# CVE-2026-5740: Mattermost Server denial of service via msgpack WebSocket frames

Source: https://junglewise.ai/threats/cve-2026-5740-mattermost-server-denial-of-service-via-msgpack-websocket-frames  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-05-22. Updated: 2026-07-23  
License: CC BY 4.0 with a link back

CVE-2026-5740 · Severity: high · CVSS 7.5 · Published 2026-05-22

Mattermost, a popular collaboration and messaging platform, is vulnerable to a flaw that allows an attacker to crash the server. By sending a specially crafted message to the server's public communication channel, an unauthenticated user can force the system to shut down. This results in a total service outage, preventing all employees from communicating until the server is manually restarted.

Technologies: [github.com/mattermost/mattermost-server (Go)](https://junglewise.ai/threats/technologies/github-com-mattermost-mattermost-server), [github.com/mattermost/mattermost-server/v5 (Go)](https://junglewise.ai/threats/technologies/github-com-mattermost-mattermost-server-v5), [github.com/mattermost/mattermost-server/v6 (Go)](https://junglewise.ai/threats/technologies/github-com-mattermost-mattermost-server-v6), [github.com/mattermost/mattermost/server/v8 (Go)](https://junglewise.ai/threats/technologies/github-com-mattermost-mattermost-server-v8), [Mattermost Server](https://junglewise.ai/threats/technologies/server-public). Vendors: [Go](https://junglewise.ai/threats/vendors/go), [Mattermost](https://junglewise.ai/threats/vendors/mattermost).

## Quick answers

**Is CVE-2026-5740 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-5740 being exploited in the wild.

**How severe is CVE-2026-5740?** High severity, with a CVSS score of 7.5, published 22 May 2026.

**Which products and versions are affected?** github.com/mattermost/mattermost-server (Go) SEMVER:introduced=10.11.0+incompatible,fixed=10.11.15+incompatible,introduced=11.4.0+incompatible,fixed=11.4.5+incompatible,introduced=11.5.0+incompatible,fixed=11.5.4+incompatible,introduced=11.6.0+incompatible,fixed=11.6.1+incompatible, SEMVER:introduced=11.6.0,fixed=11.6.1; github.com/mattermost/mattermost-server/v5 (Go) SEMVER:introduced=0; github.com/mattermost/mattermost-server/v6 (Go) SEMVER:introduced=0; github.com/mattermost/mattermost/server/v8 (Go) SEMVER:introduced=0,fixed=8.0.0-20260410202636-17939826efa2; and 1 more.

**Is there a fix for CVE-2026-5740?** Yes. github.com/mattermost/mattermost-server (Go): upgrade to 11.6.1, 11.6.1+incompatible or later; github.com/mattermost/mattermost/server/v8 (Go): upgrade to 8.0.0-20260410202636-17939826efa2 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-5740 attacked?** It is attacked over the network, in a hosted service.

**Am I affected by CVE-2026-5740?** Check whether you run github.com/mattermost/mattermost-server (Go), github.com/mattermost/mattermost-server/v5 (Go), github.com/mattermost/mattermost-server/v6 (Go) or one of the others listed 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

- github.com/mattermost/mattermost-server (Go): upgrade to 11.6.1, 11.6.1+incompatible or later

  ```sh
  go get github.com/mattermost/mattermost-server@v11.6.1+incompatible
  ```

- github.com/mattermost/mattermost/server/v8 (Go): upgrade to 8.0.0-20260410202636-17939826efa2 or later

  ```sh
  go get github.com/mattermost/mattermost/server/v8@v8.0.0-20260410202636-17939826efa2
  ```


## Technical details

A vulnerability exists in Mattermost Server's handling of WebSocket communications. The application fails to properly validate the size or structure of msgpack-encoded WebSocket frames before attempting to allocate memory for them (CWE-789). An unauthenticated remote attacker can exploit this by sending a crafted binary WebSocket message to the public WebSocket endpoint. This leads to excessive memory allocation, causing the server process to crash and resulting in a denial-of-service (DoS) condition. The issue is resolved in versions 11.7.0, 11.6.1, 11.5.4, 11.4.5, and 10.11.15.

## Affected products

- Mattermost Mattermost Server 11.6.0, 11.5.0 to 11.5.3, 11.4.0 to 11.4.4, 10.11.0 to 10.11.14

## Identifiers

- CVE-2026-5740
- GHSA-w9m8-p4cc-4qj9
- GO-2026-5838

## Timeline

- 2026-05-22: advisory: Mattermost published advisory MMSA-2026-00647
- 2026-05-22: disclosed: CVE-2026-5740 published to NVD

## References

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

## Related threats

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- [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 denial of service in WebSocket msgpack decoder](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)
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- [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)