Microsoft Warns of “PrintNightmare,” an Unpatched, Actively Exploited Flaw in the Windows Print Spooler Service

This post is going to be long because it includes a lot of information, so bear with me. First, I’ve been following this story closely for a while, and I’m glad to see it starting to get some mainstream attention. The thing is, one person was able to abuse the print spooler flaw in a way that left him completely undetectable. However, it’s a pretty scary scenario because now that someone has used this technique to go undetected, it begs the question of whether they are going to continue to exploit this flaw, or worse.

“PrintNightmare,” is a vulnerability affecting Windows systems that can be exploited by malicious actors to cause a number of system file and service crashes. The vulnerability is described in Microsoft Security Advisory 221232 as an “actively exploited vulnerability in the Windows Print Spooler Service” that has been seen in the wild.

The Print Spooler Service, or “printspool.dll,” is a Windows service that runs on every computer. It handles printing jobs and provides a library of common printing operations. As such, it is an essential part of any Windows or Office…

Image: Microsoft

Microsoft has released an advisory on a new remote code execution issue in the Windows print spooler, which abuses code present in all versions of the operating system. The vulnerability, called PrintNightmare, allows attackers to remotely execute code with system privileges to install programs, delete data and create accounts with full user privileges. Microsoft has confirmed that the security hole is being actively exploited. Although there is no official patch yet, 0patch offers a temporary fix for Windows users who want to fix Day Zero immediately. Other options include disabling the Print Spooler service or disabling incoming remote printing via Group Policy.

Microsoft has assigned a remote code execution vulnerability (CVE-2021-34527) that affects the Windows print spooler. More information can be found here:

– Microsoft Security Intelligence (@MsftSecIntel) 2. July 2021

Our patches are free until Microsoft releases an official patch. If you want to use it, create a free account at, then install and register the 0patch agent at Everything else happens automatically. A restart is not necessary.

– 0patch (@0patch) 2. July 2021

CISA also issued a PrintNightmare Zero Day advisory urging administrators to disable the Windows Print Spooler service on servers not used for printing. According to Microsoft’s previous recommendations for mitigating risk on domain controllers running the Print Spooler service, the service should be disabled on all Active Directory domain controllers and administrative systems via a Group Policy Object because it provides a larger attack surface.

Sources: Microsoft, BleepingComputer (1, 2), The Verge

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3. July 2021 3. July 2021Since Microsoft released UWN in September 2018, Microsoft has posted a number of advisories, warnings and updates to help customers protect their systems from the security risks introduced by UWN. In this post, Microsoft discusses a newly discovered risk — a zero-day vulnerability in the Windows Print Spooler Service which can be used to deliver malware.. Read more about what is print spooler service and let us know what you think.

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