Microsoft Windows Print Spooler Remote Code Execution Vulnerability
CVE-2021-34527 is a security vulnerability in Microsoft Windows, added to CISA KEV on November 3, 2021. Patch priority: Patch now. Known ransomware use: yes.
What it is
Microsoft Windows Print Spooler contains an unspecified vulnerability due to the Windows Print Spooler service improperly performing privileged file operations. Successful exploitation allows an attacker to perform remote code execution with SYSTEM privileges. The vulnerability is also known under the moniker of PrintNightmare.
Known to be used in ransomware campaigns. Active threat actors have chained this vulnerability into ransomware operations — treat patching as a same-week priority, not a "next maintenance window" task. The fix is the same one below; the urgency is higher.
VulnCheck tracked exploitation 124 days before CISA's KEV listing. VulnCheck's broader catalog of in-the-wild exploitation evidence picked this up before it landed on CISA's formal list. Per-CVE fact about this entry; not a population claim about lead time in general.
Who's affected
Affects anyone running Windows workstations or servers. In a small CPA, legal, or dental practice, Windows is typically the platform your accounting, document management, or practice management software runs on — exploitation gives an attacker access to whatever client files and credentials live on those machines.
What to do
Apply updates per vendor instructions.
If you don't have someone in-house to verify the patch deployed across every endpoint — or you're not sure whether you're affected — that's exactly the kind of triage we do. Book a free 20-minute triage call.
Insurance readiness
Your insurer will ask if this is patched.
Unpatched entries on CISA's Known Exploited Vulnerabilities list are exactly what cyber-insurance carrier questionnaires probe for, and knowing which controls they check is how you keep a renewal from stalling.
Get the free carrier questionnaire →Severity
CVSS base score: 8.8 — HIGH
Microsoft fix
- KB5004947 (Security Update)
- KB5005575 (Security Update)
- KB5004945 (Security Update)
- KB5007215 (Security Update)
- KB5008212 (Security Update)
- KB5018427 (Security Update)
- KB5019959 (Security Update)
- KB5004950 (Security Update)
- KB5004948 (Security Update)
- KB5004954 (Monthly Rollup)
- KB5004955 (Monthly Rollup)
- KB5004959 (Security Only)
- KB5004953 (Monthly Rollup)
- KB5004951 (Security Only)
- KB5004956 (Monthly Rollup)
- KB5004960 (Security Only)
- KB5004958 (Security Only)
Microsoft Exploitability Index (assessed at update release): Exploitation Detected
Microsoft has confirmed in-the-wild exploitation in the latest software release. This aligns with CISA's listing of this CVE.
Vendor workaround
Vendor-published workaround
Determine if the Print Spooler service is running
Run the following in Windows PowerShell:
Get-Service -Name Spooler
If the Print Spooler is running or if the service is not set to disabled, select one of the following options to either disable the Print Spooler service, or to Disable inbound remote printing through Group Policy:
Option 1 - Disable the Print Spooler service
If disabling the Print Spooler service is appropriate for your enterprise, use the following PowerShell commands:
Stop-Service -Name Spooler -Force
Set-Service -Name Spooler -StartupType Disabled
Impact of workaround Disabling the Print Spooler service disables the ability to print both locally and remotely.
Option 2 - Disable inbound remote printing through Group Policy
You can also configure the settings via Group Policy as follows:
Computer Configuration / Administrative Templates / Printers
Disable the “Allow Print Spooler to accept client connections:” policy to block remote attacks.
You must restart the Print Spooler service for the group policy to take effect.
Impact of workaround This policy will block the remote attack vector by preventing inbound remote printing operations. The system will no longer function as a print server, but local printing to a directly attached device will still be possible.
For more information see: Use Group Policy settings to control printers.
A workaround reduces exposure while you patch — it is not a substitute for the update. Install the fix above as soon as you can.
CISA due date
Federal deadline: May 3, 2022. Federal agencies must complete the required action by this date. For private SMBs the deadline is advisory — but treat it as a strong recommendation, especially if you handle regulated data (HIPAA, GLBA, ABA model rules).
EPSS
EPSS score: 99.8% — higher than 100% of all scored CVEs.
EPSS estimates the probability of exploitation activity in the next 30 days. Every entry on this site is already confirmed exploited, so read it as relative urgency among the things you still have open, not as permission to wait.
Other exploited Microsoft Windows entries
More CISA KEV entries for Microsoft Windows. Same product line, same actively-exploited status.
Source
This entry comes from CISA's Known Exploited Vulnerabilities catalog, public US-Government data we pull daily. The plain-language framing and the what-to-do guidance above are ours. View the original entry on cisa.gov.