Two New Unpatched Critical RCE Flaws Affect All Windows Versions

On Mar 24, 2020

Microsoft today issued a new security advisory warning billions of Windows users of two new critical, unpatched zero-day vulnerabilities that could let hackers remotely take complete control over targeted computers.

According to Microsoft, both unpatched flaws are being used in limited, targeted attacks and impact all supported versions of the Windows operating system including Windows 10, 8.1 and Server 2008, 2012, 2016, and 2019 editions, as well as Windows 7 for which Microsoft ended its support on January 14, 2020.

Both vulnerabilities reside in the Windows Adobe Type Manager Library, a font parsing software that not only parses content when open with a 3rd-party software but also used by Windows Explorer to display the content of a file in the ‘Preview Pane’ or ‘Details Pane’ without having users to open it.

The flaws exist in Microsoft Windows when the Adobe Type Manager Library improperly “handles a specially-crafted multi-master font – Adobe Type 1 PostScript format,” allowing remote attackers to execute arbitrary malicious code on targeted systems by convincing a user to open a specially crafted document or viewing it in the Windows Preview pane.

“For systems running supported versions of Windows 10, a successful attack could only result in code execution within an AppContainer sandbox context with limited privileges and capabilities,” Microsoft said.

At this moment, though it’s not clear if the flaws can also be triggered remotely over a web browser by convincing a user to visit a web-page containing specially-crafted malicious OTF fonts, there are multiple other ways an attacker could exploit the vulnerability, such as through the Web Distributed Authoring and Versioning (WebDAV) client service.

No Patch Yet Available; Apply Workarounds

Microsoft said it’s aware of the issue and working on a patch, which the company would release to all Windows users as part of its next Patch Tuesday updates, on 14th April.

“Enhanced Security Configuration does not mitigate this vulnerability,” the company added.

1) Disable the Preview Pane and Details Pane in Windows Explorer

Meanwhile, all Windows users are highly recommended to disable the Preview Pane and Details Pane feature in Windows Explorer as a workaround to reduce the risk of getting hacked by opportunistic attacks.

To disable the Preview Pane and Details Pane feature:

Open Windows Explorer, click Organize and then click Layout.
Clear both the Details pane and Preview pane menu options.
Click Organize, and then click Folder and search options.
Click the View tab.
Under Advanced settings, check the Always show icons, never thumbnails box.
Close all open instances of Windows Explorer for the change to take effect.

However, to be noted, while this workaround prevents malicious files from being viewed in Windows Explorer, it does not strict any legitimate 3rd-party software from loading the vulnerable font parsing library.

2) Disable the WebClient service

Besides this, it is also advised to disable Windows WebClient service to prevent cyberattacks through the WebDAV client service.

Click Start, click Run (or press the Windows Key and R on the keyboard), type Services.msc and then click OK.
Right-click WebClient service and select Properties.
Change the Startup type to Disabled. If the service is running, click Stop.
Click OK and exit the management application.

“After applying this workaround, it is still possible for remote attackers who successfully exploit this vulnerability to cause the system to run programs located on the targeted user’s computer or the Local Area Network (LAN), but users will be prompted for confirmation before opening arbitrary programs from the Internet,” the Microsoft warned.

3) Rename or Disable ATMFD.DLL

Microsoft is also urging users to rename Adobe Type Manager Font Driver (ATMFD.dll) file to temporarily disable the embedded font technology, which could cause certain 3rd-party apps to stop working.

Enter the following commands at an administrative command prompt:

For 32-bit system:
cd “%windir%system32”
takeown.exe /f atmfd.dll
icacls.exe atmfd.dll /save atmfd.dll.acl
icacls.exe atmfd.dll /grant Administrators:(F)
rename atmfd.dll x-atmfd.dll

For 64-bit system:
cd “%windir%system32”
takeown.exe /f atmfd.dll
icacls.exe atmfd.dll /save atmfd.dll.acl
icacls.exe atmfd.dll /grant Administrators:(F)
rename atmfd.dll x-atmfd.dll
cd “%windir%syswow64”
takeown.exe /f atmfd.dll
icacls.exe atmfd.dll /save atmfd.dll.acl
icacls.exe atmfd.dll /grant Administrators:(F)
rename atmfd.dll x-atmfd.dll

Restart the system.