CVE-2026-12882 Mattermost Markdown autolink parsing denial of service
An authenticated user can cause server CPU exhaustion and degrade availability by sending crafted Markdown autolinks with unmatched parentheses.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-12882?
CVE-2026-12882 is a medium-severity denial of service vulnerability in Mattermost Mattermost Server, published on 14 Sep 2026. Any signed-in user can exploit it over the network, with no action from the victim.
Official description
Mattermost versions 11.9.x <= 11.9.0, 11.8.x <= 11.8.4, 11.7.x <= 11.7.7, 10.11.x <= 10.11.22 fail to parse Markdown autolinks with unmatched trailing closing parentheses in linear time, which allows an authenticated user with permission to create posts to cause excessive server CPU consumption and degrade availability for other users via specially crafted post or message attachment content. Mattermost Advisory ID: MMSA-2026-00703
Weakness: CWE-407
Which versions are affected by CVE-2026-12882?
It affects 1 product.
Products include Mattermost Mattermost Server.
How do I fix CVE-2026-12882?
Upgrade to a fixed version.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-12882 being exploited?
There is no known exploitation of CVE-2026-12882 in the wild and no public exploit code.
Other vulnerabilities in Mattermost Mattermost Server
- CVE-2025-12419 Account takeover on OAuth/OpenID-enabled servers
- CVE-2025-12421 Account Takeover via Code Exchange Endpoint
- CVE-2025-4981 Path Traversal Leading to RCE by Any Authenticated Mattermost User
- CVE-2025-20051 Arbitrary file read via block duplication in Mattermost Boards
- CVE-2025-25279 Arbitrary file read in Mattermost Boards via import & export board archive
Sources
Also known as GHSA-mfv9-qx66-gxm4, MINI-j373-p25g-mx8m, MINI-w4pm-g8jp-7q39.