Microsoft offers workaround to mitigate the IE Scripting Engine Memory Corruption Vulnerability

Clément Lecigne of Google’s Threat Analysis Group reported about an IE vulnerability that allowed remote execution of code via Internet Explorer. The vulnerability lies in the way the scripting engine handles objects in memory in IE. Microsoft has disclosed the issue, and also offered a workaround for this Scripting Engine Memory Corruption Vulnerability (CVE-2020-0674).

By default, IE11, IE10, and IE9 use Jscript9.dll and are not impacted by this vulnerability. This vulnerability only affects certain websites that utilize JScrip.dll as the scripting engine.

How does an attacker execute remote code?

An attacker can set up a trap by sending a link to a specially crafted website over email. When the victim clicks on the link, the code will exploit the vulnerability through Internet Explorer

Workaround to mitigate IE Scripting Engine Memory Corruption Vulnerability

Once you have applied the patch, it will result in reduced functionality for components or features that rely on jscript.dll. However, it still doesn’t offer full protection, and it will be best not to use IE unless a permanent fix is rolled out. We highly recommend you to move to Microsoft Edge until the update becomes available. Microsoft is expected to patch this on its upcoming Patch Tuesday updates.

Restrict access to JScript.dll

For 32-bit systems, enter the following command at an administrative Command Prompt:

    takeown /f %windir%\system32\jscript.dll
    cacls %windir%\system32\jscript.dll /E /P everyone:N

For 64-bit systems, enter the following command at an administrative command prompt:

    takeown /f %windir%\syswow64\jscript.dll
    cacls %windir%\syswow64\jscript.dll /E /P everyone:N
    takeown /f %windir%\system32\jscript.dll
    cacls %windir%\system32\jscript.dll /E /P everyone:N

How to undo the workaround

Microsoft recommends reverting the mitigation steps before installing the update to return to a full state. Else any access to JScirpt.dll will suffer reduced functionality.

For 32-bit systems, enter the following command at an administrative command prompt:

    cacls %windir%\system32\jscript.dll /E /R everyone    

For 64-bit systems, enter the following command at an administrative command prompt:

    cacls %windir%\system32\jscript.dll /E /R everyone    
    cacls %windir%\syswow64\jscript.dll /E /R everyone

Microsoft also shared that Internet Explorer on Windows Server copies (2008/2012/2016) run in a restricted mode known as Enhanced Security Configuration. It reduces the likelihood of a user or administrator downloading and running specially crafted web content on a server.