# CVE-2026-93849: Mattermost denial of service in WebSocket msgpack decoder

Source: https://junglewise.ai/threats/cve-2026-93849-mattermost-versions-11-9-x-11-9-1-11-8-x-11-8-5-11-7-x-11-7-10-11  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-10-26. Updated: 2026-10-26  
License: CC BY 4.0 with a link back

CVE-2026-93849 · Severity: medium · CVSS 6.5 · Published 2026-10-26

Mattermost, a team collaboration and messaging platform, fails to properly limit memory allocation when processing WebSocket messages. An authenticated attacker can send a specially crafted binary message that causes the server to allocate excessive memory, leading to service degradation or outage.

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

## Quick answers

**Is CVE-2026-93849 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-93849 being exploited in the wild.

**How severe is CVE-2026-93849?** Medium severity, with a CVSS score of 6.5, published 26 October 2026.

**Which products and versions are affected?** Mattermost Server 11.7.x up to 11.7.10, 11.8.x up to 11.8.5, 11.9.x up to 11.9.1, 11.10.x up to 11.10.1.

**Is there a fix for CVE-2026-93849?** Yes: fixed in Mattermost Server 11.7.11, 11.8.6, 11.9.2, 11.10.2 (or later versions in each branch). Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-93849 attacked?** It is attacked over the network, in a hosted service.

**Am I affected by CVE-2026-93849?** 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.

## How to fix

- Fixed in Mattermost Server 11.7.11, 11.8.6, 11.9.2, 11.10.2 (or later versions in each branch)

## Technical details

The WebSocket msgpack decoder in Mattermost lacks allocation limits on the byte-string decode path, allowing an authenticated attacker to declare a Bin32 payload with lengths up to 4,294,967,295 bytes via a crafted binary frame. This triggers excessive server memory allocation resulting in denial of service. Authentication is required; the vulnerability is exploited over the network via the WebSocket protocol.

## Affected products

- Mattermost Mattermost Server 11.7.x up to 11.7.10, 11.8.x up to 11.8.5, 11.9.x up to 11.9.1, 11.10.x up to 11.10.1

## Timeline

- 2026-10-26: disclosed

## References

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

## Related threats

- [Mattermost CSRF via Agents plugin mention reminder validation](https://junglewise.ai/threats/cve-2026-93851-mattermost-versions-11-9-x-11-9-1-11-8-x-11-8-5-11-7-x-11-7-10)
- [Mattermost denial of service via PDF text extraction memory exhaustion](https://junglewise.ai/threats/cve-2026-93850-mattermost-versions-11-9-x-11-9-1-11-8-x-11-8-5-11-7-x-11-7-10-11)
- [Mattermost information disclosure in post deletion events](https://junglewise.ai/threats/cve-2026-19621-mattermost-versions-11-10-x-11-10-0-11-7-x-11-7-9-11-9-x-11-9-1)
- [Mattermost authentication token leak in server logs](https://junglewise.ai/threats/cve-2026-19600-mattermost-versions-11-9-x-11-9-1-11-8-x-11-8-5-11-7-x-11-7-10-11)
- [Mattermost authentication bypass in OAuth account takeover](https://junglewise.ai/threats/cve-2026-19076-mattermost-versions-11-10-x-11-10-0-11-7-x-11-7-9-11-9-x-11-9-1)