CVE-2026-6850CWE-1333

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

Medium · published July 13, 2026

CVSS v3.1
6.5
EPSS
0%
Percentile
35.2
In the wild
Unconfirmed
What it is

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

The record
Technical detail
CVSS v3.1
6.5 · MEDIUM
Vector
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
CVSS v4.0
Not supplied
EPSS
0.00420 · 35.2th percentile
Weakness
CWE-1333 · Inefficient Regular Expression Complexity
Published
2026-07-13T08:13Z
EPSS history
Timeline
  • 13 JUL 08:13Z
    Crafted message attachment causes client-side denial of service via markdown parser regex backtracking in Mattermost
    cvelistv5