# CVE-2026-78509: Microsoft Outlook heap buffer overflow allows remote code execution

Source: https://junglewise.ai/threats/cve-2026-78509-microsoft-outlook-heap-buffer-overflow-allows-remote-code  
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-2026-78509 · Severity: critical · CVSS 9.8 · Published 2026-09-08

Microsoft Outlook is the email and calendar client used by millions of enterprises and individuals worldwide. A heap buffer overflow vulnerability allows an attacker to send a specially crafted email that executes arbitrary code on a victim's computer without authentication or user interaction, potentially leading to complete system compromise and data theft.

Technologies: [Microsoft Outlook](https://junglewise.ai/threats/technologies/outlook). Vendors: [Microsoft](https://junglewise.ai/threats/vendors/microsoft).

## Quick answers

**Is CVE-2026-78509 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-78509 being exploited in the wild.

**How severe is CVE-2026-78509?** Critical severity, with a CVSS score of 9.8, published 8 September 2026.

**Which products and versions are affected?** Microsoft Outlook.

**Is there a fix for CVE-2026-78509?** 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-2026-78509 attacked?** It is attacked over the network, in a web application.

**Am I affected by CVE-2026-78509?** Check whether you run Microsoft Outlook. 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 Outlook's email message processing logic. The vulnerability is triggered when Outlook processes a maliciously crafted email attachment or message body, allowing an attacker to overwrite heap memory and execute arbitrary code. The attack is network-based and requires no authentication or user interaction beyond receiving the malicious email. Successful exploitation results in remote code execution with the privileges of the user running Outlook. Microsoft has released security patches to address this vulnerability.

## Affected products

- Microsoft Outlook

## Timeline

- 2026-09-08: disclosed

## References

- [https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-78509](https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-78509)

## Related threats

- [Microsoft Office Outlook user warning spoofing vulnerability](https://junglewise.ai/threats/cve-2026-124194-microsoft-office-outlook-missing-user-interaction-for)
- [Microsoft Outlook heap-based buffer overflow](https://junglewise.ai/threats/cve-2026-123384-heap-based-buffer-overflow-in-microsoft-office-outlook-allows-an)
- [Microsoft Outlook heap-based buffer overflow](https://junglewise.ai/threats/cve-2026-123380-heap-based-buffer-overflow-in-microsoft-office-outlook-allows-an)
- [Microsoft Office Outlook heap-based buffer overflow](https://junglewise.ai/threats/cve-2026-123377-heap-based-buffer-overflow-in-microsoft-office-outlook-allows-an)
- [Microsoft Office Outlook remote code execution](https://junglewise.ai/threats/cve-2026-70125-microsoft-office-outlook-remote-code-execution-vulnerability)