PrintNightmare — Windows Print Spooler Remote Code Execution
A remote-code-execution vulnerability in the Windows Print Spooler service allows an authenticated attacker to run arbitrary code with SYSTEM privileges, or a domain-authenticated attacker to compromise a domain controller, by abusing the spooler's remote printer-driver installation functionality.
- Vendor
- Microsoft
- Product
- Windows Print Spooler
- CVSS
- 8.8
- EPSS (exploit probability)
- 99.8%
- Status
- kevransomware use
- CISA patch-by (BOD 22-01)
- Published
Is CVE-2021-34527 being exploited?
Yes. CISA added CVE-2021-34527 to its Known Exploited Vulnerabilities catalog on . CISA only lists a CVE when it has reliable evidence of exploitation in the wild. CISA marks it as known to be used in ransomware campaigns.
FIRST's EPSS model puts the probability of exploitation activity in the next 30 days at 99.8% (scored October 14, 2026).
- CISA KEV[ LISTED ]
- ENISA EUVD (exploited)[ — ]
- CIRCL KEV[ — ]
Catalog snapshot 2026-11-13. Full cross-catalog view: /kev-comparison
CVE-2021-34527 timeline
Every event links to its source. Catalog dates are each catalog's own; day offsets count from NVD publication. Data: JSON
What to do
US federal civilian agencies must remediate by (181 days after listing) under BOD 22-01. Everyone else: that deadline is a sensible upper bound, not a target. Vendor guidance and the NVD record are linked under Source above.
CVE-2021-34527, dubbed “PrintNightmare,” is a remote-code-execution flaw in the Windows Print Spooler service, which runs by default — and typically with SYSTEM privileges — on nearly every Windows machine, including domain controllers. It came to light after researchers accidentally published proof-of-concept exploit details publicly while a related, distinct spooler bug (CVE-2021-1675) was still being patched by Microsoft, forcing an emergency out-of-band advisory on July 1, 2021.
Why it mattered
The Print Spooler service’s remote printer-driver-installation capability could be abused by an authenticated attacker — including, critically, a low-privileged domain user — to install a malicious driver and execute code as SYSTEM. On a domain controller, where the spooler service commonly runs, this translated to a path to full domain compromise.
Microsoft’s initial patch was incomplete, and further point-in-time bypasses were reported for months afterward, leading many security teams to simply disable the Print Spooler service entirely on servers where printing wasn’t required — Microsoft’s own guidance for high-security environments. The vulnerability was added to CISA’s KEV catalog given confirmed active exploitation. Full patch and mitigation guidance is in Microsoft’s security advisory and at the NVD link above.
Other Microsoft CVEs we track
| CVE / Vulnerability | Severity | CVSS | EPSS | Added to KEV |
|---|---|---|---|---|
| CVE-2026-124194 Microsoft Office Outlook Missing User Interaction for Potentially Unsafe Action Vulnerability | [ MEDIUM ] | 6.5 | 1.6% | |
| CVE-2026-96951 Microsoft Windows Command Injection Vulnerability | [ HIGH ] | 8.8 | 9.8% | |
| CVE-2026-96940 Microsoft Exchange Server privilege escalation via weak authorizationwrite-up | [ HIGH ] | 8.8 | 2.7% | — |
| CVE-2026-65660 Microsoft SharePoint Code Injection Vulnerabilitywrite-up | [ HIGH ] | 8.8 | 9.9% | |
| CVE-2026-85889 Azure AI Foundry missing authentication allows unauthorized privilege escalationwrite-up | [ CRITICAL ] | 10.0 | 0.7% | — |
| CVE-2026-69843 Microsoft Fabric authentication bypass by spoofingwrite-up | [ CRITICAL ] | 10.0 | 0.9% | — |
| CVE-2026-85880 Microsoft Windows ALPC Heap Buffer Overflow Privilege Escalationwrite-up | [ HIGH ] | 7.8 | 0.4% | |
| CVE-2026-81963 Microsoft Windows Update Stack Link-Following Privilege Escalationwrite-up | [ HIGH ] | 7.8 | 7.1% |
