CVE-2025-21391
CVE-2025-21391 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 11 Feb 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 10.0.10240.20915, 10.0.14393.7785, 10.0.17763.6893 and 10.0.19044.5487 and others.
Description
CISA calls it a storage link following in Microsoft Windows. In its words: “Microsoft Windows Storage contains a link following vulnerability that could allow for privilege escalation. This vulnerability could allow an attacker to delete data including data that results in the service being unavailable.”[1]
The underlying weakness is CWE-59, in MITRE’s list of common software weaknesses.[2]
It scores 7.1 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 | 10.0.10240.20915 |
| Windows 10 Version 1607 | 10.0.14393.7785 |
| Windows 10 Version 1809 | 10.0.17763.6893 |
| Windows 10 Version 21H2 | 10.0.19044.5487 |
| Windows 10 Version 22H2 | 10.0.19045.5487 |
| Windows 11 version 22H2 | 10.0.22621.4890 |
| Windows 11 version 22H3 | 10.0.22631.4890 |
| Windows 11 Version 23H2 | 10.0.22631.4890 |
Every device with Microsoft Windows needs the update.
Exploitation
CISA added it to its catalog of known exploited vulnerabilities on 11 Feb 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 Mar 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 2%. That’s in the top 17% 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 10.0.10240.20915 or later.Fixed in version 10.0.10240.20915, 10.0.14393.7785, 10.0.17763.6893 and 10.0.19044.5487 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 7.1.[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 2%.[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 ·
Similar flaws
- CVE-2015-5287, Red Hat Automatic Bug Reporting Tool: privilege escalation ·
- CVE-2025-48384, Git: link following ·
Added the same day
- CVE-2025-21418, Microsoft Windows: ancillary function driver for winSock heap-based buffer overflow ·
- CVE-2024-40891, Zyxel DSL CPE Devices: DSL CPE OS command injection ·
- CVE-2024-40890, Zyxel DSL CPE Devices: DSL CPE OS command injection ·
Every Microsoft flaw on CISA’s list · Everything added in February 2025
References
- ^ Cybersecurity and Infrastructure Security Agency. “Microsoft Windows Storage Link Following Vulnerability”. Known Exploited Vulnerabilities Catalog, added 11 Feb 2025.
- ^ National Institute of Standards and Technology. “CVE-2025-21391 Detail”. National Vulnerability Database, published 11 Feb 2025.
- ^ Microsoft. Advisories and fixes: msrc.microsoft.com.
- ^ FIRST. “Exploit Prediction Scoring System”. EPSS 0.023, percentile 83.2, 31 Oct 2026.
- CVE Program. “CVE-2025-21391”. 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.