Oh, so you have an antivirus… name every bug

Original text by halove23

Oh, so you have an antivirus… name every bug

After my previous disclosure with Windows defender and Windows Setup, this is the next one

First of all, why ? it’s because I can, and because I need a job.

In this blog I will be disclosing about 8 0-day vulnerability and all of them are still unknow to the vendors, don’t expect those bugs to be working for more than a week or two cause probably they will release an emergency security patches to fix those bugs.

    Avast antivirus

a.   Sandbox Escape

So avast antivirus (any paid version) have a feature called Avast sandbox, this feature allow you to test suspicious file in sandbox. But this sandbox is completely different from any sandbox I know, let’s say windows sandboxed apps are running in a special container and also by applying some mitigation to their tokens (such as: lowering token integrity, applying the create process mitigation…) and other sandboxes actually run a suspicious file in a virtual machine instead so the file will stay completely isolated. But Avast sandbox is something completely different, the sandboxed app run in the OS and with few security mitigation to the sandboxed app token, such as removing some privileges like SeDebugPrivilege, SeShutdownPrivilege… while the token integrity stay the same, while this isn’t enough to make a sandbox. Avast sandbox actually create a virtualized file stream and registry hive almostly identical to the real one, while it also force the sandboxed app to use the virtualized stream by hooking every single WINAPI call ! This sounds cool but also sound impossible, any incomplete hooking could result in sandbox escape.

Btw, the virtualized file stream is located in “C:\avast! sandbox”

While the virtualized registry hive exist in “H****\__avast! Sandbox” and it look like there’s also a virtualized object manager in “\snx-av\”

So normally to make any escape I should read any available write-ups related to avast sandbox escape, and after some research it look like I found something: CVE-2016-4025 and another bug by google project zero.

Nettitude Labs covered a crafted DeviceIoControll call in order to escape from the virtualization, they noticed after using the “Save As” feature in notepad the actual saved file is outside the sandbox (in the real filesystem) and it look like it’s my way to get out

I selected their way by clicking on “Save As” and it don’t seems to be working because the patch has disabled the feature but instead of clicking on “Save As” I clicked in “Print”

By doing that it look like we got another pop-up so normally I clicked print with the default printer “Microsoft XPS Document Writer”

And yup we will have a “Save As” window after clicking on Print

So clicked on Save, I really didn’t expected anything to happen but guess what

The file was spawned outside the virtualized file stream. That’s clearly a sandbox escape.

How ? it seems look like an external process written the file while impersonating notepad’s access token. And luckily since CVE-2020-1337 I was focused on learning the Printer API by reading every single documentation provided by Microsoft, while in other side. James Forshaw published something related to a windows sandbox escape by using the Printer API here.

So I assume we can easily escape from the sandbox if we managed to call the Printer API correctly, so we will begin with OpenPrinter function

And of course we will specify in pPrinterName the default printer that exist on a standard windows installation “Microsoft XPS Document Writer”

Next we will go for StartDocPrinter function which allow us to prepare a document for printing and the third argument looks kinda important

So we will take a look in DOC_INFO_1 struct and there’s some good news

It look like the second member will allow us to specify the actual file name so yeah it’s probably our way out of Avast sandbox.

So what now, we can probably see the file outside the sandbox, but what about writing things to the file. After further research I found another function which work like WriteFile it’s WritePrinter

Then the final result will look like this

Note: The bug was reported to the vendor but they didn’t replied as usual

a.   Privilege Escalation

It was a bit hard to find a privilege escalation but after taking some time, here you go Avast, there’s a feature in Avast called “REPAIR APP” after clicking on it, it look like a new child process is being created by Avast Antivirus Engine called “Instup.exe”

Probably there’s something worthy to look there, after attempting to repair the app. In this case we will be using a tool called Process Monitor

And as usual we got something that worth our attention, the instup process look for some non existing directories C:\stage0 and c:\stage1

So what if they exist ?

I created the c:\stage0 directory with a subfile inside it and I took a look on how the instup.exe behave against it and I observed an unexpected behaviour, instead of just deleting or ignoring the file it actually create a hardlink to the file

we can exploit the issue but the issue is that the hardlink have a random name and guessing it at the time of the hardlink we can redirect the creation to an arbitrary location but unluckily the hardlink random name is incredible hard to guess, if we attempted who know how much time it will take so I prefer to not look there instead I started looking somewhere else

In the end of the process of hardlink creation, you can see that both of them has been marked for deletion, probably we can abuse the issue to achieve an arbitrary file deletion bug.

I’ve created an exploit for the issue, the exploit will create a file inside c:\stage0 and will continuously look for the hardlink. When the hardlink is created the poc OpLock it until instup attempt to delete it, then the poc will move the file away and set c:\stage0 into junction to “\RPC CONTROL\” which there we will create a symbolic link to redirect the file deletion to an arbitrary file

Note: This bug wasn’t reported to the vendor.

PoC can be found here

McAfee Total Security

a.   Privilege Escalation

I already found bugs on this AV before and got acknowledged by vendor

For CVE-2020-7279 and CVE-2020-7282

CVE-2020-7282 was for an arbitrary file deletion issue in McAfee total protection and CVE-2020-7279 was for self-defence bypass.

The McAfee total security was vulnerable to an arbitrary file deletion, by creating a junction from C:\ProgramData\McAfee\Update to an arbitrary location result in arbitrary file deletion, the security patch was done by enforcing how McAfee total security updater handle reparse point.

But the most important is C:\ProgramData\McAfee\Update is actually protected by the self defence driver, so even an administrator couldn’t create files in this directory. The bypass was done by open the directory for GENERIC_WRITE access and then creating a mount point to the target directory so as soon the updater start it will delete the target directory subcontent.

But now a lot has changed, the directory now has subcontent (previously it was empty by default), 

After doing some analysis on how they fixed the self defence bug. Instead of preventing the directory opening (as it was expected) with GENERIC_WRITE they blocked the following control codes FSCTL_SET_REPARSE_POINT and FSCTL_SET_REPARSE_POINT_EX from being called on a protected filesystem component, I expected FSCTL_SET_REPARSE_POINT_EX but no they did a smart move in this case, so if we didn’t bypass the self defence we don’t have any actual impact on the component.

So this is it, this is as far as I can go… or no ?

a.   Novel way to bypass the self defence

This method work for all antiviruses which the filesystem filter.

So how does the kernel filter work ?

The filesystem filter restrict the access to the antivirus owned objects, by intercepting the user mode file I/O request, if the request coming from an antivirus component it will be granted, if not it will return access denied.

You can read more about that here, I already wrote some of them but for some private usage so for the moment I can’t disclose them, but there’s a bunch of examples you can find by example: here

So as far as I know there’s 2 way to bypass the filter

1.     Do a special call so it will be conflicted by what the driver see

2.     Request access from a protected component

So the special way was patched in CVE-2020-7279, the option that remain is the second one. How can we do that ?

The majority of the AV’s GUI support file dialog to select something let’s take by example McAfee file shredder which open a file dialog in order to let you choose to pick something

While the file dialog is used to pick files it be weaponized against the AV, to better understand the we need to make an example code, so I had to look for the API provided by Microsoft to do that. Generically apps use either GetOpenFileNameW or IFileDialog interface and since GetOpenFileNameW seems to be a bit deprecated we will focusing in IFileDialog Interface.

So I created a sample code (it look horrible but still doing the job)

After running the code

It look like that the job is being done from the process not from an external process (such as explorer), so technically anything we do is considered to be done as the process.

Hold on, if the things are done by the process. Doesn’t that mean that we can create a folder in a protected location ? Yes we can

c.   Weaponizing the self-protection bypass

The CVE-2020-7282 patch was a simple check against reparse points, before managing to delete any directory.

There’s a simple check to be done, if FSCTL_GET_REPARSE_POINT control on a directory return anything except STATUS_NOT_A_REPARSE_POINT the target will removed else the updater will delete the subcontent as

“NT AUTHORITY\SYSTEM”

Chaining it together, I’ve an exploit which demonstrate the bug, first a directory in C:\updmgr will be created and then you should manually move it to C:\ProgramData\McAfee\Update an opportunistic lock will trigger the poc to create a reparse point to the target as soon as the AV GUI attempt to move the folder, the poc will set it to reparse point and will lock the moved directory so it will prevent the reparse point deletion.

PoC can be found here

Avira Antivirus

 I’m gonna do the tests on Avira Prime, not gonna lie Avira has the easiest way to download their antivirus. Not like other vendors they crack your head before they give the trial, I really feel bad for disclosing this bug

Anyway it look like there’s a feature come with Avira Prime called Avira System Speedup Pro, I can’t still explain why this behaviour exist in Avira System Speedup feature but yeah it exist.

When starting the Avira System Speedup Pro GUI there’s an initialization done by the service “Avira.SystemSpeedup.Service.exe” which is written in C# which make it easier to reverse the service but I reversed the service and things just doesn’t make any sense so I guess it’s better to show process monitor output to understand the issue.

 When opening the GUI I assume that there’s an RPC communication between the GUI and the service to make the required initialization in order to serve the user needs. While the service begin the initialization process it will create and remove a directory in C:\Windows\Temp\Avira.SystemSpeedup.Service.madExcept

without even checking for reparse point. It’s extremely easy to abuse the issue.

This time instead of writing a c++ PoC I’ll be writing a simpler one as a batch script. The PoC in this case doesn’t need any user interaction, and will delete the targeted directory subcontent.

PoC can be found here

1    Trend Micro Maximum security

 One of the best AV’s I’ve ever seen, but unluckily this disclosure include this antivirus to the black list.

I already discovered an issue in trend micro and it was patched in CVE-2020-25775, I literally just found a high severity issue on trend micro. But I was contracted for so I can’t disclose it here.

Moving out, as other AV’s there’s a PC Health Checkup feature, it probably worth our attention.

While browsing trough the component, I noticed that there’s a feature “Clean Privacy Data” feature.

I clicked on MS Edge and cleaned, the output from process monitor was:

And as you see Trend Micro Platinum Host Service is deleting a directory in a user write-able location without proper check against reparse point while running as “NT AUTHORITY\SYSTEM” which is easily abuse-able by a user to delete arbitrary files.

There’s nothing to say more, I created a proof of concept as a batch script after running it expect the target directory subcontent to be deleted.

PoC can be found here

MalwareBytes

 Yup, another good AV, Already engaged with the antivirus and as usual I got a bug. 5 months has passed since I reported the bug, they still didn’t patched the issue and since they paid the bounty, I can’t disclose the bug but as usual PAPA has candies for you !

I will using the same technique explained above to bypass the self protection.

While checking for updates, the antivirus look for a non existing directory

Hmmmm, let’s take a look

The pic shown above, show us that Malwarebytes antivirus engine is deleting every subcontent of C:\ProgramData\Malwarebytes\MBAMService\ctlrupdate\test.txt.txt and since there’s no impersonation of the user and literally no proper check against reparse point we can probably abuse that, by creating a directory there and creating a reparse point inside  C:\ProgramData\Malwarebytes\MBAMService\ctlrupdate we can redirect the file deletion to an arbitrary location.

The PoC can be found here

1    Kaspersky

The AV which I engaged with the most, about 11 bugs were reported and 3 of them were fixed.

For the moment I will be talking about a bug which I already disclosed here, this PoC will spawn a SYSTEM shell as soon as it succeed, the bug seems to be still existing on Kaspersky Total Security with December 2020 latest security patches, the only issue you will have is the AV will detect the exploit as a malware, you must do some modification to prevent your exploit from being deleting. Let’s I can confirm that the issue still exist.

One more thing

Another issue I discovered in all Kaspersky’s antiviruses which allow arbitrary file overwrite without user interaction. I’ve already reported the bug to Kaspersky but they didn’t gave me a bug bounty

They said that the issue isn’t eligible for bug bounty because the reproduction of the issue is unstable, ain’t gonna lie I gave them a horrible proof of concept but still do the job so I guess it should be rewarded and since they wrote that they gave bounties. I won’t give bugs for free like a foo.

So let’s dive inside the bug, when any user start Mozilla Firefox, Kaspersky write a special in %Firefox_Dir%\defaults\pref while not impersonating the user or not even doing proper links check, if abused correctly it can be used against the AV to trigger arbitrary file overwrite on-demand without user interaction.

A proof of concept is attached implement the issue, I’ve rewritten a new one which will trigger your needs on demand thanks me later.

PoC can be found here

Windows

 I was about to disclose bugs in Eset and Bitdefender but I don’t have time to write more, so here’s the last one.

First, if you’re not familiar with windows installer CVE-2020-16902, it’s literally the 6th time I am bypassing the security patch and they still don’t hire security researchers. I will be using the same package as CVE-2020-16902

Microsoft has patched the issues by checking if c:\config.msi exist, if not it will be used to generate rollback directory otherwise if it exist c:\windows\installer\config.msi will be used as a folder to generate rollback files.

A tweet by sandboxescaper mentioned that if a registry key “HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Installer\Folders\C:\Config.Msi” existed when the installation begin, the windows installer will use c:\config.msi as a directory rollback files. As an unprivileged user I guess there’s no way to prevent the deletion or create “HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Installer\Folders\C:\Config.Msi”

And as usual there’s always something that worth our attention.

When the directory is deleted, there’s an additional check if the directory exist or not. Which is kinda strange, since the RemoveDirectory returned TRUE

I guess there’s no need to make additional checks. I am pretty sure that there’s a bug there, I managed to create the directory as soon the installer delete and this happened

The installer did a check if the directory exist and it return that the directory exist, so the windows installer won’t delete the registry key  “HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Installer\Folders\C:\Config.Msi” because the directory wasn’t delete.

In the next installation the C:\Config.Msi will be used to save rollback files on it, which can be easily abused (I’ve already done that in CVE-2020-1302 and CVE-2020-16902).

I’ve provided a PoC as c++ project to exploit the issue, it’s a double click to SYSTEM shell, thank me later again.

PoC can be found hereNote: I am not responsible for any usage for those disclosures, you’re on your own.

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