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MEDIUM severity

CVE-2026-4055 — v8

MEDIUM

CVE-2026-4055 is a medium-severity (CVSS 4.3) CWE-863 vulnerability in github.com/mattermost/mattermost/server/v8. A fix is available for github.com/mattermost/mattermost/server/v8 — see the affected versions and patch details below.

Insufficient permission validation on cross-team playbook run creation

Also known asGHSA-6cfr-wp44-6qmvGO-2026-5173
Published
May 21, 2026
Updated
Aug 12, 2026
Affected
1 pkg
Patched
1 / 1
Exploits
None indexed
Exploitation data as of Oct 2, 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-2026-4055.

EPSS Exploitation Probability

via FIRST.org ↗
0.3%probability of exploitation in next 30 days
Lower Risk0.00%
Lower risk than most CVEs15th percentile — riskier than 15% of all scored CVEsHighest risk
0.00%0.25%0.50%0.75%0.0%0.3%Jun 26Sep 26Oct 26

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

How urgent is this, really

CVE-2026-4055 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 382,205 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.

Real-World Exposure

1 pkg affected
🐹github.com/mattermost/mattermost/server/v8

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 versions 11.5.x <= 11.5.1 fail to validate team-level run_create permission against the target team when creating a playbook run which allows an authenticated team member to create runs in teams where they lack permission via specifying a different team ID in the run creation API request. Mattermost Advisory ID: MMSA-2026-00629

Affected Packages

1 total 1 fixed
EcosystemPackageVulnerable rangeFix
🐹Gogithub.com/mattermost/mattermost/server/v8≥ 8.0.0-20260304132957-9f2616376582&&< 8.0.0-20260320113102-f2b3d1c6a9458.0.0-20260320113102-f2b3d1c6a945go get github.com/mattermost/mattermost/server/v8@v8.0.0-20260320113102-f2b3d1c6a945

Affected Products

1 product · 3 configurations
Application
mattermost servermattermost
≥ 11.6.0 && < 11.6.2
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/server/v8, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

  2. Fix

    Update github.com/mattermost/mattermost/server/v8 to 8.0.0-20260320113102-f2b3d1c6a945 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-4055 is resolved across your whole dependency graph.

  3. Workarounds

    Put an independent control in front of the weakness: restrict the affected endpoint or interface to trusted networks, require an additional authentication factor or proxy-level check, and invalidate existing sessions and credentials in case the flaw has already been used.

Frequently Asked Questions

Mattermost versions 11.5.x <= 11.5.1 fail to validate team-level run_create permission against the target team when creating a playbook run which allows an authenticated team member to create runs in teams where they lack permission via specifying a different team ID in the run creation API request. Mattermost Advisory ID: MMSA-2026-00629
O3 Security · Impact-Aware SCA

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CVE-2026-4055: v8 — Fix Available | O3 Security