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"
PATCH_ID 606
PATCH_FORMAT_VER 2
VULN_ID 7112
PLATFORM win64
patchlet_start
    PATCHLET_ID 1
    PATCHLET_TYPE 2
    PATCHLET_OFFSET 0xbe342
    N_ORIGINALBYTES 5
    JUMPOVERBYTES 0
    PIT jscript9!0x8be60 ; ByteCodeGenerator::EmitLocalPropInit
    
    code_start
        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
    code_end
    
patchlet_end

 

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.

To learn more about 0patch, please visit our Help Center. 

 

0patch Security-Adopts Windows 10 v1803 and v1809 to Keep them Running Securely

Towards Micropatching the "Security Update Gap"

 

by Mitja Kolsek, the 0patch Team

 

[Update: We initially security-adopted only Windows 10 v1809, but were then approached by customers needing micropatches for v1803 as well, so we security-adopted that version too.]

The May 2021 Windows Updates will contain the last official security fixes for many editions of three Windows 10 operating system versions:

1. Windows 10 v1803
2. Windows 10 v1809
3. Windows 10 v1909

For organizations with any of these versions installed on their computers, this means the end of official security patches, and a pressure to upgrade to a supported Windows 10 version. Such organization-wide operating system upgrade may seem like a simple, mostly automated task - but in reality, updates break things:

• Windows 10 October 2020 Update biggest problems and complaints
• Windows 10 May 2020 Update biggest problems and complaints
• Windows 10 May 2019 Update biggest problems and complaints

In addition, with many users working from home these days, upgrading an operating system involves users downloading a huge update via their home Internet connection and difficult remote assistance in case something goes wrong with the upgrade.

Consequently, customers were approaching us in recent months asking whether we were planning to security-adopt some of these Windows 10 versions (mostly version 1809, later also version 1803), as they were looking for ways to keep using them securely.

And so we've decided to security-adopt Windows 10 v1803 (build 10.0.17134) and v1809 (build 10.0.17763) - as we have previously security-adopted Windows 7, Windows Server 2008 R2, and Office 2010.

Starting this month, initially for one year, we will actively gather information about vulnerabilities affecting Windows 10 v1803/v1809 and, based on our risk criteria, create micropatches for this operating system. We will be particularly interested in any vulnerabilities patched by Microsoft in still-supported Windows 10 versions, and whether they might affect v1803/v1809 as well.

These micropatches will be included in 0patch PRO and Enterprise licenses along with all other micropatches we're issuing - which means that users protecting their Windows 10 v1803/v1809 with 0patch will also receive our occasional micropatches for "0day" vulnerabilities in various products.

In order to have our Windows 10 v1803/v1809 micropatches applied, users will have to have their computers fully updated with the latest (May 2021) official Windows Updates provided by Microsoft.

We welcome all interested organizations with Windows 10 v1803/v1809 to contact sales@0patch.com for information about pricing, deployment, or setting up a trial. If you happen to be using a large number of v1909 versions in your environment, also let us know as given sufficient demand we will security-adopt those too.

 

Addressing The Security Update Gap 

Our security-adoption of an unsupported Windows 10 version is an important milestone on our journey towards addressing the "security update gap" on supported Windows versions, which aims to allow organizations to protect themselves with our micropatches while thoroughly testing monthly Windows Updates before deploying them. And eventually even skipping one or two monthly updates under our protection.

 

To learn more about 0patch, please visit our Help Center.  

 

 

 

Another Windows Installer Local Privilege Escalation Bug Gets a Micropatch (CVE-2021-26415)

 

by Mitja Kolsek, the 0patch Team

On April 21, security researcher Adrian Denkiewicz published an in-depth analysis of a local privilege escalation vulnerability in Windows Installer that was fixed by April 2021 Windows Updates. Adrian's analysis included a proof-of-concept.

The vulnerability is a classical symbolic-link issue, whereby a privileged process (in this case, msiexec.exe) works with a file (in this case, installer log file) that the attacker is able to "redirect" to another location where the they do not have permissions to create or modify files.

Since attacker has limited control over the content of installer log file, and cannot modify the redirected log file after it has been created, Adrian had to be creative and found a working attack scenario in creating/overwriting PowerShell profile file (C:\Windows\System32\WindowsPowerShell\v1.0\profile.ps1) that gets loaded whenever anyone, ideally admin, uses PowerShell.

In essence, Microsoft's fix included a call to function IsAdmin from function CreateLog, which is in charge of creating installer log file. Some permissions checking was already in place before in this function but was not resilient to the "bait-and-switch" symbolic link trick that has been successful against many Windows products before, and will surely be successful against many more to come.

Our micropatch does logically the same as Microsoft's fix. Here is its source code for 64-bit Windows 7 and Server 2008 R2 with its 7 CPU instructions.

MODULE_PATH "..\Affected_Modules\msi.dll_5.0.7601.24535_64bit\msi.dll"
PATCH_ID 604
PATCH_FORMAT_VER 2
VULN_ID 7058
PLATFORM win64

patchlet_start
 PATCHLET_ID 1
 PATCHLET_TYPE 2
 PATCHLET_OFFSET 0xf5a55               ; First GetCurrentThread block in CreateLog function
                                       ; instruction lea r9, [rsp+98h+TokenHandle]
    N_ORIGINALBYTES 5
    JUMPOVERBYTES 0
    PIT msi.dll!0xf5b31,msi.dll!0xef7f8   ; Address of block to jump to; IsAdmin function
    
    code_start
        push rax                      ;Save the GetCurrentThread return
        push rax                      ;Push one more time to fix stack alignment
        call PIT_0xef7f8              ;Call IsAdmin (ret 1 if admin, 0 if not)
        cmp rax, 0                    ;Check if user is admin
        pop rax                       ;Restore the GetCurrentThread return and fix stack alignment again
        pop rax
        je PIT_0xf5b31                ;If user is not an admin, jump over the scond createfile block
    code_end
    
patchlet_end

 

See the micropatch in action here:

We'd like to thank Adrian Denkiewicz 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. 

And don't forget, if your organization has Windows 7 or Server 2008 R2 machines with Extended Security Updates and wouldn't mind saving lots of money on less expensive low-risk security patches in 2021 that don't even need your machines to be restarted, contact sales@0patch.com.

To learn more about 0patch, please visit our Help Center.