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CVE-2026-6046 Mattermost CVE debrief

CVE-2026-6046 is a medium-severity vulnerability in Mattermost, a popular communication platform. The issue arises from the platform's failure to validate that a username returned during bot registration belongs to a bot account. This oversight allows an unprivileged attacker to intercept private messages sent by plugins via direct message channels by pre-registering a user account with a predictable plugin bot username.

Vendor
Mattermost
Product
Unknown
CVSS
MEDIUM 5.3
CISA KEV
Not listed in stored evidence
Original CVE published
2026-06-12
Original CVE updated
2026-06-18
Advisory published
2026-06-12
Advisory updated
2026-06-18

Who should care

Users of Mattermost, particularly those who have plugins installed that send private messages via direct message channels, should be aware of this vulnerability. Administrators of Mattermost instances should prioritize patching to prevent potential exploitation.

Technical summary

The vulnerability is caused by inadequate validation of usernames during bot registration. Specifically, Mattermost versions 11.6.x <= 11.6.1, 11.5.x <= 11.5.4, 10.11.x <= 10.11.15, and 10.11.x <= 10.11.16 are affected. An attacker can exploit this by pre-registering a user account with a predictable plugin bot username, allowing them to intercept private messages sent by plugins.

Defensive priority

Medium

Recommended defensive actions

  • Apply patches or updates provided by Mattermost to address the vulnerability.
  • Review and update Mattermost instances to ensure they are running a version that has addressed this issue.
  • Monitor for any suspicious activity related to bot registrations and private message channels.

Evidence notes

The CVE-2026-6046 vulnerability has been documented by Mattermost and is tracked under Mattermost Advisory ID: MMSA-2026-00649. For more information, refer to the official CVE record [cve-org] and the NVD detail page [nvd].

Sources and references

Verified primary and authoritative sources

  • CVE-2026-6046 CVE Program record

    Publisher, destination, and source semantics verified

    URL: https://www.cve.org/CVERecord?id=CVE-2026-6046

    CVE Program - Official CVE Program record with source-provided CVE metadata.

  • CVE-2026-6046 NVD vulnerability detail

    Publisher, destination, and source semantics verified

    URL: https://nvd.nist.gov/vuln/detail/CVE-2026-6046

    NIST National Vulnerability Database - Official NIST NVD detail page and source-specific vulnerability assessment.

Supplemental references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.