Quick answers
- Is CVE-2026-86349 exploited in the wild?
- No. Junglewise Threat Intelligence has no report of CVE-2026-86349 being exploited in the wild.
- How severe is CVE-2026-86349?
- Medium severity, with a CVSS score of 4.3, published 14 September 2026.
- Which products and versions are affected?
- Mattermost Server 11.9.x <= 11.9.0, 11.8.x <= 11.8.4, 11.7.x <= 11.7.8, 10.11.x <= 10.11.22.
- Is there a fix for CVE-2026-86349?
- No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.
- How is CVE-2026-86349 attacked?
- It is attacked over the network.
- Am I affected by CVE-2026-86349?
- Check whether you run Mattermost Server at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.
Technical details
The vulnerability exists in the server-side Markdown parser, which fails to enforce a limit on nesting depth for Markdown constructs such as blockquotes and list items. An authenticated attacker can craft a malicious post containing deeply nested structures that cause excessive computational work during parsing, leading to CPU resource exhaustion and denial of service. The attack requires authentication but no additional preconditions beyond the ability to post messages. An attacker can achieve temporary service degradation or unavailability. Patches are available in fixed versions: 11.9.1+, 11.8.5+, 11.7.9+, and 10.11.23+.
Affected products
- Mattermost Mattermost Server 11.9.x <= 11.9.0, 11.8.x <= 11.8.4, 11.7.x <= 11.7.8, 10.11.x <= 10.11.22
Timeline
- 2026-09-14: disclosed