CVE-2026-21509
Microsoft Office 2016 … 2019
Raw vector
CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:HSummary
CVE-2026-21509 is a high-severity Reliance on Untrusted Inputs in a Security Decision (CWE-807) vulnerability in Microsoft Office. Its CVSS base score is 7.8 (High).
Operationally, exploitation aligns with the MITRE ATT&CK technique Exploit Public-Facing Application (T1190); ranked in the top 0.6% of CVEs by exploit likelihood; CISA has added it to the Known Exploited Vulnerabilities catalog.
The strongest mitigations our analysis identified map to AC-24 (Access Control Decisions) and SI-10 (Information Input Validation) — see the control section below for these in your framework.
Deeper analysis AI-assisted summary
Synthesised by an AI model from the NVD description and linked references — a reading aid, not an authoritative source.
CVE-2026-21509 is a vulnerability in Microsoft Office arising from reliance on untrusted inputs in a security decision, as defined by CWE-807. Published on 2026-01-26, it enables an unauthorized attacker to bypass a security feature locally. The issue carries a CVSS v3.1 base score of 7.8 (AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H), reflecting high severity due to substantial impacts on confidentiality, integrity, and availability.
Exploitation requires local access to the affected system, low attack complexity, and no user privileges, though it demands user interaction. An unauthorized attacker can leverage this to circumvent Microsoft Office security mechanisms, potentially leading to high-level compromise of the local environment.
Microsoft’s Security Response Center provides an update guide at https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-21509. Vicarius offers a detection script and a mitigation script via their blog posts at https://www.vicarius.io/vsociety/posts/cve-2026-21509-detection-script-microsoft-office-security-feature-bypass-vulnerability and https://www.vicarius.io/vsociety/posts/cve-2026-21509-mitigation-script-microsoft-office-security-feature-bypass-vulnerability, respectively. The vulnerability also appears in CISA’s Known Exploited Vulnerabilities Catalog at https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2026-21509.
OWASP Top 10 for Web (2025)
EU & UK References
- 🇪🇺 ENISA EUVD: EUVD-2026-4666
Vulnerability Data
Reliance on untrusted inputs in a security decision in Microsoft Office allows an unauthorized attacker to bypass a security feature locally.
- CWE(s)
- KEV Date Added
- 26 January 2026
Related Threats
MITRE ATT&CK Enterprise Techniques
CVEs Like This One
Affected Assets
Mitigating Controls
Control response
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- 1 hardening rule · 1 OS baseline
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Mitigating Controls (NIST 800-53 r5) AI
Explicitly requires that every access decision be based on authoritative, protected decision data rather than caller-supplied inputs.
Input validation can reject malformed or attacker-controlled values before they reach a security decision point.
Access enforcement requires decisions to be made from trusted policy data rather than modifiable client-supplied inputs.
Information-flow enforcement applies rules to validated, internal attributes instead of untrusted external values.
Mitigating Controls (NIST CSF 2.0) AI
Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→CSF cross-walk (authority under review) — links open the control.
Strong authentication mechanisms directly prevent security decisions from depending on modifiable, untrusted inputs.
Verification and protection of identity assertions stops reliance on attacker-controlled values for authorization decisions.
Proper management of identities/credentials reduces the chance that security decisions will be driven by untrusted inputs.
Policy-driven, least-privilege authorization reduces opportunities to bypass controls via tampered inputs.
Mitigating Controls (ISO/IEC 27001:2022 Annex A) AI
Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→ISO cross-walk (authority under review) — links open the control.
Secure coding standards directly prohibit using untrusted inputs for security-critical decisions.
Security testing in development catches input-validation flaws before deployment.
Monitoring can detect exploitation but does not stop the underlying weakness.
Application security requirements explicitly call for validation of inputs used in security decisions.
Secure architecture principles mandate treating all external inputs as untrusted.
Access-control rules can be bypassed if they rely on untrusted inputs.