CVERadar
HIGH · CVSS 3.1 7.8CISA KEV · added 2026-01-26Public PoCEPSS 70.80% (p99)Threat level 75

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

Microsoft · Office · NVD published 2026-01-27 · posted 2026-01-26 · last modified 2026-06-25

What it is

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

WeaknessCWE-807
CVSS vectorCVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H (source: [email protected])
CISA due date2026-02-16 (for US federal agencies under BOD; a good deadline for everyone)
ATT&CK tacticsn/a

Exploitation evidence

First exploit evidence seen: 2026-01-26 (earliest date across the sources below).

Exploited in the wild

  • CISA KEV: listed since 2026-01-26. Source

Public exploit code

GitHub proof-of-concept repositories

13 public repositories from 11 accounts, first seen 2026-01-27. Most starred: gavz/CVE-2026-21509-PoC (42 stars), view repository.

Verify before running: public PoCs can be malicious. Read the code and only run it in an isolated lab. A PoC means exploit code is public, not that this CVE is exploited in the wild.

Sources: CISA KEV · nomi-sec/PoC-in-GitHub.

No ATT&CK techniques could be mapped or inferred for this CVE yet.

How to fix it

CISA 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. (source: CISA KEV)

Fixed versions

NVD does not list a fixed version for this CVE. Use the vendor advisory below.

Derived from the NVD CPE configuration (versionEndExcluding / versionEndIncluding). Always confirm with the vendor advisory.

Vendor advisories and patches

Detection Generic guidance

    Response checklist Generic guidance

    1. Inventory: find every instance of the affected product and version (include internet-facing and forgotten ones).
    2. Patch or mitigate per the vendor advisory; if you cannot patch, remove internet exposure or apply the vendor workaround.
    3. Assume compromise if the system was exposed before patching: hunt for the indicators in the advisory and review logs back to the disclosure date.
    4. Rotate credentials and secrets stored on or used by the affected system if compromise cannot be ruled out.

    Sources