What is CVE-2026-19603?

CVE-2026-19603 is a medium-severity vulnerability in Mattermost, classified under Insertion of Sensitive Information into Sent Data. CVSS score: 4.3/10. Published 2026-10-16.

How severe is CVE-2026-19603?

Medium severity. CVSS v3 base score is 4.3 out of 10.

Vulnerability in Mattermost

CVE-2026-19603

Mattermost versions 11.9.x <= 11.9.1, 11.8.x <= 11.8.5, 11.7.x <= 11.7.10, 11.10.x <= 11.10.1 fail to sanitize the active channel field returned by the user status API endpoints, which allows an authenticated user to learn the channel ID of a private channel another user is actively viewing via the status endpoints, without being a member of that channel. Mattermost Advisory ID: MMSA-2026-00765

Published 2026-10-16 · last modified 2026-10-16.

CVSS v3 metric

CVSS v3 base score 4.3 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:N.

EPSS exploit prediction

No EPSS score from FIRST.org yet. New CVEs usually get one within a few days; no score means no signal yet, not low risk. How EPSS works.

Affected products

Mattermost

Weakness classification (CWE)

CWE-201 — Insertion of Sensitive Information into Sent Data

References

mattermost.com/security-updates (vendor-advisory)

Frequently asked questions

What is CVE-2026-19603?: CVE-2026-19603 is a medium-severity vulnerability in Mattermost, classified under Insertion of Sensitive Information into Sent Data. CVSS score: 4.3/10. Published 2026-10-16.
How severe is CVE-2026-19603?: Medium severity. CVSS v3 base score is 4.3 out of 10.