CVE-2026-21509: Microsoft Office High-Severity Vulnerability

What is CVE-2026-21509?

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

Technical Analysis

Weakness Classification

This vulnerability is classified as CWE-807.

Attack Vector Breakdown

• Attack Vector: Local
• Attack Complexity: Low
• Privileges Required: None
• User Interaction: Required
• Scope: Unchanged
• Confidentiality: High
• Integrity: High
• Availability: High

Impact Analysis

Exploitation requires local access to the target system, such as a local user session or a malicious file opened by a user. The attack complexity is low, meaning exploitation does not depend on specialized conditions or race windows. Successful exploitation can result in full confidentiality breach (complete data disclosure), complete integrity compromise (data modification or code execution), total availability loss (denial of service).

Exploitation Probability (EPSS): 12.1% chance of exploitation in the next 30 days (94th percentile). This is a significantly elevated exploitation probability — prioritize patching.

Exploit Availability

Public exploit code or proof-of-concept is available for CVE-2026-21509. This significantly increases the risk of exploitation, as attackers can use or adapt existing exploit code rather than developing their own. A public exploit reference has been identified. Organizations should treat this as an urgent patching priority.

Is CVE-2026-21509 being exploited?

Yes. CVE-2026-21509 is listed in CISA's Known Exploited Vulnerabilities (KEV) catalog, restricted to CVEs with confirmed active exploitation in the wild. CISA added this vulnerability on 2026-01-26. Inclusion in KEV triggers the patching mandate under BOD 22-01 for U.S. federal civilian agencies, with a remediation deadline of 2026-02-16. Additionally, public exploit code is available, which lowers the barrier for attackers.

How to fix CVE-2026-21509

Apply the latest security update via Windows Update, WSUS, or Microsoft Update Catalog. For enterprise environments, use SCCM/Intune for managed deployment. If the patch cannot be applied immediately, check the MSRC advisory for workarounds such as registry-based mitigations or disabling affected features. Check the Microsoft Security Response Center page for the specific patch or update. CISA's Known Exploited Vulnerabilities catalog requires U.S. federal agencies to remediate this vulnerability by the due date shown below under Binding Operational Directive 22-01. Private organizations should treat KEV entries as priority-one patches because active exploitation has been confirmed in the wild.

Related Vulnerabilities

Other actively exploited vulnerabilities affecting Office or sharing the same weakness (CWE-807):

• CVE-2026-21514 (High 7.8) [same product + same CWE] — Reliance on untrusted inputs in a security decision in Microsoft Office Word…
• CVE-2009-0238 (High 8.8) [same product] — Microsoft Office Excel 2000 SP3, 2002 SP3, 2003 SP3, and 2007 SP1; Excel…
• CVE-2009-0556 (High 8.8) [same product] — Microsoft Office PowerPoint 2000 SP3, 2002 SP3, and 2003 SP3, and PowerPoint…
• CVE-2007-0671 (High 8.8) [same product] — Unspecified vulnerability in Microsoft Excel 2000, XP, 2003, and 2004 for Mac…
• CVE-2008-4250 (Critical 9.8) [same vendor] — The Server service in Microsoft Windows 2000 SP4, XP SP2 and SP3, Server 2003…

Frequently asked questions

Is CVE-2026-21509 critical?

CVE-2026-21509 has a CVSS 3.1 base score of 7.8 — High severity. It should be treated as a priority patch, especially given the active exploitation confirmed by CISA. The availability of public exploit code further increases the urgency.

Is CVE-2026-21509 being actively exploited?

Yes — CISA added CVE-2026-21509 to the Known Exploited Vulnerabilities catalog on 2026-01-26. Inclusion in KEV means CISA has confirmed in-the-wild exploitation. Public exploit code has also been identified, making exploitation accessible to a broader range of threat actors.

How do I patch CVE-2026-21509?

Apply the security update from Microsoft for Office. Federal agencies must remediate by 2026-02-16 under BOD 22-01. Refer to the Microsoft Security Response Center for the official patch.

What type of vulnerability is CVE-2026-21509?

CVE-2026-21509 is classified under CWE-807.

What is the CVSS score for CVE-2026-21509?

CVE-2026-21509 has a CVSS 3.1 base score of 7.8, classifying it as High severity. The attack vector is Local, attack complexity is Low, and no privileges are required.