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CVE-2026-6342 Mattermost CVE debrief

CVE-2026-6342 is a low-complexity authorization flaw in Mattermost Plugins that can let a plugin user create subscriptions to groups they were not supposed to access. The issue stems from insufficient validation of namespaces: if a user can create a group whose name shares a prefix with a whitelisted group, the plugin may treat it as valid. Mattermost’s advisory and the NVD record both tie this to a permission-check failure rather than code execution, so the main impact is unauthorized access/control over group subscriptions, not system compromise.

Vendor
Mattermost
Product
Unknown
CVSS
MEDIUM 4.3
CISA KEV
Not listed in stored evidence
Original CVE published
2026-05-18
Original CVE updated
2026-05-18
Advisory published
2026-05-18
Advisory updated
2026-05-18

Who should care

Mattermost administrators, plugin maintainers, and teams that rely on plugin-driven group subscription controls or namespace-based allowlists. Environments using the affected Mattermost Plugins releases should treat this as an integrity and access-control issue, especially where group names are user-controlled or can be created by less-trusted users.

Technical summary

The vulnerability is described as a missing/incorrect namespace validation check in Mattermost Plugins. According to the NVD description, affected versions fail to appropriately verify valid namespaces, enabling a user to create a group that shares a prefix with a whitelisted group and thereby subscribe to groups that were not whitelisted. NVD lists the weakness as CWE-863 (Incorrect Authorization) and provides a CVSS v3.1 vector of AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:N, which aligns with a network-reachable, low-privilege authorization bypass with limited integrity impact.

Defensive priority

Medium. The CVSS score is 4.3 and the impact is limited to integrity/authorization, but the flaw can undermine trust boundaries in plugin-managed group access. Prioritize if the affected plugin path is exposed to broader user populations or if group membership drives access to sensitive collaboration spaces.

Recommended defensive actions

  • Check whether your Mattermost deployment uses the affected Plugins releases referenced in the advisory and NVD record, and plan remediation promptly.
  • Apply the vendor-recommended update path from the Mattermost security updates advisory for CVE-2026-6342.
  • Review any plugin or custom logic that authorizes groups by prefix or partial-match rules; require exact namespace validation instead of prefix-based matching.
  • Audit existing group names and subscriptions for suspicious near-collision naming patterns that could have bypassed allowlists.
  • Restrict who can create groups or manage plugin subscriptions until patched, and monitor for unauthorized subscription changes.

Evidence notes

Source evidence is limited to the supplied NVD record and the Mattermost security updates reference. The NVD description states that Mattermost Plugins versions listed as affected fail to appropriately check valid namespaces, allowing subscriptions to non-whitelisted groups via prefix collisions. NVD also records CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:N and CWE-863 (Incorrect Authorization). The CVE publication and modification timestamps supplied are 2026-05-18T08:16:14.717Z and 2026-05-18T17:32:38.127Z, respectively.

Official resources

Publicly disclosed on 2026-05-18, with the supplied CVE and source record both showing the same publication timestamp and a same-day modification update.