Quick answers
- Is CVE-2026-18223 exploited in the wild?
- No. Junglewise Threat Intelligence has no report of CVE-2026-18223 being exploited in the wild.
- How severe is CVE-2026-18223?
- Medium severity, with a CVSS score of 5, published 16 October 2026.
- Which products and versions are affected?
- Mattermost Server 11.10.x up to 11.10.0, 11.7.x up to 11.7.9, 11.9.x up to 11.9.1, 11.8.x up to 11.8.5.
- Is there a fix for CVE-2026-18223?
- 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-2026-18223 attacked?
- It is attacked over the network, in a web application.
- Am I affected by CVE-2026-18223?
- 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
Mattermost fails to validate the recipient's email address when redeeming team and guest invitation tokens. An authenticated attacker can exploit this by reusing or forwarding invitation links to join teams or gain access to private channels they were not intended to access. No user interaction beyond existing authentication is required.
Affected products
- Mattermost Mattermost Server 11.10.x up to 11.10.0, 11.7.x up to 11.7.9, 11.9.x up to 11.9.1, 11.8.x up to 11.8.5
Timeline
- 2026-10-16: disclosed