CVE-2026-21513
CVE-2026-21513 is a security flaw in Microsoft Windows, Microsoft’s operating system for PCs and servers. Attackers can get past a security protection in Microsoft Windows. The US Cybersecurity and Infrastructure Security Agency (CISA) added it to its catalog of flaws known to be used in attacks on 10 Feb 2026.[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 10.0.14393.8868, 10.0.17763.8389, 10.0.19044.6937 and 10.0.19045.6937 and others.
Description
CISA calls it an MSHTML framework protection mechanism failure in Microsoft Windows. In its words: “Microsoft MSHTML Framework contains a protection mechanism failure vulnerability that could allow an unauthorized attacker to bypass a security feature over a network.”[1]
The underlying weakness is protection mechanism failure (CWE-693), in MITRE’s list of common software weaknesses.[2]
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 1607 | 10.0.14393.8868 |
| Windows 10 Version 1809 | 10.0.17763.8389 |
| Windows 10 Version 21H2 | 10.0.19044.6937 |
| Windows 10 Version 22H2 | 10.0.19045.6937 |
| Windows 11 version 22H3 | 10.0.22631.6649 |
| Windows 11 Version 23H2 | 10.0.22631.6649 |
| Windows 11 Version 24H2 | 10.0.26100.7840 |
| Windows 11 Version 25H2 | 10.0.26200.7840 |
Every device with Microsoft Windows needs the update.
Exploitation
CISA added it to its catalog of known exploited vulnerabilities on 10 Feb 2026, 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 3 Mar 2026 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 16%. That’s in the top 4% 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, follow applicable BOD 22-01 guidance for cloud services, 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 10.0.14393.8868 or later.Fixed in version 10.0.14393.8868, 10.0.17763.8389, 10.0.19044.6937 and 10.0.19045.6937 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.[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 16%.[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 protection mechanism failure flaws
- CVE-2026-58704, Google Pixel: improper authorization ·
- CVE-2025-40536, SolarWinds Web Help Desk: security control bypass ·
- CVE-2025-0411, 7-Zip: mark of the web bypass ·
- CVE-2013-0431, Oracle Java Runtime Environment (JRE): JRE sandbox bypass ·
- CVE-2013-2465, Oracle Java SE: unspecified ·
Added the same day
- CVE-2026-21533, Microsoft Windows: improper privilege management ·
- CVE-2026-21525, Microsoft Windows: NULL pointer dereference ·
- CVE-2026-21519, Microsoft Windows: type confusion ·
- CVE-2026-21514, Microsoft Office: word reliance on untrusted inputs in a security decision ·
- CVE-2026-21510, Microsoft Windows: shell protection mechanism failure ·
Every Microsoft flaw on CISA’s list · Everything added in February 2026
References
- ^ Cybersecurity and Infrastructure Security Agency. “Microsoft MSHTML Framework Protection Mechanism Failure Vulnerability”. Known Exploited Vulnerabilities Catalog, added 10 Feb 2026.
- ^ National Institute of Standards and Technology. “CVE-2026-21513 Detail”. National Vulnerability Database, published 10 Feb 2026.
- ^ Microsoft. Advisories and fixes: msrc.microsoft.com, msrc.microsoft.com, vicarius.io.
- ^ FIRST. “Exploit Prediction Scoring System”. EPSS 0.159, percentile 96.9, 31 Oct 2026.
- CVE Program. “CVE-2026-21513”. 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.