Patch now! Emergency fix for PrintNightmare released by Microsoft

Last week we wrote about PrintNightmare, a vulnerability that was supposed to be patched but wasn’t. After June’s Patch Tuesday, researchers found that the patch did not work in every case, most notably on modern domain controllers. Yesterday, Microsoft issued a set of out-of-band patches that sets that aims to set that right by fixing the Windows Print Spooler Remote Code Execution vulnerability listed as CVE-2021-34527.

Serious problem

For Microsoft to publish an out-of-band patch a week before July’s Patch Tuesday shows just how serious the problem is.

PrintNightmare allows a standard user on a Windows network to execute arbitrary code on an affected machine, and to elevate their privileges as far as domain admin, by feeding a vulnerable machine a malicious printer driver. The problem was exacerbated by confusion around whether PrintNightmare was a known, patched problem or an entirely new problem. In the event it turned out to be a bit of both.

Last week the Cybersecurity and Infrastructure Security Agency (CISA) urged administrators to disable the Windows Print Spooler service in domain controllers and systems that don’t print.

However, the installation of the Domain Controller (DC) role adds a thread to the spooler service that is responsible for removing stale print queue objects. If the spooler service is not running on at least one domain controller in each site, then Active Directory has no means to remove old queues that no longer exist.

So, many organizations were forced to keep the Print Spooler service enabled on some domain controllers, leaving them at risk to attacks using this vulnerability.

Set of patches

Depending on the Windows version the patch will be offered as:

• KB5004945 for Windows 10 version 2004, version 20H1, and version 21H1
• KB5004946 for Windows 10 version 1909
• KB5004947 for Windows 10 version 1809 and Windows Server 2019
• KB5004949 for Windows 10 version 1803 which is not available yet
• KB5004950 for Windows 10 version 1507
• Older Windows versions (Windows 7 SP1, Windows 8.1 Server 2008 SP2, Windows Server 2008 R2 SP1, and Windows Server 2012 R2) will receive a security update that disallows users who are not administrators to install only signed print drivers to a print server.

Security updates have not yet been released for Windows 10 version 1607, Windows Server 2016, or Windows Server 2012, but they will also be released soon, according to Microsoft.

The updates are cumulative and contain all previous fixes as well as protections for CVE-2021-1675.

Not a complete fix

It is important to note that these patches and updates only tackle the remote code execution (RCE) part of the vulnerability. Several researchers have confirmed that the local privilege escalation (LPE) vector still works. This means that threat actors and already active malware can still locally exploit the vulnerability to gain SYSTEM privileges.

Advice

Microsoft recommends that you install this update immediately on all supported Windows client and server operating systems, starting with devices that currently host the print server role. You also have the option to configure the RestrictDriverInstallationToAdministrators registry setting to prevent non-administrators from installing signed printer drivers on a print server. See KB5005010 for more details.

“The attack vector and protections in CVE-2021-34527 reside in the code path that installs a printer driver to a Server. The workflow used to install a printer driver from a trusted print server on a client computer uses a different path. In summary, protections in CVE-2021-34527 including the RestrictDriverInstallationToAdministrators registry key do not impact this scenario.”

CISA encourages users and administrators to review the Microsoft Security Updates as well as CERT/CC Vulnerability Note VU #383432 and apply the necessary updates or workarounds.

Impact of the updates

So, the vulnerability lies in the normal procedure that allows users to install a printer driver on a server. A printer driver is in essence an executable like any other. And allowing users to install an executable of their choice is asking for problems. Especially combined with a privilege escalation vulnerability that anyone can use to act with SYSTEM privileges. The updates, patches, and some of the workarounds are all designed to limit the possible executables since they need to be signed printer drivers.

For a detailed and insightful diagram that shows GPO settings and registry keys administrators can check whether their systems are vulnerable, have a look at this flow chart diagram, courtesy of @gentilkiwi.

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