CVE-2026-21509
Published: January 26, 2026
Official Description
Microsoft Office contains a security feature bypass vulnerability in which reliance on untrusted inputs in a security decision in Microsoft Office could allow an unauthorized attacker to bypass a security feature locally. Some of the impacted product(s) could be end-of-life (EoL) and/or end-of-service (EoS). Users are advised to discontinue use and/or transition to a supported version.
CISA KEV Advisory
Microsoft Office Security Feature Bypass Vulnerability
Microsoft Office contains a security feature bypass vulnerability in which reliance on untrusted inputs in a security decision in Microsoft Office could allow an unauthorized attacker to bypass a security feature locally. Some of the impacted product(s) could be end-of-life (EoL) and/or end-of-service (EoS). Users are advised to discontinue use and/or transition to a supported version.
Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.
Risk Analysis
This vulnerability in Microsoft Office allows an attacker to bypass security features by tricking the software into relying on untrusted input. With a CVSS score of 7.8 and confirmed exploitation in the wild, this is a high-severity issue that requires immediate attention. The high EPSS score also indicates a significant likelihood of exploitation.
Active exploitation of this vulnerability has been observed in the wild, and it is included in CISA's Known Exploited Vulnerabilities catalog. The attack vector is local, requiring user interaction to trigger the bypass.
Users should discontinue the use of end-of-life or end-of-service Microsoft Office products and transition to supported versions. Apply all available security updates and patches for Microsoft Office to address this flaw.
Technical Analysis
CVE-2026-21509 requires local access, meaning attackers must already have a foothold on the target system.
Exploitation does not require any privileges, though user interaction (Required) is needed, which slightly reduces the risk of mass automated attacks.
A successful exploit results in complete confidentiality breach (data exposure), full integrity compromise (data manipulation), availability disruption (denial of service), with a CVSS base score of 7.8.
CISA has added CVE-2026-21509 to the Known Exploited Vulnerabilities (KEV) catalog, confirming active exploitation in the wild. U.S. federal agencies are required to patch this within the mandated timeframe, and all organizations should treat remediation as urgent.
CVSS v3.1 Vector Breakdown
Affected Vendors & Products
Exploit & PoC Resources
News & Research Mentioning CVE-2026-21509
Microsoft has published three out-of-band (OOB) updates so far in January 2026. One of these updates was released to address a vulnerability, CVE-2026-21509, affecting Microsoft Office that has been reportedly exploited in the wild. [xlite_meta score:44 src:Cisco Talos Blog xlite_fp:5076a023a0dfe8a2d35421bedbb2500da40c4a8ab8bd6a7dbd6962b3f7e020e6]
Categories: Threat Research Tags: Microsoft Office, vulnerability, advisory [xlite_meta score:36 src:Sophos X-Ops xlite_fp:b9cfe53e806d91ea717667a4ab6c9f6b3abd512a5594133ec6e719d213b460ae]
All References (1)
Quick Facts
Known Threat Actors
Recommended Actions
- →Apply vendor patches immediately
- →Monitor CVE-2026-21509 in threat intel feeds
- →Review IDS/IPS signatures for exploitation attempts
- !CISA KEV: Federal agencies must patch per BOD 22-01 timeline
- !Active exploitation confirmed — treat as P1