What is CVE-2026-6339?

CVE-2026-6339 is a medium-severity vulnerability in Mattermost, classified under Origin Validation Error. CVSS score: 4.3/10. Published 2026-05-18.

How severe is CVE-2026-6339?

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

Vulnerability in Mattermost

CVE-2026-6339

Mattermost versions 11.5.x <= 11.5.1, 11.4.x <= 11.4.3 fail to validate the X-Requested-With header on the burn-on-read reveal endpoint which allows an authenticated channel member to force the reveal of a burn-on-read message without recipient consent via a crafted Markdown image tag.. Mattermost Advisory ID: MMSA-2026-00636

Published 2026-05-18 · last modified 2026-06-17.

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:N/I:N/A:L.

EPSS exploit prediction

EPSS: 0.002 (4.1th percentile), scored 2026-09-30.

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 4th percentile — 4.1% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.001 · newest: 0.002 · change: +0.000

Affected products

Mattermost — versions 11.5.0, 11.4.0, 11.6.0
Mattermost Server

Weakness classification (CWE)

CWE-346 — Origin Validation Error

References

mattermost.com/security-updates (vendor-advisory, Vendor Advisory)

Frequently asked questions

What is CVE-2026-6339?: CVE-2026-6339 is a medium-severity vulnerability in Mattermost, classified under Origin Validation Error. CVSS score: 4.3/10. Published 2026-05-18.
How severe is CVE-2026-6339?: Medium severity. CVSS v3 base score is 4.3 out of 10.