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NOVEL EXPLOIT CHAIN ENABLES WINDOWS UAC BYPASSNOVEL EXPLOIT CHAIN ENABLES
WINDOWS UAC BYPASS

Adversaries can exploit CVE-2024-6769 to jump from regular to admin access
without triggering UAC, but Microsoft says it's not really a vulnerability.

Nate Nelson, Contributing Writer

September 27, 2024

5 Min Read
Source: Panther Media GmbH via Alamy Stock Photo


Researchers have flagged a weakness they're tracking as CVE-2024-6769, calling
it a combination user access control (UAC) bypass/privilege escalation
vulnerability in Windows. It could allow an authenticated attacker to obtain
full system privileges, they warned.

That's according to Fortra, which assigned the issue a medium severity score of
6.7 out of 10 on the Common Vulnerability Scoring System (CVSS) scale. Its
proof-of-concept exploit demonstrates that "you have the ability to shut down
the system," stressed Tyler Reguly, associate director of security R&D at
Fortra. "There are certain locations on the drive where you can write and delete
files that you couldn't previously." That includes, for example, C:\Windows, so
an attacker could take ownership over files owned by SYSTEM.



For its part, Microsoft acknowledged the research but said it does not consider
this an actual vulnerability, because it falls under its concept of
acceptability to have "non-robust" security boundaries.


UNDERSTANDING INTEGRITY LEVELS IN WINDOWS



To understand Fortra's findings, we have to go back to Windows Vista, when
Microsoft introduced the model of Mandatory Integrity Control (MIC). Simply put,
MIC assigned every user, process, and resource a level of access, called an
integrity level. Low integrity levels were afforded to all, medium for
authenticated users, high for administrators, and system for only the most
sensitive and powerful.

Alongside those integrity levels came UAC, a security mechanism that runs most
processes and applications at the medium level by default, and requires explicit
permission for any actions that require greater privileges than that. Typically,
an admin-level user can upgrade simply by right-clicking a command prompt and
selecting "Run as Administrator."



By combining two exploit techniques, Fortra researchers demonstrated in their
proof of concept how an already-authorized user could slither through this
system, jumping across the security boundary imposed on the medium integrity
level to obtain full administrative privileges, all without triggering UAC.




USING CVE-2024-6769 TO JUMP ACROSS USER BOUNDARIES

To exploit CVE-2024-6769, an attacker first must have a foothold in a targeted
system. This requires the medium integrity-level privileges of an average user,
and the account from which the attack is triggered must belong to the system's
administrative group (the type of account that could level up to admin
privileges, if not for UAC being in its way).

The first step in the attack involves remapping the targeted system's root drive
— such as "C:" — to a location under their control. This will also shift the
"system32" folder, which many services rely on to load critical system files.

One such service is the CTF Loader, ctfmon.exe, which runs without administrator
privileges at a high integrity level. If the attacker places a specially
crafted, copycat DLL in the copycat system32 folder, ctfmon.exe will load it and
execute the attacker's code at that high integrity level.

Next, if the attacker wishes to obtain full administrative privileges, they can
poison the activation context cache, which Windows uses to load specific
versions of libraries. To do this, they craft an entry in the cache pointing to
a malicious version of a legitimate system DLL, contained in an
attacker-generated folder. Through a specially crafted message to the
Client/Server Runtime Subsystem (CSRSS) server, the fake file is loaded by a
process that has administrator privileges, granting the attacker full control
over the system.




MICROSOFT: NOT A VULNERABILITY

Despite the potential for privilege escalation, Microsoft refused to accept the
issue as a vulnerability. After Fortra reported it, the company responded by
pointing to the "non-boundaries" section of the Microsoft Security Servicing
Criteria for Windows, which outlines how "some Windows components and
configurations are explicitly not intended to provide a robust security
boundary." Under the pertinent "Administrator to Kernel" section, it reads:

> Administrative processes and users are considered part of the Trusted
> Computing Base (TCB) for Windows and are therefore not strongly isolated from
> the kernel boundary. Administrators are in control of the security of a device
> and can disable security features, uninstall security updates, and perform
> other actions that make kernel isolation ineffective.

Essentially, Reguly explains, "They see the admin-to-system boundary as a
nonexistent boundary, because admin is trusted on a host." In other words,
Microsoft doesn't consider CVE-2024-6769 a vulnerability if an admin user could
ultimately perform the same system-level actions anyway, subject to UAC
approval.

In a statement to Dark Reading, a Microsoft spokesperson highlighted that "The
method requires membership in the Administrator group, so the so-called
technique is just leveraging an intended permission or privilege which does not
cross a security boundary."

Reguly and Fortra disagree with Microsoft's perspective. "When UAC was
introduced, I think we were all sold on the idea that UAC was this great new
security feature, and Microsoft has a history of fixing bypasses for security
features," he says. "So if they're saying that this is a trust boundary that is
acceptable to traverse, really what they're saying to me is that UAC is not a
security feature. It's some sort of helpful mechanism, but it's not actually
security related. I think it's a really strong philosophical difference."


WINDOWS SHOPS SHOULD STILL BEWARE UAC BYPASS RISK



Philosophical differences aside, Reguly stresses that businesses need to be
aware of the risk in allowing lower-integrity admins to escalate their
privileges to attain full system controls.

At the end of a CVE-2024-6769 exploit, an attacker would have full reign to
manipulate or delete critical system files, upload malware, establish
persistence, disable security features, access potentially sensitive data, and
more.

"Thankfully, only administrators are impacted by this, which means that most of
your standard users are unaffected," Fortra noted in an FAQ to reporters. "For
administrators, it is important to ensure that you are not running binaries
whose origins cannot be verified. For those admins, however, vigilance is the
best defense at the moment."




ABOUT THE AUTHOR

Nate Nelson, Contributing Writer



Nate Nelson is a freelance writer based in New York City. Formerly a reporter at
Threatpost, he contributes to a number of cybersecurity blogs and podcasts. He
writes "Malicious Life" -- an award-winning Top 20 tech podcast on Apple and
Spotify -- and hosts every other episode, featuring interviews with leading
voices in security. He also co-hosts "The Industrial Security Podcast," the most
popular show in its field.

See more from Nate Nelson, Contributing Writer
Keep up with the latest cybersecurity threats, newly discovered vulnerabilities,
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