CVE-2026-28759 json
Insufficient authorization in shared channel membership sync allows remote cluster to remove users from arbitrary channels
published 2026-05-18 5 mo ago modified 2026-10-14 25 d ago also GHSA-8h9w-w78c-vvr3 , GO-2026-5256 , EUVD-2026-30738
- cvss
- 4.3
- version 3.1, CNA
- cisa kev
- Not listed
- not in CISA's catalog
- epss
- 0.2%
- EPSS 0.2% chance of exploitation in 30 days, percentile 4%
- public exploits
- 0
- none known
Summary
Mattermost versions 11.5.x <= 11.5.1, 10.11.x <= 10.11.13, 11.4.x <= 11.4.3 fail to validate that a remote cluster has access to a channel before processing membership removal requests during shared channel membership sync, which allows a malicious remote cluster to remove any user from any channel, including private channels, via crafted membership sync messages targeting channels the remote cluster is not authorized to access. Mattermost Advisory ID: MMSA-2026-00576
Patch / fix
- go github.com/mattermost/mattermost/server/v8 8.0.0-20260216150504-8738f8c4b3d4, 10.11.14, 11.4.4, 11.5.2
- go github.com/mattermost/mattermost-server 5.3.2-0.20260216150504-8738f8c4b3d4, 10.11.14+incompatible, 11.4.4+incompatible, 11.5.2+incompatible
- Mattermost Mattermost 10.11.14, 11.4.4, 11.5.2, 11.6.0
- govulncheck-vulndb openSUSE Leap 16.0 0.0.20260723T184607-160000.1.1
opensuse advisories openSUSE-SU-2026:21483-1
CISA SSVC
- Exploitation
- none
- Automatable
- no
- Technical impact
- partial
- Assessed
- 5 mo ago
Exploit prediction
0.15% chance of exploitation in the next 30 days, higher than 4% of all scored CVEs (FIRST EPSS, 2026-11-08).
CVSS scores
| assigned by | version | score | vector |
|---|---|---|---|
| CNA | 3.1 | 4.3 | CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:N |
| GitHub | 3.1 | 4.3 | CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:N |
Affected packages
| ecosystem | package | affected | fixed in |
|---|---|---|---|
| go | github.com/mattermost/mattermost/server/v8 | >=11.5.0, <11.5.2 >=10.11.0, <10.11.14 >=11.4.0, <11.4.4 <8.0.0-20260216150504-8738f8c4b3d4 |
11.5.2 10.11.14 11.4.4 8.0.0-20260216150504-8738f8c4b3d4 |
| go | github.com/mattermost/mattermost-server | <5.3.2-0.20260216150504-8738f8c4b3d4 >=10.11.0+incompatible, <10.11.14+incompatible >=11.4.0+incompatible, <11.4.4+incompatible >=11.5.0+incompatible, <11.5.2+incompatible |
5.3.2-0.20260216150504-8738f8c4b3d4 10.11.14+incompatible 11.4.4+incompatible 11.5.2+incompatible |
| go | github.com/mattermost/mattermost-server/v5 | all versions |
no fix listed |
| go | github.com/mattermost/mattermost-server/v6 | all versions |
no fix listed |
Affected products
- Mattermost Mattermost
Classification
- Weakness
-
- CWE-863 Incorrect Authorization
References
Sources
- CVE List CVE-2026-28759
- GitHub Advisory GHSA-8h9w-w78c-vvr3
- NVD CVE-2026-28759
- OSV GO-2026-5256
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