Quick answers
- Is CVE-2026-95666 exploited in the wild?
- No. Junglewise Threat Intelligence has no report of CVE-2026-95666 being exploited in the wild.
- How severe is CVE-2026-95666?
- Medium severity, with a CVSS score of 4.3, published 22 September 2026.
- Which products and versions are affected?
- Mattermost Server 11.7.x <= 11.7.10, 11.8.x <= 11.8.5, 11.9.x <= 11.9.1, 11.10.x <= 11.10.1.
- Is there a fix for CVE-2026-95666?
- 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-2026-95666 attacked?
- It is attacked over the network, in a hosted service.
- Am I affected by CVE-2026-95666?
- 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 POST /api/v4/posts/ids/reactions endpoint in affected Mattermost versions fails to validate the length of the post ID array parameter, allowing authenticated attackers to submit unbounded arrays that consume excessive database resources. This is a denial-of-service vulnerability requiring valid authentication and network access to the Mattermost API. The vulnerability impacts versions 11.7.x through 11.10.x within specified patch levels.
Affected products
- Mattermost Mattermost Server 11.7.x <= 11.7.10, 11.8.x <= 11.8.5, 11.9.x <= 11.9.1, 11.10.x <= 11.10.1
Timeline
- 2026-09-22: disclosed: Security advisory published