CVE-2026-21509
CVE-2026-21509 is a security flaw in Microsoft Office, part of Microsoft Office. Attackers can get past a security protection in Microsoft Office. The US Cybersecurity and Infrastructure Security Agency (CISA) added it to its catalog of flaws known to be used in attacks on 26 Jan 2026.[1]
Microsoft Office 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 https://aka.ms/OfficeSecurityReleases, 16.0.5539.1001 and 16.0.10417.20095.
Description
CISA calls it a security feature bypass in Microsoft Office. In its words: “Microsoft Office contains a security feature bypass vulnerability in which reliance on untrusted inputs in a security decision in Microsoft Office could allow an unauthorized attacker to bypass a security feature locally. Some of the impacted product(s) could be end-of-life (EoL) and/or end-of-service (EoS). Users are advised to discontinue use and/or transition to a supported version.”[1]
The underlying weakness is CWE-807, 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 |
|---|---|
| Microsoft 365 Apps for Enterprise | https://aka.ms/OfficeSecurityReleases |
| Microsoft Office 2016 | 16.0.5539.1001 |
| Microsoft Office 2019 | 16.0.10417.20095 |
| Microsoft Office LTSC 2021 | https://aka.ms/OfficeSecurityReleases |
| Microsoft Office LTSC 2024 | https://aka.ms/OfficeSecurityReleases |
Every device with Microsoft Office needs the update.
Exploitation
CISA added it to its catalog of known exploited vulnerabilities on 26 Jan 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 16 Feb 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 71%. 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 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 Office 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 https://aka.ms/OfficeSecurityReleases or later.Fixed in version https://aka.ms/OfficeSecurityReleases, 16.0.5539.1001 and 16.0.10417.20095. 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 71%.[4] |
See also
Other Microsoft Office flaws
- CVE-2009-0238: remote code execution ·
- CVE-2026-21514: word reliance on untrusted inputs in a security decision ·
- CVE-2009-0556: powerPoint code injection ·
- CVE-2007-0671: excel remote code execution ·
- CVE-2023-23397: outlook privilege escalation ·
- CVE-2023-21715: publisher security feature bypass ·
Other Microsoft flaws
- CVE-2026-96951, Microsoft Windows: command injection ·
- CVE-2026-65660, Microsoft SharePoint: code injection ·
- CVE-2026-85880, Microsoft Windows: heap-based buffer overflow ·
- CVE-2026-81963, Microsoft Windows: link following ·
- CVE-2019-1068, Microsoft SQL Server: remote code execution ·
Added the same day
- CVE-2026-24061, GNU InetUtils: argument injection ·
- CVE-2026-23760, SmarterTools SmarterMail: authentication bypass using an alternate path or channel ·
- CVE-2025-52691, SmarterTools SmarterMail: unrestricted upload of file with dangerous type ·
- CVE-2018-14634, Linux Kernel: integer overflow ·
Every Microsoft flaw on CISA’s list · Everything added in January 2026
References
- ^ Cybersecurity and Infrastructure Security Agency. “Microsoft Office Security Feature Bypass Vulnerability”. Known Exploited Vulnerabilities Catalog, added 26 Jan 2026.
- ^ National Institute of Standards and Technology. “CVE-2026-21509 Detail”. National Vulnerability Database, published 26 Jan 2026.
- ^ Microsoft. Advisories and fixes: msrc.microsoft.com, vicarius.io.
- ^ FIRST. “Exploit Prediction Scoring System”. EPSS 0.708, percentile 99.4, 31 Oct 2026.
- CVE Program. “CVE-2026-21509”. 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.