Mattermost has session spoofing due to lack of single-use consumption of guest magic link tokens enforcement

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

6.5 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N

EPSS

• CVE-2026-3590: 0.15% estimated exploitation activity in the next 30 days; percentile 4.33 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Mattermost versions 10.11.x <= 10.11.12, 11.5.x <= 11.5.0, 11.4.x <= 11.4.2, 11.3.x <= 11.3.2 fail to enforce atomic single-use consumption of guest magic link tokens, which allows an attacker with access to a valid magic link to establish multiple independent authenticated sessions via concurrent requests. Mattermost Advisory ID: MMSA-2026-00624.

Published by the source

17 Apr 2026, 15:31 UTC

Last modified by the source

28 Apr 2026, 22:57 UTC

Source last checked

26 Oct 2026, 15:19 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-mh4x-rmrx-3hp4

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# Patch Next advisory brief: CVE-2026-3590

Advisory: Mattermost has session spoofing due to lack of single-use consumption of guest magic link tokens enforcement
IDs: CVE-2026-3590, GHSA-mh4x-rmrx-3hp4
Record: https://patchnext.com/advisories/cve-2026-3590 (revision 1, source checked 26 Oct 2026, 15:19 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 6.5)
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 8.0.0-20250723052842-4cb8d8940332; github.com/mattermost/mattermost-server 11.5.0, 11.4.3, 11.3.3, 10.11.13. A vendor fix does not mean a deployment is fixed.

## Affected components
- Go github.com/mattermost/mattermost/server/v8: affected >= 8.0.0-20250721062209-4952acea88ce, < 8.0.0-20250723052842-4cb8d8940332; fixed in 8.0.0-20250723052842-4cb8d8940332
- Go github.com/mattermost/mattermost-server: affected >= 10.11.0-rc1, < 10.11.13; >= 11.5.0-rc1, <= 11.5.0-rc1; >= 11.4.0-rc1, < 11.4.3; >= 11.3.0-rc1, < 11.3.3; fixed in 11.4.3, 11.5.0, 11.3.3, 10.11.13

## Suggested inspection (read-only)
- Go github.com/mattermost/mattermost/server/v8: check the installed version with your package manager.
- Go github.com/mattermost/mattermost-server: 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, github.com/mattermost/mattermost-server.
- 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-mh4x-rmrx-3hp4 (GitHub-reviewed advisory GHSA-mh4x-rmrx-3hp4 lists affected ranges: Go github.com/mattermost/mattermost/server/v8 >=8.0.0-20250721062209-4952acea88ce =10.11.0-rc1 =11.5.0-rc1 =11.4.0-rc1 =11.3.0-rc1 <11.3.3)

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