CVE-2026-12882: Mattermost Markdown autolink parsing denial of service
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
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
Mattermostto a version that resolves this vulnerability.Fixed in 11.10.0 - Upgrade
Upgrade
Mattermostto a version that resolves this vulnerability.Fixed in 11.9.1 - Upgrade
Upgrade
Mattermostto a version that resolves this vulnerability.Fixed in 11.8.5 - Upgrade
Upgrade
Mattermostto a version that resolves this vulnerability.Fixed in 11.7.8 - Upgrade
Upgrade
Mattermostto a version that resolves this vulnerability.Fixed in 10.11.23 - Compensating control
While upgrading, restrict access so that only trusted/authenticated users who need to create posts can post or attach specially crafted message content that could trigger the Markdown autolink parsing DoS.
Event History
Frequently Asked Questions
Who can exploit this issue?
An authenticated Mattermost user who has permission to create posts can exploit it. The crafted content can be placed in a post or message attachment.
What configurations and versions are affected?
Affected releases are Mattermost 11.9.0 and earlier in the 11.9.x line, 11.8.4 and earlier in 11.8.x, 11.7.7 and earlier in 11.7.x, and 10.11.22 and earlier in 10.11.x. The issue is triggered during Markdown autolink parsing of content containing unmatched trailing closing parentheses.
What is the practical impact?
Specially crafted content can cause excessive server CPU consumption, degrading Mattermost availability for other users. The provided severity vector indicates no confidentiality or integrity impact.