice as we may need it

           ; in remediation too
  call PIT_PathIsUNCA ; is it a UNC path?
  test eax, eax
  jz skip ; not a UNC path - skip the warning dialog

  push 0x00000134 ; uType = MB_YESNO | MB_ICONEXCLAMATION | MB_DEFBUTTON2
  call get_lpCaption
  db "Security Warning",0
 get_lpCaption:
  call get_lpText
  db "This document is trying to access a file on the network. If you trust this document, choose Yes. If you do not trust this document, choose No.",0
 get_lpText:
  push 0 ; hWnd = NULL
  call PIT_MessageBoxA
  cmp eax, 6 ; Did user click "Yes"?
  je skip ; if so, skip remediation

  ; Remediation: we'll simply delete the entire URL string by putting
  ; a 0 at the beginning
  pop eax ; pop the previously pushed path address
  mov byte [eax], 0 ; put a 0 at the start of path
  push eax ; we need to re-push something because we have a pop in the epilogue

 skip:
  pop eax ; pop the additional address of path 
 
  pop ecx ; restore original ecx and continue to execute the call
          ; towards loading the provided URL

 code_end

patchlet_end
 

With this micropatch in place, opening the malicious document results in the following warning before a request is made to the remote server:


The security dialog that was micropatched into Adobe Reader.


If the user selects "Yes", the remote style sheet is loaded (whereby the SMB request delivers your NTLM hash to the remote server), and if they select "No", Reader complains about not being able to load the style sheet from and empty path. Done.

We asked Alex, the security researcher who found this issue, to test our micropatch and he was kind enough to confirm its efficiency against this attack. (Thanks, Alex, for your help! We encourage all security researchers to collaborate on creating and testing micropatches.)

For the visual types among you, here is a video of our micropatch in action:




Conclusion


As always, if you have our Agent installed and registered, this micropatch is already on your computer - and applied to Adobe Reader whenever it loads AcroForm.api (that only happens when certain documents are opened). Otherwise, you can register a free 0patch account and install 0patch Agent to get this micropatch applied to your Reader.

Note that Adobe might issue an official fix for this issue tomorrow as they release their monthly Reader update, but if not, we'll quickly port the micropatch to the new Reader version to keep the vulnerability patched on your computers. [Update 2/12/2019: Adobe did issue an official fix the next day.]

Cheers!

@mkolsek
@0patch