Thursday, May 6, 2021

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"
VULN_ID 7058

 PATCHLET_OFFSET 0xf5a55               ; First GetCurrentThread block in CreateLog function
                                       ; instruction lea r9, [rsp+98h+TokenHandle]
    PIT msi.dll!0xf5b31,msi.dll!0xef7f8   ; Address of block to jump to; IsAdmin function
        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


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

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