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Critical, CVSS 9.3

Mattermost failed to properly validate that the channel that comes from the sync message is a shared channel

CVE-2024-39274GHSA-cmc8-222c-vqp9

What to do

Vendor fix available. Upgrade each affected package to the fixed version on its release line (github.com/mattermost/mattermost/server/v8 9.9.1, 9.8.2, 9.7.6, 9.5.7), redeploy, then verify what is actually installed.

This page has no inventory attached, so it cannot say whether you are affected. A vendor fix is not proof that your deployment is fixed.

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Affected packages

Sources

Severity and exploitation
Exploitation
No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.
CVSS 4.0
9.3 · CVSS:4.0/AV:N/AC:L/AT:N/PR:H/UI:N/VC:H/VI:H/VA:N/SC:H/SI:H/SA:N · score source
EPSS
  • CVE-2024-39274: 0.34% estimated exploitation activity in the next 30 days; percentile 27.67 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.
Source description

As published by the source; long descriptions are shortened.

Mattermost versions 9.9.x <= 9.9.0, 9.5.x <= 9.5.6, 9.7.x <= 9.7.5 and 9.8.x <= 9.8.1 fail to properly validate that the channel that comes from the sync message is a shared channel, when shared channels are enabled, which allows a malicious remote to add users to arbitrary teams and channels

Dates and revisions
Published by the source
Last modified by the source
Source last checked
  1. Revision 1 · created ·

    Published from ghsa record GHSA-cmc8-222c-vqp9

Agent brief

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# Patch Next advisory brief: CVE-2024-39274

Advisory: Mattermost failed to properly validate that the channel that comes from the sync message is a shared channel
IDs: CVE-2024-39274, GHSA-cmc8-222c-vqp9
Record: https://patchnext.com/advisories/cve-2024-39274 (revision 1, source checked 26 Oct 2026, 15:05 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Critical (CVSS 9.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: github.com/mattermost/mattermost/server/v8 9.9.1, 9.8.2, 9.7.6, 9.5.7. A vendor fix does not mean a deployment is fixed.

## Affected components
- Go github.com/mattermost/mattermost/server/v8: affected >= 9.5.0, < 9.5.7; >= 9.7.0, < 9.7.6; >= 9.8.0, < 9.8.2; >= 9.9.0, <= 9.9.0; fixed in 9.7.6, 9.9.1, 9.5.7, 9.8.2

## Suggested inspection (read-only)
- Go github.com/mattermost/mattermost/server/v8: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on github.com/mattermost/mattermost/server/v8.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-cmc8-222c-vqp9 (GitHub-reviewed advisory GHSA-cmc8-222c-vqp9 lists affected ranges: Go github.com/mattermost/mattermost/server/v8 >=9.5.0 =9.7.0 =9.8.0 =9.9.0 <=9.9.0)

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