CVE-2025-21333
CVE-2025-21333 is a security flaw in Microsoft Windows, Microsoft’s operating system for PCs and servers. Someone who already has a foothold can use Microsoft Windows to take full control of the machine. The US Cybersecurity and Infrastructure Security Agency (CISA) added it to its catalog of flaws known to be used in attacks on 14 Jan 2025.[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 10.0.19044.5371, 10.0.19045.5371, 10.0.22621.4751 and 10.0.22631.4751 and others.
Description
CISA calls it a hyper-V NT kernel integration VSP heap-based buffer overflow in Microsoft Windows. In its words: “Microsoft Windows Hyper-V NT Kernel Integration VSP contains a heap-based buffer overflow vulnerability that allows a local attacker to gain SYSTEM privileges.”[1]
The underlying weakness is heap buffer overflow (CWE-122), in MITRE’s list of common software weaknesses.[2]
It scores 7.8 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 21H2 | 10.0.19044.5371 |
| Windows 10 Version 22H2 | 10.0.19045.5371 |
| Windows 11 version 22H2 | 10.0.22621.4751 |
| Windows 11 version 22H3 | 10.0.22631.4751 |
| Windows 11 Version 23H2 | 10.0.22631.4751 |
| Windows 11 Version 24H2 | 10.0.26100.2894 |
| Windows Server 2022, 23H2 Edition (Server Core installation) | 10.0.25398.1369 |
| Windows Server 2025 | 10.0.26100.2894 |
Every device with Microsoft Windows needs the update.
Exploitation
CISA added it to its catalog of known exploited vulnerabilities on 14 Jan 2025, within a day of it being made public. CISA lists a flaw only when it has reliable evidence of it being exploited, and gave US federal agencies until 4 Feb 2025 to fix it.[1]
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 10%. That’s in the top 5% 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 mitigations per vendor instructions or discontinue use of the product if mitigations are unavailable.”[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 10.0.19044.5371 or later.Fixed in version 10.0.19044.5371, 10.0.19045.5371, 10.0.22621.4751 and 10.0.22631.4751 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 7.8.[2] | |
| CISA adds it to its catalog of flaws known to be used in attacks.[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 10%.[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 heap buffer overflow flaws
- CVE-2026-94127, F5 BIG-IP APM: heap-based buffer overflow ·
- CVE-2025-25249, Fortinet products: heap-based buffer overflow ·
- CVE-2024-38812, VMware vCenter Server: heap-based buffer overflow ·
- CVE-2023-4911, GNU C Library: c library buffer overflow ·
- CVE-2023-27997, Fortinet FortiOS and FortiProxy SSL-VPN: heap-based buffer overflow ·
Added the same day
- CVE-2025-21335, Microsoft Windows: hyper-V NT kernel integration VSP use-after-free ·
- CVE-2025-21334, Microsoft Windows: hyper-V NT kernel integration VSP use-after-free ·
- CVE-2024-55591, Fortinet FortiOS and FortiProxy: authentication bypass ·
Every Microsoft flaw on CISA’s list · Everything added in January 2025
References
- ^ Cybersecurity and Infrastructure Security Agency. “Microsoft Windows Hyper-V NT Kernel Integration VSP Heap-based Buffer Overflow Vulnerability”. Known Exploited Vulnerabilities Catalog, added 14 Jan 2025.
- ^ National Institute of Standards and Technology. “CVE-2025-21333 Detail”. National Vulnerability Database, published 14 Jan 2025.
- ^ Microsoft. Advisories and fixes: msrc.microsoft.com, msrc.microsoft.com, vicarius.io, vicarius.io.
- ^ FIRST. “Exploit Prediction Scoring System”. EPSS 0.101, percentile 95.6, 31 Oct 2026.
- CVE Program. “CVE-2025-21333”. 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.