# CVE-2026-78521: Microsoft Office Word heap-based buffer overflow

Source: https://junglewise.ai/threats/cve-2026-78521-microsoft-office-word-heap-based-buffer-overflow  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-09-08. Updated: 2026-09-26  
License: CC BY 4.0 with a link back

CVE-2026-78521 · Severity: high · CVSS 8.8 · Published 2026-09-08

Microsoft Office Word is widely used for document creation and editing across organizations. A heap-based buffer overflow vulnerability in Word allows an attacker to execute arbitrary code by sending a specially crafted document over the network, potentially compromising user systems and exposing sensitive business data.

Technologies: [Microsoft Office Word](https://junglewise.ai/threats/technologies/office-word). Vendors: [Microsoft](https://junglewise.ai/threats/vendors/microsoft).

## Quick answers

**Is CVE-2026-78521 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-78521 being exploited in the wild.

**How severe is CVE-2026-78521?** High severity, with a CVSS score of 8.8, published 8 September 2026.

**Which products and versions are affected?** Microsoft Office Word.

**Is there a fix for CVE-2026-78521?** No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.

**How is CVE-2026-78521 attacked?** It is attacked over the network.

**Am I affected by CVE-2026-78521?** Check whether you run Microsoft Office Word. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## Technical details

A heap-based buffer overflow exists in Microsoft Office Word that can be triggered by a specially crafted document. The vulnerability is reachable over the network without requiring user authentication to the affected system, though user interaction (opening a malicious document) is typically required. An attacker can exploit this flaw to achieve remote code execution with the privileges of the user running Word. The vulnerability has a CVSS score of 8.8, indicating high severity, though no active exploitation in the wild has been reported as of the publication date.

## Affected products

- Microsoft Office Word

## Timeline

- 2026-09-08: disclosed

## References

- [https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-78521](https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-78521)

## Related threats

- [Microsoft Office Word buffer over-read information disclosure](https://junglewise.ai/threats/cve-2026-124233-buffer-over-read-in-microsoft-office-word-allows-an-unauthorized)
- [Microsoft Office Word use of uninitialized resource](https://junglewise.ai/threats/cve-2026-124185-use-of-uninitialized-resource-in-microsoft-office-word-allows-an)
- [Microsoft Office Word heap-based buffer overflow](https://junglewise.ai/threats/cve-2026-123394-heap-based-buffer-overflow-in-microsoft-office-word-allows-an)
- [Microsoft Office Word heap buffer overflow](https://junglewise.ai/threats/cve-2026-116925-heap-based-buffer-overflow-in-microsoft-office-word-allows-an)
- [Microsoft Office Word heap-based buffer overflow](https://junglewise.ai/threats/cve-2026-116923-heap-based-buffer-overflow-in-microsoft-office-word-allows-an)