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🛡️ CVE-2026-27659 — mattermost-server

🟡 CVSS 4.6 — Medium ✅ No Known Exploit NVD
4.6
CVSS Score
0 Low4 Medium7 High9 Critical10

Description

Mattermost doesn't properly validate CSRF tokens

Mattermost versions 11.2.x <= 11.2.2, 10.11.x <= 10.11.10, 11.4.x <= 11.4.0, 11.3.x <= 11.3.1 fail to properly validate CSRF tokens in the /api/v4/access_control_policies/{policy_id}/activate endpoint, which allows an attacker to trick an admin into changing access control policy active status via a crafted request. Mattermost Advisory ID: MMSA-2026-00578

How this vulnerability can be exploited

This issue can be reached over the network, attack complexity is low, an attacker needs low-level privileges on the target. A user must be tricked into taking some action. The scope is unchanged, so the impact stays within the vulnerable component. Rated impact: confidentiality low, integrity low, availability none.

CVSS metrics in full

The score comes from this vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:L/I:L/A:N

  • Attack vector: Network — reachable from anywhere that can route to the service.
  • Attack complexity: Low — the attack works reliably, with no preparation.
  • Privileges required: Low — an ordinary user account is enough.
  • User interaction: Required — someone has to click, open or visit something.
  • Scope: Unchanged — the damage stays inside the vulnerable component.
  • Confidentiality impact: Low — limited, and the attacker does not choose what is affected.
  • Integrity impact: Low — limited, and the attacker does not choose what is affected.
  • Availability impact: None.

Affected software

CVE-2026-27659 is recorded against 5 packages.

  • github.com/mattermost/mattermost-server
  • github.com/mattermost/mattermost-server/v5
  • github.com/mattermost/mattermost-server/v6
  • github.com/mattermost/mattermost/server/v8
  • mattermost-server (from 11.4.0 up to 11.4.1)

Timeline and source

Published on 25 June 2026. No public exploit is currently recorded for this entry. Record sourced from NVD.

References

github.com (Advisory)
nvd.nist.gov (Advisory)
mattermost.com (Web)

Other advisories for this package

github.com/mattermost/mattermost-server has other advisories on record. If you are patching this one, these are worth checking on the same host:

Details

Severity Medium
CVSS Score 4.6
CVSS Vector CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:L/I:L/A:N
CWE N/A
Public Exploit ✅ No
Source NVD
Published 2026-06-25
Updated 2026-08-20
Modified 2026-06-25
Fix URL N/A

Affected Packages

Software From version Fixed in
github.com/mattermost/mattermost-server — —
github.com/mattermost/mattermost-server/v5 — —
github.com/mattermost/mattermost-server/v6 — —
github.com/mattermost/mattermost/server/v8 — —
mattermost-server 11.4.0 11.4.1

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ⓘ Data Notice: The information presented above has been compiled from publicly available internet sources. Boteraser aggregates this data solely for informational purposes and does not independently classify, evaluate, or endorse any findings about the vulnerabilities listed. The accuracy and completeness of this information is the sole responsibility of the original publishers. Boteraser and its operators accept no liability for any decisions made based on this data.

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