# CVE-2026-9859 — Mattermost

> As of 2026-08-18, CVE-2026-9859 is a MEDIUM-severity vulnerability in Mattermost, CVSS v3.1 6.5, EPSS 0.2% (16.9th percentile). No Threadlinqs-tracked threat campaign has been attributed to CVE-2026-9859 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-2026-9859?

Mattermost versions 11.7.x <= 11.7.6, 10.11.x <= 10.11.21, 11.8.x <= 11.8.3 fail to enforce PermissionManageBoardRoles on the channelId field of the batch endpoint, which allows an authenticated board editor to relink any board they can edit to an arbitrary channel via a crafted PATCH request. Mattermost Advisory ID: MMSA-2026-00686

The record classifies CVE-2026-9859 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 confidentiality. 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:** 6.5 — MEDIUM (`CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N`)
- **EPSS (FIRST):** 0.2% probability of exploitation in the next 30 days, higher than 16.9% of all scored CVEs
- **CISA KEV:** Not listed in the CISA Known Exploited Vulnerabilities catalog
- **Threadlinqs priority:** 3.9/10 — 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-2026-9859 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-2026-9859

No vendor patch reference has been recorded for CVE-2026-9859 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-2026-9859

No threat campaign in the Threadlinqs corpus currently references CVE-2026-9859, 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-2026-9859
Full detection coverage and IOCs for threats exploiting CVE-2026-9859 via the Threadlinqs MCP server (Purple tier): https://intel.threadlinqs.com/mcp