CVE-2026-21509
Microsoft Office Security Feature Bypass Vulnerability
Exploited in the wild. In CISA KEV since 2026‑01‑26. Fixed versions are out.
What to do
Find your product and version, then upgrade to the build on the right.
- Microsoft Office 2016Affected: ≥ 16.0.0, < 16.0.5539.1001Upgrade to: 16.0.5539.1001Affected: ≥ 19.0.0, < 16.0.10417.20095Upgrade to: 16.0.10417.20095Affected: ≥ 16.0.1, < https://aka.ms/OfficeSecurityReleasesUpgrade to: https://aka.ms/OfficeSecurityReleasesAffected: ≥ 16.0.0, < https://aka.ms/OfficeSecurityReleasesUpgrade to: https://aka.ms/OfficeSecurityReleasesStated by: CVE record
- Microsoft 365 Apps for EnterpriseAffected: ≥ 16.0.1, < https://aka.ms/OfficeSecurityReleasesUpgrade to: https://aka.ms/OfficeSecurityReleasesStated by: CVE record
Versions as the sources state them. Full version matrix after sign-in
Coverage timeline
4 publishersA Russian-focused campaign used malicious RTF files to exploit CVE-2026-21509 in Microsoft Office, enabling bypass of Office protections via an OLE Shell Explorer component.
Exatrack reported analysis of PixyNetLoader malware linked to APT28 that hides an in-memory payload in PNG LSBs and stays active per a CERT-UA advisory.
Added to CISA KEV catalog
Sourced from public security reporting — each date is when it was published, not when it happened. 1 further article named this CVE in wider roundups without reporting it as a distinct event.
What it is
From the CVE record
Reliance on untrusted inputs in a security decision in Microsoft Office allows an unauthorized attacker to bypass a security feature locally.
In plain language
No plain-language summary for this CVE yet.
Exploitation
Where each signal puts this CVE on the scale from published to confirmed exploited.
- CISA KEV
Listed as exploited in the wild since 2026-01-26.
US federal agencies must remediate by 2026-02-16.
Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.
- Public exploits
No public exploit or proof of concept found in the sources we track.
- EPSS
71% chance of exploitation activity in the next 30 days, which ranks it in the 99th percentile of scored CVEs.
Exploit Prediction Scoring System, FIRST.org. A probability, not a confirmation.
- Attention now
Rising.
Lifecycle
9 events over 150 days, from the signal feeds we watch.
- OpenVAS check addedpatch available, workaround available, record updated
- EPSS band changemoderate → high
- EPSS band changelow → moderate
- Added to CISA KEVpublished, weakness classified
Affected products
Technical detail
CVSS 3.1 vector
Open in the CVSS calculatorCVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H
Scored 7.8 by NVD.
How it is reached
- Attack Vector LocalRequires local access to the vulnerable system (e.g. local login, malicious file)
- Attack Complexity LowNo special conditions — the attack can be reliably reproduced
- Privileges Required NoneNo authentication required — anyone can exploit this
- User Interaction RequiredA user must click a link, open a file, or perform some action
Scope
- Scope UnchangedImpact is limited to the vulnerable component itself
Impact if exploited
- Confidentiality HighTotal information disclosure — all data in the component is compromised
- Integrity HighTotal loss of integrity — attacker can modify any data in the component
- Availability HighTotal denial of service — the component is completely unavailable
Weaknesses
Sources
References in the record
- msrc.microsoft.com/update-guide/vulnerability/CVE-2026-21509
- vicarius.io/vsociety/posts/cve-2026-21509-detection-script-microsoft-office-security-feature-bypass-vulnerability
- vicarius.io/vsociety/posts/cve-2026-21509-mitigation-script-microsoft-office-security-feature-bypass-vulnerability
And 4 more references. See all after sign-in
In the news
- Russian APT28 Breaches NATO Naval Shipbuilder in NATO Country, Deploys New Reflective Loader
- PixyNetLoader Malware Analysis: How APT28 Hides Payloads Inside PNG Files
- Gamaredon Exploits WinRAR to Deliver GammaWorm and GammaSteel Against Ukraine
- Sednit reloaded: Back in the trenches
- Проникнуть в офис через Office
- Февральский «В тренде VM»: уязвимости в продуктах Microsoft
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