Quick answers
- Is CVE-2026-19867 exploited in the wild?
- No. Junglewise Threat Intelligence has no report of CVE-2026-19867 being exploited in the wild.
- How severe is CVE-2026-19867?
- Medium severity, with a CVSS score of 4.3, published 16 October 2026.
- Which products and versions are affected?
- Mattermost Server 11.10.x through 11.10.0, 11.9.x through 11.9.1, 11.8.x through 11.8.5, 11.7.x through 11.7.10.
- Is there a fix for CVE-2026-19867?
- 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-2026-19867 attacked?
- It is attacked over the network, in a web application.
- Am I affected by CVE-2026-19867?
- 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 versions 11.10.0 and earlier branches because the application fails to enforce limits on the number and size of mention keys stored in user notification properties (notify_props.mention_keys), and does not bound the quadratic complexity of multibyte-keyword matching performed during post processing. An authenticated attacker stores an oversized mention_keys list on their own profile, and when any post is made in a shared channel, the unoptimized matching logic consumes excessive CPU for all users. No network elevation is required; the attacker only needs valid channel membership.
Affected products
- Mattermost Mattermost Server 11.10.x through 11.10.0, 11.9.x through 11.9.1, 11.8.x through 11.8.5, 11.7.x through 11.7.10
Timeline
- 2026-10-16: disclosed