Vulnerability · Published Jul 13, 2026 · Assigned by Mattermost

CVE-2026-6850Crafted message attachment causes client-side denial of service via markdown parser regex backtracking in Mattermost

Medium 6.5

In plain English

A medium-severity regular-expression denial of service in Mattermost from Mattermost. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.4% (higher than 37% of all known vulnerabilities).

Severity (CVSS 3.1)6.5Medium

Exploit odds (30 days)0.4%top 63.5% of all CVEs

Exploited in the wildNo reportsCISA SSVC: none

Attack toolingNoneno public tooling known

Official description

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

Affected products (as reported)

Vendor	Product	Versions
Mattermost	Mattermost	11.7.0 – through 11.7.211.6.0 – through 11.6.410.11.0 – through 10.11.19

Weakness: CWE-1333 · Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

References

MMSA-2026-00658 vendor-advisory