Update July 6, 2021: Microsoft has released a patch for CVE 2021-34527, available here.

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Another week, another critical vulnerability. The latest critical security flaw is dubbed “PrintNightmare,” a reference to two vulnerabilities in the Windows Print Spooler service—CVE 2021-1675 & CVE 2021-34527, published between June và July 2021.

CVE 2021-1675 is a patched vulnerability that enables remote code execution and privilege escalation on servers & computers running the Print Spooler. CVE 2021-34527 also allows remote code execution and privilege escalation on the same service through somewhat different means. (Update on July 6, 2021: Microsoft released a patch for CVE-2021-34527.) At this moment, the exact relationship is unclear between the two vulnerabilities & whether the patch was incomplete or if another attack method was discovered.

Before I go any further, let’s start with the bottom line—if you haven’t already done so, disable any Windows Print Spooler service running on a domain controller. (anhchien.vn’ Directory Services Protector (DSP) includes indicators that continuously scan for exposures và signs of compromise, including enabled Print Spoolers on domain controllers.)

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anhchien.vn Directory Services Protector continuously scans AD và alerts on indicators of exposure và compromise, including an indicator khổng lồ look for domain name controllers with the active sầu print spooler service running

Print Spooler background

Print Spooler is a Windows service enabled by default in all Windows clients and servers. The service manages print jobs by loading printer drivers, receiving files lớn be printed, queuing them, scheduling, và so on.

The Print Spooler service is required when a computer is physically connected lớn a printer that provides printing services khổng lồ additional computers on the network. It’s possible to lớn use third-tiệc nhỏ drivers & software (like those offered by the printer manufacturers), but many organizations rely on the mặc định Print Spooler service.

On domain controllers, Print Spoolers are mainly used for printer pruning—removing printers that have been published to lớn Active sầu Directory and that are no longer available on the network. Printer pruning is especially important in larger environments with many printers as it “cleans up” the printer list available khổng lồ domain users. For this approach khổng lồ work, though, an appropriate Group Policy must be in place.

From a security perspective sầu, the Windows Print Spooler, and printers in general, have been a juicy target for exploitation by attackers for many years. The 2010 Stuxnet worm used against Iranian nuclear facilities exploited a vulnerability in the service khổng lồ escalate privileges và propagate malware across the network. The same Print Spooler vulnerability re-surfaced in 20trăng tròn when researchers uncovered new ways khổng lồ exploit it. Combined with the fact that printers are notoriously hackable, maybe it’s time we truly go paperless?

PrintNightmare Zero-Day bug

The June 2021 iteration of the Print Spooler vulnerability began with Microsoft’s June 2021 Patch Tuesday that included a patch for CVE 2021-1675, which Microsoft considered a privilege escalation vulnerability with exploitation “less likely” at the time—ranking a 6.8 (Medium Risk) CVSS score. Microsoft updated this CVE on June 21 lớn include Remote Code Execution & upgraded the CVSS score to lớn 7.8 (High Risk). A few days later, PoC exploit code appeared on GitHub. The code was quickly taken offline but was already forked several times.

On June 30, it became apparent that the patch was not sufficient, và fully patched systems were still vulnerable to remote code execution và privilege escalation to lớn SYSTEM. The original exploit code was modified, making the patch only somewhat effective.

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On July 1, Microsoft created a new vulnerability, CVE 2021-34527. (July 6, 2021 – Microsoft released a patch.)

Print Spooler vulnerability breakdown

The Print Spooler remote code execution vulnerability takes advantage of the RpcAddPrinterDriver function Call in the Print Spooler service that allows clients lớn add arbitrary dll files as printer drivers và load them as SYSTEM (the spooler service context).

The function is designed to lớn allow users to update printers remotely—for example, the IT person remotely installing your brand-new office printer. However, a logical flaw in how this works allows any user to lớn inject their own unsigned dll inlớn the process, bypassing authentication or validation of the tệp tin.

Mitigating PrintNightmare

Update July 7, 2021 – Microsoft released a patch for CVE 2021-34527. It is still recommended to lớn disable the Print Spooler where they are not required. You can disable the Print Spooler service across all your DCs (or any machine, for that matter) by using either the Group Policy setting under Computer ConfigurationWindows SettingsSecurity SettingsSystem Services or, better yet, using GPhường. Preferences under Computer ConfigurationPreferencesControl Panel SettingsServices. An additional, unofficial mitigation is lớn limit access to the driver thư mục where the service loads from. This would need khổng lồ be done on every VPS and has not been fully tested lớn the best of my knowledge, so use at your own risk.

Protecting tên miền controllers

Domain controllers should never be used as print servers. The only legitimate reason that I know of to lớn have Print Spooler running on a DC is for printer pruning mentioned above sầu. In addition to the numerous vulnerabilities in Windows Print Spooler, an attack vector against this service known as “the printer bug” enables an attacker who can compromise a resource with Kerberos unconstrained delegation to also compromise the DC with SYSTEM privileges. A good explanation of this attaông chồng is here.

Continuously monitor for vulnerabilities like PrintNightmare

We’ve sầu seen vulnerabilities in commonly deployed IT infrastructure on a weekly basis. The short cycle times from disclosure to lớn weaponization necessitates a rapid mitigation approach that includes continuous monitoring of the environment combined with prioritization and actionable remediation information.

anhchien.vn’ Directory Services Protector (DSP) hàng hóa includes indicators that continuously scan for exposures & signs of compromise in hybrid identity environments (including detecting enabled Print Spoolers on domain name controllers) và provides prioritization, metrics, and prescriptive sầu guidance for remediation.

More resources

Want to learn more about how to lớn address PrintNightmare, PetitPotam, và other new attacks that target Active Directory? Tune in lớn Stepping Up Your Active sầu Directory Defenses: Lessons Learned from Recent Attacks Like PrintNightmare, a miễn phí on-demvà web seminar hosted by Sean Deuby, anhchien.vn Director of Services.