Is CVE-2026-4055 exploited in the wild?

No. Junglewise Threat Intelligence has no report of CVE-2026-4055 being exploited in the wild.

How severe is CVE-2026-4055?

Medium severity, with a CVSS score of 4.3, published 21 May 2026.

Which products and versions are affected?

github.com/mattermost/mattermost-server (Go) SEMVER:introduced=0; 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) >= 8.0.0-20260304132957-9f2616376582, < 8.0.0-20260320113102-f2b3d1c6a945, SEMVER:introduced=8.0.0-20260304132957-9f2616376582,fixed=8.0.0-20260320113102-f2b3d1c6a945; and 1 more.

Is there a fix for CVE-2026-4055?

Yes. github.com/mattermost/mattermost/server/v8 (Go): upgrade to 8.0.0-20260320113102-f2b3d1c6a945 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

How is CVE-2026-4055 attacked?

It is attacked over the network, in a hosted service.

Am I affected by CVE-2026-4055?

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.

CVE-2026-4055: Mattermost incorrect authorization in Playbook run creation

Q: Is there a fix for CVE-2026-4055?

Yes. github.com/mattermost/mattermost/server/v8 (Go): upgrade to 8.0.0-20260320113102-f2b3d1c6a945 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

Q: How is CVE-2026-4055 attacked?

It is attacked over the network, in a hosted service.

Q: Am I affected by CVE-2026-4055?

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.

CVE-2026-4055 · Severity: medium · CVSS 4.3 · Published 2026-05-21

Mattermost, a collaboration platform for teams, contains a flaw in its Playbooks feature. An authorized user could bypass security restrictions to create playbook runs in teams they do not belong to. This could lead to unauthorized activity or organizational disruption within teams where the user should not have access.

Technologies: github.com/mattermost/mattermost-server (Go), github.com/mattermost/mattermost-server/v5 (Go), github.com/mattermost/mattermost-server/v6 (Go), github.com/mattermost/mattermost/server/v8 (Go), Mattermost Server. Vendors: Go, Mattermost.

Quick answers

Is CVE-2026-4055 exploited in the wild?: No. Junglewise Threat Intelligence has no report of CVE-2026-4055 being exploited in the wild.
How severe is CVE-2026-4055?: Medium severity, with a CVSS score of 4.3, published 21 May 2026.
Which products and versions are affected?: github.com/mattermost/mattermost-server (Go) SEMVER:introduced=0; 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) >= 8.0.0-20260304132957-9f2616376582, < 8.0.0-20260320113102-f2b3d1c6a945, SEMVER:introduced=8.0.0-20260304132957-9f2616376582,fixed=8.0.0-20260320113102-f2b3d1c6a945; and 1 more.
Is there a fix for CVE-2026-4055?: Yes. github.com/mattermost/mattermost/server/v8 (Go): upgrade to 8.0.0-20260320113102-f2b3d1c6a945 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-4055 attacked?: It is attacked over the network, in a hosted service.
Am I affected by CVE-2026-4055?: 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/v8 (Go): upgrade to 8.0.0-20260320113102-f2b3d1c6a945 or later
```
go get github.com/mattermost/mattermost/server/v8@v8.0.0-20260320113102-f2b3d1c6a945
```

Technical details

An incorrect authorization vulnerability (CWE-863) exists in Mattermost Server versions 11.5.x <= 11.5.1. The application fails to validate the 'run_create' permission against the specific target team during a playbook run creation request. An authenticated attacker can exploit this by manually specifying a different team ID in the API request, allowing them to initiate playbook runs in teams where they lack the necessary privileges. The issue is addressed in versions 11.5.2, 11.6.0, and 10.11.14.

Affected products

Mattermost Mattermost Server 11.5.0 - 11.5.1

Identifiers

CVE-2026-4055
GHSA-6cfr-wp44-6qmv
GO-2026-5173

Timeline

2026-05-21: disclosed
2026-05-21: advisory

References

https://mattermost.com/security-updates

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