Heap-based buffer overflow in Microsoft products - CVE-2026-78521

Vulnerability details

The vulnerability allows a remote attacker to execute arbitrary code.

The vulnerability exists due to a heap-based buffer overflow in Microsoft Office Word when processing malicious mail-merge data. A remote attacker can send a specially crafted Word document containing malicious mail-merge data to execute arbitrary code.

User interaction is required to open the document and process or update its mail-merge data. The Preview Pane is not an attack vector.

Affected software

How to mitigate CVE-2026-78521

External References

• https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2026-78521
• https://docs.microsoft.com/en-us/officeupdates/microsoft365-apps-security-updates

Related Security Bulletins

• Multiple vulnerabilities in Microsoft Office