Update

As of July 24th 2020, there is a bypass for this patch, with a public writeup available at https://0x00sec.org/t/windows-defender-av-zero-day-vulnerability/22258 along with a working exploit at https://github.com/klinix5/WinDefend_ZeroDay. The bypass takes advantage of the fact that the patch only checks whether C:\Windows\Temp\MpCmdRun.log.bak is a junction directory, rather than accounting for the fact that any one of the subfolders could be a directory junction. Its possible also that one could also further nest folders within one another, which basically means the new patch will need to account for potentially near infinite nesting possibilities should it choose to still honor junction directories. I am still not 100% sure if one could exploit this via files, but my initial feeling says no as I think you would need to create symbolic links which requires administrator privileges, at which point there are much better and easier ways of getting SYSTEM access.

Summary/TLDR

This is a vulnerability within the MpCmdRun.exe component of Windows Defender which, in MpCmdRun.exe versions prior to 4.18.2005.1, did not appropriately validate that the file at C:\Windows\Temp\MpCmdRun.log.bak was not a junction directory prior to trying to delete it. As a result, attackers could exploit this vulnerability to delete arbitrary files as the SYSTEM user, which could allow for an elevation of privilege.

Rating as medium as it require authenticated access, but leaving exploitability as a medium as in theory this could work if the LPE vector does indeed work like the article states, but wasn’t able to confirm this. Also need to fill a log file with 16 MB of data which can take some time to do when your only writing about 2 KB per attempt.

Edit: Originally put this as easy to weaponize but tbh the file deletion trick stumped a lot of people as many people have stopped there before getting to LPE so even if we do get it working, its not exactly a “simple trick”.

Longer Explanation

Now that the summary of this vulnerability is out of the way, lets dive into the details a bit. The original discoverer of this bug is itm4n, who wrote a writeup at https://itm4n.github.io/cve-2020-1170-windows-defender-eop/ explaining his thought process and steps for discovering this vulnerability and the various different things that he found worked and didn’t work during his research. If you haven’t read it already I would highly recommend taking a look at it. Its not a challenging read compared to most technical blog posts, and it provides a great overview of how to actually query deeper to find interesting bugs.

With that being said I’m not going to repeat itm4n’s blog verbatum here, but rather explain some of the notes I made whilst reading his blog. The first interesting point, and perhaps the most important, is that Windows Defender has a log file that it creates when updating signatures, located at C:\Windows\Temp\MpCmdRun.log, which is then backed up to a file at C:\Windows\Temp\MpCmdRun.log.bak when its size exceeds 16 MB. The interesting thing to note though is that if one runs icacls on these files as an administrator, they will notice that these files can only be deleted or otherwise modified by SYSTEM or one of the computer’s administrators. This leads us to an interesting point: If the C:\Windows\Temp\MpCmdRun.log.bak file already exists, this vulnerability can only be exploited by a local administrator.

If alternatively, the file does not exist, an attacker can create a Directory Junction at C:\Windows\Temp\MpCmdRun.log.bak and link this to an arbitrary directory by using the command cmd.exe /C 'mklink /J C:\Windows\Temp\MpCmdRun.log.bak *target directory*. Following this they will then need to fill up the contents of the file at C:\Windows\Temp\MpCmdRun.log so that is 16 MB or larger, which can be done by repetitively running the command for ($i=0; $i -lt 2000; $i++) { Update-MpSignature -ErrorAction SilentlyContinue -UpdateSource InternalDefinitionUpdateServer }. Note that this command may need some pauses to prevent locking up MpCmdRun.exe as during tests there where times if we ran too many tests we either had to open another PowerShell command, or wait about 40 seconds or so before continuing. More testing may need to be performed to check how to solve this issue and make things more reliable.

Whilst executing this loop the attacker will most likely end up triggering the bug, which will cause all files and folders in the *target directory* specified earlier, no matter how deeply nested they are inside *target directory*, to be deleted. In the exploit itm4n briefly shows within his blog, he set *target directory* to C:\ProgramData\Microsoft\Windows\WER. The reason for this is that by deleting this directory and all files and folders located underneath it, we can then abuse a flaw in the WER service whereby if the C:\ProgramData\Microsoft\Windows\WER folder doesn’t exist, then when running the task \Microsoft\Windows\Windows Error Reporting\QueueReporting, the folder will be recreated albeit with read, write and delete permissions enabled for all authenticated users.

You can then create the C:\ProgramData\Microsoft\Windows\WER folder as a junction folder that links to \??\c:\windows\system32\wermgr.exe.local. This folder does not exist by default on Windows systems. By doing this C:\Windows\System32\wermgr.exe.local\ will be created as a directory with the same open permissions that grant all authenticated users to read, write and delete permissions. From there one can create the amd64_microsoft.windows.common-controls_6595b64144ccf1df_6.0.18362.778_none_e6c6b761130d4fb8 directory inside of C:\Windows\System32\wermgr.exe.local\ , and then place a malicious copy of comctl32.dll, which will then get loaded and run as the SYSTEM user when wermgr.exe is run, granting the attacker arbitrary code execution as the SYSTEM user.

Update #2: I have managed to confirm this is a legitimate LPE technique and after some further help from @jonasLyk on Twitter, I have managed to clear up a few misunderstandings on his technique of going from an arbitrary file deletion bug to SYSTEM level code execution using the technique mentioned above. This has resulted in a LPE module being created for Metasploit which is presently being worked on at https://github.com/rapid7/metasploit-framework/pull/13800, where we have managed to achieve a full LPE chain. Its just awaiting some refinement and reorganization before it can be landed into the framework.