CVE-2026-4646: Mattermost Server denial of service in PR details endpoint

CVE-2026-4646 · Severity: medium · CVSS 4.3 · Published 2026-05-22

Mattermost, a popular enterprise collaboration and messaging platform, is affected by a vulnerability that allows a logged-in user to crash specific plugin processes. By sending a specially crafted request to the Pull Request (PR) details interface, an attacker can cause a denial-of-service condition for that component. This disrupts normal operations and may require administrative intervention to restore full functionality.

Technologies: github.com/mattermost/mattermost-plugin-github (Go), github.com/mattermost/mattermost-server (Go), Mattermost GitHub Plugin for Mattermost, Mattermost Server. Vendors: Go, Mattermost.

Quick answers

Is CVE-2026-4646 exploited in the wild?
No. Junglewise Threat Intelligence has no report of CVE-2026-4646 being exploited in the wild.
How severe is CVE-2026-4646?
Medium severity, with a CVSS score of 4.3, published 22 May 2026.
Which products and versions are affected?
github.com/mattermost/mattermost-plugin-github (Go) SEMVER:introduced=0,fixed=1.0.1-0.20260330164815-c2840e980b3c; 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; Mattermost GitHub Plugin for Mattermost < 1.0.1-0.20260330164815-c2840e980b3c; 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.
Is there a fix for CVE-2026-4646?
Yes. github.com/mattermost/mattermost-plugin-github (Go): upgrade to 1.0.1-0.20260330164815-c2840e980b3c or later; github.com/mattermost/mattermost-server (Go): upgrade to 11.6.1, 11.6.1+incompatible or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-4646 attacked?
It is attacked over the network, in a network appliance.
Am I affected by CVE-2026-4646?
Check whether you run github.com/mattermost/mattermost-plugin-github (Go), github.com/mattermost/mattermost-server (Go), Mattermost GitHub Plugin for Mattermost 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-plugin-github (Go): upgrade to 1.0.1-0.20260330164815-c2840e980b3c or later
    go get github.com/mattermost/mattermost-plugin-github@v1.0.1-0.20260330164815-c2840e980b3c
  • github.com/mattermost/mattermost-server (Go): upgrade to 11.6.1, 11.6.1+incompatible or later
    go get github.com/mattermost/mattermost-server@v11.6.1+incompatible

Technical details

A denial-of-service vulnerability exists in Mattermost Server due to improper validation of specified input types (CWE-1287) within API request handlers. An authenticated attacker with network access to the Mattermost instance can send a specially crafted HTTP request to the Pull Request (PR) details endpoint. This malformed input causes the associated plugin process to crash. The vulnerability affects multiple version branches including 10.11.x, 11.4.x, 11.5.x, and 11.6.x. Patches have been released in versions 10.11.15, 11.4.5, 11.5.4, 11.6.1, and 11.7.0.

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-2026-4646
  • GHSA-rmvv-8v8w-rf7x
  • GO-2026-5833

Timeline

  • 2026-05-22: advisory: Mattermost published advisory MMSA-2026-00638
  • 2026-05-22: disclosed: CVE-2026-4646 published to NVD

References

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