CVE-2021-34527
CVE-2021-34527 is a security flaw in Microsoft Windows, Microsoft’s operating system for PCs and servers. Attackers can run their own code on Microsoft Windows. The US Cybersecurity and Infrastructure Security Agency (CISA) added it to its catalog of flaws known to be used in attacks on 3 Nov 2021, and says ransomware gangs have used it.[1]
Microsoft Windows is in things households have, such as phones, computers, browsers and everyday apps, so the fix matters at home as much as at work. Fixed in version 10.0.10240.18969, 10.0.14393.4470, 10.0.17763.2029 and 10.0.19042.1083 and others.
Description
CISA calls it a print spooler remote code execution in Microsoft Windows. In its words: “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.”[1]
The CVE record adds: “A remote code execution vulnerability exists when the Windows Print Spooler service improperly performs privileged file operations. An attacker who successfully exploited this vulnerability could run arbitrary code with SYSTEM privileges. An attacker could then install programs; view, change, or delete data; or create new accounts with full user rights. UPDATE July 7, 2021: The security update for Windows Server 2012, Windows Server 2016 and Windows 10, Version 1607 have been released. Please see the Security Updates table for the applicable update for your system. We recommend that you install these updates immediately. If you are unable to install these updates, see the FAQ and Workaround sections in this CVE for information on how to help protect your system from this vulnerability. In addition to installing the updates, in order to secure your system, you must…”[2]
The underlying weakness is improper privilege management (CWE-269), in MITRE’s list of common software weaknesses.
It scores 8.8 out of 10 on the Common Vulnerability Scoring System, which rates it high, scored by the organisation that published the CVE using CVSS 3.1.[2]
Affected versions
NVD’s record lists these fixed versions: in each line, everything before the version shown is affected.[2]
| Product | Fixed in |
|---|---|
| Windows 10 Version 1507 | 10.0.10240.18969 |
| Windows 10 Version 1607 | 10.0.14393.4470 |
| Windows 10 Version 1809 | 10.0.17763.2029 |
| Windows 10 Version 20H2 | 10.0.19042.1083 |
| Windows 10 Version 21H2 | 10.0.19044.1415 |
| Windows 10 Version 22H2 | 10.0.19045.2251 |
| Windows 11 version 21H2 | 10.0.22000.318 |
| Windows 11 version 22H2 | 10.0.22621.674 |
Every device with Microsoft Windows needs the update.
Exploitation
CISA added it to its catalog of known exploited vulnerabilities on 3 Nov 2021, 124 days after it was made public. CISA lists a flaw only when it has reliable evidence of it being exploited, and gave US federal agencies until 3 May 2022 to fix it.[1] CISA says ransomware gangs have used it.
FIRST’s Exploit Prediction Scoring System, which estimates how likely a flaw is to be used in attacks in the next 30 days from what it sees across the internet, puts it at over 99%. That’s in the top 1% of the flaws it scores.[4]
CISA assesses it as not automatable: each attack takes some work.
CISA doesn’t say who was attacked, or how.
Fix
Microsoft has published a fix or advice.[3] CISA’s required action: “Apply updates per vendor instructions.”[1]
- Update Microsoft Windows now.On Windows: Settings, Windows Update, Check for updates, then restart. Office updates come the same way, or from any Office app: File, Account, Update Options. Then turn on automatic updates, so the next fix arrives on its own.
- Check you’re on 10.0.10240.18969 or later.Fixed in version 10.0.10240.18969, 10.0.14393.4470, 10.0.17763.2029 and 10.0.19042.1083 and others. Anything older is what attackers are going after.
- If there’s no update, stop using it.CISA’s advice when a flaw being used in attacks has no fix is to stop using the product until it does.
Timeline
| Date | Event |
|---|---|
| The flaw is made public in the National Vulnerability Database, with a severity score of 8.8.[2] | |
| CISA adds it to its catalog of flaws known to be used in attacks, noting that ransomware gangs have used it.[1] | |
| CISA’s deadline for US government agencies to fix it.[1] | |
| FIRST puts the odds of it being used in attacks in the next 30 days at over 99%.[4] |
See also
Other Microsoft Windows flaws
- CVE-2026-96951: command injection ·
- CVE-2026-85880: heap-based buffer overflow ·
- CVE-2026-81963: link following ·
- CVE-2008-4250: buffer overflow ·
- CVE-2026-32202: protection mechanism failure ·
- CVE-2025-60710: link following ·
Other improper privilege management flaws
- CVE-2026-102490, Zammad GmbH Zammad: improper privilege management ·
- CVE-2026-84869, ConnectWise ScreenConnect: improper privilege management and missing authorization ·
- CVE-2026-46817, Oracle E-Business Suite: improper privilege management ·
- CVE-2024-8068, Citrix Session Recording: improper privilege management ·
- CVE-2023-28434, MinIO: security feature bypass ·
Added the same day
- CVE-2021-42258, BQE BillQuick Web Suite: SQL injection ·
- CVE-2021-42013, Apache HTTP Server: path traversal ·
- CVE-2021-41773, Apache HTTP Server: path traversal ·
- CVE-2021-40539, Zoho ManageEngine: aDSelfService plus authentication bypass ·
- CVE-2021-40444, Microsoft MSHTML: remote code execution ·
Every Microsoft flaw on CISA’s list · Everything added in November 2021
References
- ^ Cybersecurity and Infrastructure Security Agency. “Microsoft Windows Print Spooler Remote Code Execution Vulnerability”. Known Exploited Vulnerabilities Catalog, added 3 Nov 2021.
- ^ National Institute of Standards and Technology. “CVE-2021-34527 Detail”. National Vulnerability Database, published 2 Jul 2021.
- ^ Microsoft. Advisories and fixes: msrc.microsoft.com, portal.msrc.microsoft.com.
- ^ FIRST. “Exploit Prediction Scoring System”. EPSS 0.998, percentile 100.0, 31 Oct 2026.
- CVE Program. “CVE-2021-34527”. The official CVE record.
This article is built from CISA’s catalog (version 2026.10.30), NVD’s record and FIRST’s scores, kept in HARD’s database and checked every morning. The plain English is HARD’s.