This week, PrintNightmare – Microsoft’s Print Spooler vulnerability (CVE-2021-34527) was upgraded from a ‘Low’ criticality to a ‘Crucial’ criticality.
This is because of a Proof of Idea revealed on GitHub, which attackers may doubtlessly leverage for having access to Area Controllers.
As we, Microsoft already launched a patch in June 2021, however it wasn’t sufficient to cease exploits. Attackers can nonetheless use Print Spooler when connecting remotely. You could find all you might want to learn about this vulnerability on this article and how one can mitigate it (and you may).
Print Spooler in a nutshell: Print Spooler is Microsoft’s service for managing and monitoring recordsdata printing. This service is amongst Microsoft’s oldest and has had minimal upkeep updates because it was launched.
Each Microsoft machine (servers and endpoints) has this characteristic enabled by default.
PrintNightmare vulnerability: As quickly as an attacker positive aspects restricted person entry to a community, he’ll be capable to join (straight or remotely) to the Print Spooler. Because the Print Spooler has direct entry to the kernel, the attacker can use it to achieve entry to the working system, run distant code with system privileges, and in the end assault the Area Controller.
The best choice relating to mitigating the PrintNightmare vulnerability is to disable the Print Spooler on each server and/or delicate workstation (akin to directors’ workstations, direct internet-facing workstations, and non-printing workstations).
That is what Dvir Goren’s, hardening professional and CTO at, suggests as your first transfer in the direction of mitigation.
Observe these steps to disable the Print Spooler service on Home windows 10:
- Open Begin.
- Seek for PowerShell, right-click on it and choose the Run as administrator.
- Sort the command and press Enter: Cease-Service -Identify Spooler -Pressure
- Use this command to stop the service from beginning again up once more throughout restart: Set-Service -Identify Spooler -StartupType Disabled
In line with Dvir’s expertise, 90% of servers don’t require Print Spooler. It’s the default configuration for many of them, so it’s normally enabled. Because of this, disabling it may possibly resolve 90% of your drawback and have little affect on manufacturing.
In giant and sophisticated infrastructures, it may be difficult to find the place Print Spooler is used.
Listed below are just a few examples the place Print Spooler is required:
- When utilizing Citrix providers,
- Fax servers,
- Any software requiring digital or bodily printing of PDFs, XPSs, and many others. Billing providers and wage purposes, for instance.
Listed below are just a few examples when Print Spooler isn’t wanted however enabled by default:
- Area Controller and Energetic Listing – the principle danger on this vulnerability may be neutralized by training fundamental cyber hygiene. It is not sensible to have Print Spooler enabled in DCs and AD servers.
- Member servers akin to SQL, File System, and Trade servers.
- Machines that don’t require printing.
Just a few different hardening steps urged by Dvir for machines depending on Print Spooler embody:
- Change the susceptible Print Spooler protocol with a non-Microsoft service.
- By altering ‘Enable Print Spooler to simply accept shopper connections’, you possibly can limit customers’ and drivers’ entry to the Print Spooler to teams that should use it.
- Disable Print Spooler caller in Pre-Home windows 2000 compatibility group.
- Be sure that Level and Print isn’t configured to No Warning – examine registry key SOFTWARE/Insurance policies/Microsoft/Home windows NT/Printers/PointAndPrint/NoElevationOnInstall for DWORD worth 1.
- Flip off EnableLUA – examine registry key SOFTWARE/Microsoft/Home windows/CurrentVersion/Insurance policies/System/EnableLUA for DWORD worth 0.
This is what you might want to do subsequent to make sure your group is safe:
- Determine the place Print Spooler is getting used in your community.
- Map your community to search out the machines that should use Print Spooler.
- Disable Print Spooler on machines that don’t use it.
- For machines that require Print Spooler – configure them in a option to decrease its assault floor.
Beside this, to search out potential proof of exploitation, you also needs to monitor Microsoft-Home windows-PrintService/Admin log entries. There may be entries with error messages that point out Print Spooler cannot load plug-in module DLLs, though this will additionally occur if an attacker packaged a respectable DLL that Print Spooler calls for.
The ultimate advice from Dvir is to implement these suggestions by. With out automation, you’ll spend numerous hours trying to harden manually and will find yourself susceptible or inflicting techniques to go down
After selecting your plan of action, awill uncover the place Print Spooler is enabled, the place they’re truly used, and disable or reconfigure them routinely.