# CVE-2026-86348: Mattermost denial of service via handler panic in post-action request

Source: https://junglewise.ai/threats/cve-2026-86348-mattermost-denial-of-service-via-handler-panic-in-post-action  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-09-14. Updated: 2026-09-26  
License: CC BY 4.0 with a link back

CVE-2026-86348 · Severity: medium · CVSS 4.3 · Published 2026-09-14

Mattermost Server fails to properly handle unexpected data types in post-action requests, allowing an authenticated user to crash the plugin. This results in temporary unavailability of the affected plugin functionality until the server is restarted, disrupting team communication and collaboration features.

Technologies: [Mattermost Server](https://junglewise.ai/threats/technologies/server-public). Vendors: [Mattermost](https://junglewise.ai/threats/vendors/mattermost).

## Quick answers

**Is CVE-2026-86348 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-86348 being exploited in the wild.

**How severe is CVE-2026-86348?** Medium severity, with a CVSS score of 4.3, published 14 September 2026.

**Which products and versions are affected?** Mattermost Server <=11.9 including 11.0.9, 11.4.8, 11.7.7, 10.22.11.0.

**Is there a fix for CVE-2026-86348?** 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-2026-86348 attacked?** It is attacked over the network, in a hosted service.

**Am I affected by CVE-2026-86348?** Check whether you run Mattermost Server at an affected version. 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

The vulnerability exists in Mattermost's post-action handler, which does not properly recover from panics when processing requests containing unexpected field types. An authenticated user can craft a malicious post-action request with an invalid field type to trigger an unhandled panic in the handler, causing the plugin to crash. The attack requires valid authentication credentials and network access to the Mattermost instance. Affected versions include 11.9 and earlier, with specific vulnerable releases: 11.0.9, 11.4.8, 11.7.7, and 10.22.11.0. The vendor has released patches; users should upgrade to patched versions to remediate the issue.

## Affected products

- Mattermost Mattermost Server <=11.9 including 11.0.9, 11.4.8, 11.7.7, 10.22.11.0

## Timeline

- 2026-09-14: disclosed

## References

- [https://mattermost.com/security-updates](https://mattermost.com/security-updates)

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