Vulnerability · Published Sep 14, 2026 · Assigned by Mattermost

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

Medium 4.3

In plain English

A medium-severity security flaw 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 31% of all known vulnerabilities).

Severity (CVSS 3.1)4.3Medium

Exploit odds (30 days)0.4%top 69.3% of all CVEs

Exploited in the wildNo reportsCISA SSVC: none

Attack toolingNoneno public tooling known

Official 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 (as reported)

Vendor	Product	Versions
Mattermost	Mattermost	11.9.0 – through 11.9.011.8.0 – through 11.8.411.7.0 – through 11.7.810.11.0 – through 10.11.22

Weakness: CWE-407 · Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L

References

MMSA-2026-00707 vendor-advisory