CVE-2026-26233: Denial of Service via HTTP/2 single packet attack on login endpoint

Q: What is the severity of CVE-2026-26233?

CVE-2026-26233 is classified as a denial of service vulnerability.

Q: How do I fix CVE-2026-26233?

To fix CVE-2026-26233, upgrade Mattermost to versions 11.4.1 or later, 11.3.2 or later, 11.2.4 or later, or 10.11.12 or later.

Q: Which versions of Mattermost are affected by CVE-2026-26233?

Mattermost versions 11.4.0 and earlier, 11.3.1 and earlier, 11.2.3 and earlier, and 10.11.11 and earlier are affected.

Q: What type of attack is facilitated by CVE-2026-26233?

CVE-2026-26233 facilitates a denial of service attack via HTTP/2 single packet attacks on the login endpoint.

Q: Who is vulnerable to CVE-2026-26233?

Unauthenticated remote attackers are able to exploit CVE-2026-26233 to cause server crashes and restarts.

Published Mar 25, 2026

Updated

Mattermost versions 11.4.x <= 11.4.0, 11.3.x <= 11.3.1, 11.2.x <= 11.2.3, 10.11.x <= 10.11.11 fail to rate limit login requests which allows unauthenticated remote attackers to cause denial of service (server crash and restart) via HTTP/2 single packet attack with 100+ parallel login requests.. Mattermost Advisory ID: MMSA-2025-00566

Affected Software

5 affected components

Mattermost Mattermost<=11.4.0, <=11.3.1, <=11.2.3, <=10.11.11

Mattermost Mattermost Server>=10.11.0<10.11.12

Mattermost Mattermost Server>=11.2.0<11.2.4

Mattermost Mattermost Server>=11.3.0<11.3.2

Mattermost Mattermost Server>=11.4.0<11.4.1

Remediation

Information

Update Mattermost to versions 11.5.0, 11.4.1, 11.3.2, 11.2.4, 10.11.12 or higher.

Event History

Mar 25, 2026

CVE Published

via MITRE·04:24 PM

Data Sourced

via MITRE·04:24 PM

RemedyDescriptionSeverityWeakness

Data Sourced

via NVD·05:16 PM

DescriptionSeverityWeaknessAffected Software

Frequently Asked Questions

What is the severity of CVE-2026-26233?

CVE-2026-26233 is classified as a denial of service vulnerability.

How do I fix CVE-2026-26233?