CVE-2026-86349: Mattermost Server Algorithmic DoS via Unbounded Markdown Block Nesting

Severity: medium (CVSS 4.3 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.4% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-09-14 · updated 2026-10-07
Weakness: CWE-407

Description

Mattermost versions 11.9.x <= 11.9.0, 11.8.x <= 11.8.4, 11.7.x <= 11.7.8, 10.11.x <= 10.11.22 fail to limit the nesting depth in the server-side Markdown parser which allows an authenticated attacker to cause a denial of service (CPU resource exhaustion) via a crafted post containing deeply nested blockquotes or list items.. Mattermost Advisory ID: MMSA-2026-00707

Affected products

Mattermost: from 10.11.0 before 10.11.23; from 11.7.0 before 11.7.9; from 11.8.0 before 11.8.5; from 11.9.0 before 11.9.1 — fixed in 10.11.23, 11.7.9, 11.8.5, 11.9.1

How to fix

Upgrade to a fixed version: Mattermost 10.11.23, 11.7.9, 11.8.5, 11.9.1. Check the vendor advisory for mitigations.

References

https://mattermost.com/security-updates

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