CVE-2026-6342: Group prefix matching bypass for subscriptions

Severity: medium (CVSS 4.3 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.2% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-05-18 · updated 2026-10-14
Weakness: CWE-863

Description

Mattermost Plugins versions <=11.5 11.1.5 10.13.11 11.3.4.0 fail to appropriately check for valid namespaces which allows plugin users to create subscriptions to groups that were not whitelisted via creating groups that share the same prefix as a whitelisted group. Mattermost Advisory ID: MMSA-2026-00601

Affected products

Mattermost: from 10.13.0 through 10.13.11; from 11.1.0 through 11.1.5; from 11.3.0 through 11.3.4

References

https://mattermost.com/security-updates

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