CVE-2026-6339

Missing request origin validation on burn-on-read reveal endpoint

State: Published
Severity: 4.3 Medium
Published: May 18, 2026
Updated: May 18, 2026
Reserved: Apr 15, 2026

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

Scores

CVSS	Score	Vector	Source
3.1	4.3 Medium	CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L	Mattermost

Weaknesses

CWE-346 Origin Validation Error

Affected Products

Product	Versions
MattermostMattermost	11.5.0 through 11.5.1Affected 11.4.0 through 11.4.3Affected 11.6.0Not affected 11.5.2Not affected 11.4.4Not affected

References

MMSA-2026-00636vendor-advisory

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