# CVE-2026-9816 — Mattermost

> As of 2026-08-18, CVE-2026-9816 is a HIGH-severity vulnerability in Mattermost, CVSS v3.1 8.3, EPSS 0.2% (15.9th percentile). No Threadlinqs-tracked threat campaign has been attributed to CVE-2026-9816 as of 2026-08-18; the identifier is re-checked against the Threadlinqs threat corpus on every daily ingest.

**Last updated:** 2026-08-18

## What is CVE-2026-9816?

Mattermost versions 11.7.x <= 11.7.6, 10.11.x <= 10.11.21, 11.8.x <= 11.8.3 fail to validate BoardMember.Scheme* fields server-side on insert and archive-import paths which allows a board editor or non-guest team member to grant board admin to arbitrary users via POST /api/v2/boards/{boardID}/members and POST /api/v2/teams/{teamID}/archive/import.. Mattermost Advisory ID: MMSA-2026-00685

The record classifies CVE-2026-9816 under weakness class [CWE-863](https://intel.threadlinqs.com/cwe/CWE-863). Its CVSS v3 base vector states that the flaw is reachable remotely over the network, needs low-privilege credentials, needs no user interaction, and has high impact on integrity, availability. 1 affected-product entry is recorded, across 1 vendor, listed below. The identifier was first published 49 days ago.

## Severity and exploitation probability

- **CVSS v3.1 base score:** 8.3 — HIGH (`CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:H/A:H`)
- **EPSS (FIRST):** 0.2% probability of exploitation in the next 30 days, higher than 15.9% of all scored CVEs
- **CISA KEV:** Not listed in the CISA Known Exploited Vulnerabilities catalog
- **Threadlinqs priority:** 4.8/10 — a Threadlinqs composite of the CVSS base score, the EPSS percentile and public exploit availability
- **Published:** 2026-08-17, last modified 2026-08-18

## Is CVE-2026-9816 being exploited?

It currently carries a trending score of 34 in the Threadlinqs vulnerability feed.

## Affected products and versions

- [Mattermost](https://intel.threadlinqs.com/vendors/mattermost): Mattermost

## How to fix CVE-2026-9816

No vendor patch reference has been recorded for CVE-2026-9816 in the tracked sources. Follow the references below for a fix, and treat the products listed above as exposed until the vendor states otherwise.

## Threat activity tracking CVE-2026-9816

No threat campaign in the Threadlinqs corpus currently references CVE-2026-9816, in its CVE list or as an indicator. The linkage is recomputed from the live corpus on every daily ingest, so this section fills in as soon as a tracked campaign cites the identifier.

## Sources

Enriched from CVE.org, NVD, FIRST EPSS, GitHub Security Advisories. Last verified by Threadlinqs on 2026-08-18. This product uses the NVD API but is not endorsed or certified by the NVD.

**Vendor advisory and patch**

- [mattermost.com — vendor advisory](https://mattermost.com/security-updates)

Canonical: https://intel.threadlinqs.com/cve/CVE-2026-9816
Full detection coverage and IOCs for threats exploiting CVE-2026-9816 via the Threadlinqs MCP server (Purple tier): https://intel.threadlinqs.com/mcp