Medium severity · published Mar 26, 2026
CVE-2026-4274
Insufficient authorization in shared channel membership sync grants team-level access instead of channel-level access
5.4
CVSS
0.1%
EPSS
No
On KEV
Description
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 restrict team-level access when processing membership sync from a remote cluster, which allows a malicious remote cluster to grant a user access to an entire private team instead of only the shared channel via sending crafted membership sync messages that trigger team membership assignment. Mattermost Advisory ID: MMSA-2026-00574
CVSS vector
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:N
Attack vector
Network
Complexity
Low
Privileges
Low
User interaction
None
Confidentiality
Low
Integrity
Low
Availability
None
Scope
Unchanged
Exploitation
Is it being used?
Exploit prediction (EPSS)0.1% · higher than 4% of CVEs
Exploited in the wildNo evidence yet
CISA KEVNot listed
Ransomware campaign useNot known
Affected products
Which versions
MA
Mattermost · Mattermost
11.2.0 through 11.2.2; 10.11.0 through 10.11.10; 11.4.0 through 11.4.0; 11.3.0 through 11.3.1 · source: vendor
MS
Mattermost Server · Mattermost
10.11.0 before 10.11.11; 11.2.0 before 11.2.3; 11.3.0 before 11.3.2; 11.4.0 before 11.4.1 · source: NVD
References
NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-4274
CVE.org
https://www.cve.org/CVERecord?id=CVE-2026-4274
Vendor Advisory
https://mattermost.com/security-updates
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