CVE-2026-9816
Authenticated users can escalate to board admin in Mattermost via malformed API requests.
Is CVE-2026-9816 being exploited?
Not confirmed. CVE-2026-9816 does not appear in CISA's Known Exploited Vulnerabilities catalog, which records only exploitation that has been observed and reported publicly. That is evidence of absence of a report, not evidence the flaw is unattacked. EPSS currently estimates a 0.26% probability of exploitation in the next 30 days.
How severe is CVE-2026-9816?
CVE-2026-9816 is rated High with a CVSS score of 8.3. Severity describes how bad exploitation would be, not how likely it is: pair it with exploitation evidence before deciding what to patch first.
Is there a patch for CVE-2026-9816?
Yes. A fix has been recorded for CVE-2026-9816. The vendor advisory is the authority on the exact fixed version — apply it from there rather than from a summary.
What does CVE-2026-9816 affect?
CVE-2026-9816 affects Mattermost Server 11.7.x <= 11.7.6, Mattermost Server 11.8.x <= 11.8.3, Mattermost Server 10.11.x <= 10.11.21, API: POST /api/v2/boards/{boardID}/members, and 1 further product or version. Confirm the exact affected versions against the vendor advisory before deciding you are exposed.
What should I do about CVE-2026-9816?
Upgrade to vendor fixed versions (>11.7.6, >11.8.3, >10.11.21) and temporarily restrict board editor rights.
Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.
Authenticated board editors or non-guest team members can grant board admin rights via POST APIs, enabling board takeover and elevated actions.
Immediate action required
- affected>= 11.7.0 and <= 11.7.6
- affected>= 10.11.0 and <= 10.11.21
- affected>= 11.8.0 and <= 11.8.3
As published in the CVE Program record. A version outside these ranges is not a statement that it is unaffected — vendors sometimes understate a range, and distribution-backported builds carry upstream numbers that do not reflect what was patched into them.