What is CVE-2026-6850?

CVE-2026-6850 is a medium-severity vulnerability in Mattermost, classified under Inefficient Regular Expression Complexity. CVSS score: 6.5/10. Published 2026-07-13.

How severe is CVE-2026-6850?

Medium severity. CVSS v3 base score is 6.5 out of 10.

Vulnerability in Mattermost

CVE-2026-6850

Mattermost versions 11.7.x <= 11.7.2, 11.6.x <= 11.6.4, 10.11.x <= 10.11.19 fail to validate the length and content of message attachment field values, which allows an authenticated attacker to cause a denial of service for all users in a channel via a post containing a specially crafted payload that triggers catastrophic backtracking in the client-side markdown parser.. Mattermost Advisory ID: MMSA-2026-00658

Vulnerability class: ReDoS (Regular Expression Denial of Service)

Published 2026-07-13 · last modified 2026-07-13.

CVSS v3 metric

CVSS v3 base score 6.5 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H.

EPSS exploit prediction

EPSS: 0.004 (34.4th percentile), scored 2026-10-12.

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 34th percentile — 34.4% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.004 · newest: 0.004 · change: 0.000

Affected products

Mattermost — versions 11.7.0, 11.6.0, 10.11.0
Mattermost Server

Weakness classification (CWE)

CWE-1333 — Inefficient Regular Expression Complexity

References

mattermost.com/security-updates (vendor-advisory, Vendor Advisory)

Frequently asked questions

What is CVE-2026-6850?: CVE-2026-6850 is a medium-severity vulnerability in Mattermost, classified under Inefficient Regular Expression Complexity. CVSS score: 6.5/10. Published 2026-07-13.
How severe is CVE-2026-6850?: Medium severity. CVSS v3 base score is 6.5 out of 10.