Tuesday, May 18, 2021

Micropatch for Remote Code Execution Issue in Internet Explorer (CVE-2021-26419)


by Mitja Kolsek, the 0patch Team

May 2021 Windows Updates brought a fix for an "Exploitation More Likely" memory corruption vulnerability in Scripting Engine (CVE-2021-26419) discovered by Ivan Fratric of Google Project Zero. Ivan published details and a proof-of-concept the next day, and we took these to reproduce the vulnerability in our lab and create a micropatch for it.

Since Microsoft's patch was available, we reviewed it and found they only changed function ByteCodeGenerator::LoadCachedHeapArguments such that instead of calling ByteCodeGenerator::EmitPropStore, it now calls ByteCodeGenerator::EmitLocalPropInit.These are undocumented and largely unknown functions but their names imply the vulnerability resides in just-in-time compiler's code generation logic, where the generated code gets an improper level of access to the arguments object.

Our micropatch is logically identical to Microsoft's:

MODULE_PATH "..\Affected_Modules\jscript9.dll_11.0.9600.19867_64bit\jscript9.dll"
VULN_ID 7112
    PIT jscript9!0x8be60 ; ByteCodeGenerator::EmitLocalPropInit
        mov r9, rbp ; Some instructions are erased and a new function call added
        mov r8, rdi
        mov edx, esi
        mov rcx, rbx
        mov rbx, [rsp+70h]
        add rsp, 40h
        pop rdi
        pop rsi
        pop rbp
        jmp PIT_0x8be60 ; New call to EmitLocalPropInit


See the micropatch in action:

We'd like to thank  Ivan Fratric for sharing their analysis and POC, which allowed us to create this micropatch for Windows users without official security updates. We also encourage security researchers to privately share their analyses with us for micropatching.

This micropatch is immediately available to all 0patch users with a PRO license, and is already downloaded and applied on all online 0patch-protected Windows 7 and Windows Server 2008 R2 machines without Extended Security Updates, or with year 1 of Extended Security Updates.

To obtain the micropatch and have it applied on your computer(s) along with other micropatches included with a PRO license, create an account in 0patch Central, install 0patch Agent and register it to your account. Note that no computer restart is needed for installing the agent or applying/un-applying any 0patch micropatch. 

By the way, if your organization has either Windows 10 v1809 or Office 2010 installations that stopped receiving security updates, and would like to continue using them, it could be useful to know we've security-adopted both for at least 12 months. To save lots of money and step into the age of reboot-less security patching, contact sales@0patch.com.

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