Microsoft Office Zero-Day CVE-2026-21509

Executive Summary

Microsoft Office zero-day CVE-2026-21509 is being actively exploited. Learn the risks, affected versions, attack methods, and mitigation guidance.

Key Details

Affected Platform

Microsoft Office

Attack Surface

Recommended Action

Affected Environment

Operational Impact

Risk is High for government and business environments. Impact ranges from service-level DoS to potential remote code execution depending on how OpenSSL is used and whether applications parse attacker-controlled cryptographic inputs. The most operationally dangerous scenarios involve automated processing of untrusted CMS/PKCS#7 or PKCS#12 content, which can expose services to pre-auth parsing vulnerabilities.

Technical Root Cause

Recommended Mitigation

Prioritise patching on systems that parse untrusted cryptographic content. Reduce exposure by restricting certificate/CMS ingestion workflows, applying input validation where feasible, and segmenting critical services. Enforce timely vulnerability management and ensure unsupported OpenSSL branches (<1.1.1) are removed or covered by an extended support plan.

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Full Technical Report

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References

https://openssl-library.org/news/vulnerabilities/index.html#CVE-2025-15467
https://www.securityweek.com/high-severity-remote-code-execution-vulnerability-patched-in-openssl/