CVE-2026-21388: Mattermost MS Teams plugin doesn't limit the request body size on the /lifecycle webhook endpoint

CVE-2026-21388 · Severity: low · CVSS 3.1 · Published 2026-06-25

Mattermost MS Teams plugin doesn't limit the request body size on the /lifecycle webhook endpoint in github.com/mattermost/mattermost-plugin-msteams

Technologies: github.com/mattermost/mattermost-plugin-msteams (Go). Vendors: Go.

Quick answers

Is CVE-2026-21388 exploited in the wild?
No. Junglewise Threat Intelligence has no report of CVE-2026-21388 being exploited in the wild.
How severe is CVE-2026-21388?
Low severity, with a CVSS score of 3.1, published 25 June 2026.
Which products and versions are affected?
github.com/mattermost/mattermost-plugin-msteams (Go) SEMVER:introduced=0,fixed=1.15.1-0.20260213190728-6fe4d295592e.
Is there a fix for CVE-2026-21388?
Yes. github.com/mattermost/mattermost-plugin-msteams (Go): upgrade to 1.15.1-0.20260213190728-6fe4d295592e or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
Am I affected by CVE-2026-21388?
Check whether you run github.com/mattermost/mattermost-plugin-msteams (Go) 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-msteams (Go): upgrade to 1.15.1-0.20260213190728-6fe4d295592e or later
    go get github.com/mattermost/mattermost-plugin-msteams@v1.15.1-0.20260213190728-6fe4d295592e

Affected products

  • github.com/mattermost/mattermost-plugin-msteams (Go) SEMVER:introduced=0,fixed=1.15.1-0.20260213190728-6fe4d295592e

Identifiers

  • CVE-2026-21388
  • GHSA-x274-8qfc-hrgf
  • GO-2026-5734

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