# CVE-2026-19603: Mattermost information disclosure in user status API

Source: https://junglewise.ai/threats/cve-2026-19603-mattermost-versions-11-9-x-11-9-1-11-8-x-11-8-5-11-7-x-11-7-10-11  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-10-16. Updated: 2026-10-16  
License: CC BY 4.0 with a link back

CVE-2026-19603 · Severity: medium · CVSS 4.3 · Published 2026-10-16

Mattermost is a team collaboration platform similar to Slack. An authenticated user can learn the private channel IDs that other users are viewing through the status API, even if they don't have access to those channels. This leaks information about which private conversations other team members are having.

Technologies: [Mattermost Server](https://junglewise.ai/threats/technologies/server-public). Vendors: [Mattermost](https://junglewise.ai/threats/vendors/mattermost).

## Quick answers

**Is CVE-2026-19603 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-19603 being exploited in the wild.

**How severe is CVE-2026-19603?** Medium severity, with a CVSS score of 4.3, published 16 October 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-2026-19603?** Yes: fixed in Mattermost Server 11.7.11, 11.8.6, 11.9.2, 11.10.2. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-19603 attacked?** It is attacked over the network, in a hosted service.

**Am I affected by CVE-2026-19603?** 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.

## How to fix

- Fixed in Mattermost Server 11.7.11, 11.8.6, 11.9.2, 11.10.2

## Technical details

The user status API endpoints fail to sanitize the active channel field in responses, allowing an authenticated attacker to enumerate private channel IDs of channels they are not a member of by observing other users' status information. This is an information disclosure vulnerability requiring valid authentication but no special privileges. Fixes are available in later versions.

## 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

- 2026-10-16: disclosed

## References

- [https://mattermost.com/security-updates](https://mattermost.com/security-updates)

## Related threats

- [Mattermost CSRF via Agents plugin mention reminder validation](https://junglewise.ai/threats/cve-2026-93851-mattermost-versions-11-9-x-11-9-1-11-8-x-11-8-5-11-7-x-11-7-10)
- [Mattermost denial of service via PDF text extraction memory exhaustion](https://junglewise.ai/threats/cve-2026-93850-mattermost-versions-11-9-x-11-9-1-11-8-x-11-8-5-11-7-x-11-7-10-11)
- [Mattermost denial of service in WebSocket msgpack decoder](https://junglewise.ai/threats/cve-2026-93849-mattermost-versions-11-9-x-11-9-1-11-8-x-11-8-5-11-7-x-11-7-10-11)
- [Mattermost information disclosure in post deletion events](https://junglewise.ai/threats/cve-2026-19621-mattermost-versions-11-10-x-11-10-0-11-7-x-11-7-9-11-9-x-11-9-1)
- [Mattermost authentication token leak in server logs](https://junglewise.ai/threats/cve-2026-19600-mattermost-versions-11-9-x-11-9-1-11-8-x-11-8-5-11-7-x-11-7-10-11)