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

> An attacker can bypass security features in Microsoft Office by exploiting reliance on untrusted inputs, requiring local access.

- **Severity:** High 7.8 (`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`)
- **Exploited in the wild:** Yes, since 26 Jan 2026 (CISA KEV)
- **Login needed:** Not needed
- **Attackable:** Local or same network only
- **Victim action needed:** A user has to click or open something
- **Fix available:** No
- **Published:** 26 Jan 2026
- **Updated:** 30 Jul 2026
- **Also known as:** GHSA-ch84-h92g-mw93

## What is CVE-2026-21509?

CVE-2026-21509 is a high-severity vulnerability in Microsoft 365 Apps, published on 26 Jan 2026. It can only be exploited from the same machine or local network, and only if a user opens a file or follows a link.

### Official description

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

Weakness: CWE-807 (Reliance on Untrusted Inputs in a Security Decision)

## Which versions are affected by CVE-2026-21509?

It affects 3 products.


Products: Microsoft 365 Apps, Microsoft Office, Microsoft Office Long Term Servicing Channel.

## How do I fix CVE-2026-21509?

No fix has been published for CVE-2026-21509 yet. Limit exposure of the affected software until one is available. US federal agencies must fix it by 16 Feb 2026.

## Is CVE-2026-21509 being exploited?

Yes. Attackers have exploited CVE-2026-21509 in the wild since at least 26 Jan 2026.

Public exploit code:

- Educational PoC for CVE‑2026‑21509 (Microsoft Office security feature bypass). Generates a harmless DOCX with dummy OLE artifacts to study EDR/AV visibility. Not an exploit. For isolated labs only; see README for 7‑Zip inspection steps and mitigation references. (github_poc): https://github.com/gavz/CVE-2026-21509-PoC
- CVE-2026-21509 (github_poc): https://github.com/kimstars/Ashwesker-CVE-2026-21509
- PowerShell script to check, apply, and test the Kill-Bit protection for the CVE-2026-21509 Microsoft Office zero-day vulnerability affecting Office 2016/2019/LTSC. (github_poc): https://github.com/suuhm/CVE-2026-21509-handler
- New Physics Disclosure This repository contains a full weaponized exploit for **CVE-2026-21509**, targeting the Windows Network File System (NFSv4.1) kernel-mode driver (`nfssvr.sys`).  (github_poc): https://github.com/SimoesCTT/CTT-NFS-Vortex-RCE
- CVE-2026-21509 is a critical bypass in the Microsoft Office OLE (Object Linking and Embedding) validation engine. While standard "laminar" exploits attempt to manipulate static COM objects, this repository utilizes Theorem 4.2 to achieve a speculative race-condition bypass of the Object Definition Rule (ODR). (github_poc): https://github.com/SimoesCTT/CTT-MICROSOFT-OFFICE-OLE-MANIFOLD-BYPASS-CVE-2026-21509
- YARA rule and python script to detect potential exploits for the CVE-2026-21509 vulnerability in MS Office (github_poc): https://github.com/decalage2/detect_CVE-2026-21509

## Other vulnerabilities in Microsoft 365 Apps

- [CVE-2024-21413](https://insights.sbomify.com/CVE-2024-21413/): Microsoft Outlook Improper Input Validation (critical 9.8, exploited in the wild)
- [CVE-2023-23397](https://insights.sbomify.com/CVE-2023-23397/): Microsoft Office Outlook Privilege Escalation (critical 9.8, exploited in the wild)
- [CVE-2024-38189](https://insights.sbomify.com/CVE-2024-38189/): Microsoft Project Remote Code Execution Vulnerability (high 8.8, exploited in the wild)
- [CVE-2023-35311](https://insights.sbomify.com/CVE-2023-35311/): Microsoft Outlook Security Feature Bypass (high 8.8, exploited in the wild)
- [CVE-2026-21514](https://insights.sbomify.com/CVE-2026-21514/): Microsoft Office Word Reliance on Untrusted Inputs in a Security Decision (high 7.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-21509 and fix it if it is affected.

What it is: CVE-2026-21509 is a high-severity vulnerability in Microsoft 365 Apps, published on 26 Jan 2026. It can only be exploited from the same machine or local network, and only if a user opens a file or follows a link.
Exploitation: Yes. Attackers have exploited CVE-2026-21509 in the wild since at least 26 Jan 2026.

Affected product: Microsoft 365 Apps. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-21509/ (Markdown: https://insights.sbomify.com/CVE-2026-21509/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-21509
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-21509
- OSV: https://osv.dev/vulnerability/CVE-2026-21509

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-21509/
Summaries are generated and may contain mistakes; the official description above is the record.