CVE-2026-6339: Missing request origin validation on burn-on-read reveal endpoint

Severity: medium (CVSS 4.3 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.1% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-05-18 · updated 2026-10-14
Weakness: CWE-346

Description

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

Affected products

Mattermost: from 11.4.0 before 11.4.4; from 11.5.0 before 11.5.2 — fixed in 11.4.4, 11.5.2

How to fix

Upgrade to a fixed version: Mattermost 11.4.4, 11.5.2. Check the vendor advisory for mitigations.

References

https://mattermost.com/security-updates

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