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LOW severity

CVE-2026-21388 — mattermost-plugin-msteams

LOWFix: mattermost/mattermost-plugin-msteams@6fe4d29

CVE-2026-21388 is a low-severity (CVSS 3.7) CWE-770 vulnerability in github.com/mattermost/mattermost-plugin-msteams. A fix is available for github.com/mattermost/mattermost-plugin-msteams — see the affected versions and patch details below.

Mattermost MS Teams plugin doesn't limit the request body size on the /lifecycle webhook endpoint

Also known asGO-2026-5734
Published
Updated
Affected
1 pkg
Patched
1 / 1
Exploits
None indexed
Exploitation data as of Nov 3, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

No confirmed exploitation observed yet

  • CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.

Exploitation and automatability from CISA’s SSVC triage for CVE-2026-21388.

EPSS Exploitation Probability

via FIRST.org ↗
0.3%probability of exploitation in next 30 days
Lower Risk0.00%
Lower risk than most CVEs24th percentile — riskier than 24% of all scored CVEsHighest risk
0.00%0.27%0.54%0.81%0.0%0.3%May 26Sep 26Nov 26

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

Where this sits among everything scored

Real-World Exposure

1 pkg affected
🐹github.com/mattermost/mattermost-plugin-msteams

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Go packages — download data is not available via public APIs for these ecosystems.

Description

Mattermost Plugins versions <=2.3.1 fail to limit the request body size on the {{/lifecycle}} webhook endpoint which allows an authenticated attacker to cause memory exhaustion and denial of service via sending an oversized JSON payload. Mattermost Advisory ID: MMSA-2026-00610.

Affected Packages

1 total 1 fixed
EcosystemPackageVulnerable rangeFix
🐹Gogithub.com/mattermost/mattermost-plugin-msteamsall versions1.15.1-0.20260213190728-6fe4d295592ego get github.com/mattermost/mattermost-plugin-msteams@v1.15.1-0.20260213190728-6fe4d295592e

Affected Products

1 product · 1 configurations
Application
mattermost servermattermost
≤ 2.3.1
range

Detection & mitigation playbook

Open-source dependency

  1. Detect

    Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/mattermost/mattermost-plugin-msteams, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

  2. Fix

    Update github.com/mattermost/mattermost-plugin-msteams to 1.15.1-0.20260213190728-6fe4d295592e or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-21388 is resolved across your whole dependency graph.

  3. Workarounds

    Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.

Frequently Asked Questions

Mattermost Plugins versions <=2.3.1 fail to limit the request body size on the {{/lifecycle}} webhook endpoint which allows an authenticated attacker to cause memory exhaustion and denial of service via sending an oversized JSON payload. Mattermost Advisory ID: MMSA-2026-00610.
O3 Security · Impact-Aware SCA

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CVE-2026-21388: mattermost-plugin DoS — Fix Available