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…

PriorityP421medium4.3CVSS 3.1

AVNACLPRLUINSUCLINAN

EPSS

0.11%

1.6th percentile

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

8 ranges

Vendor	Product	Version range	Fixed in
github.com	mattermost_mattermost-server	>= 0 < 5.3.2-0.20260327001745-7a339a6438f5	5.3.2-0.20260327001745-7a339a6438f5
github.com	mattermost_mattermost_server_v8	>= 0 < 8.0.0-20260327001745-7a339a6438f5	8.0.0-20260327001745-7a339a6438f5
github.com	mattermost_mattermost_server_v8	>= 11.4.0 < 11.4.4	11.4.4
github.com	mattermost_mattermost_server_v8	>= 11.5.0 < 11.5.2	11.5.2
mattermost	mattermost	11.4.0 – 11.4.3	—
mattermost	mattermost	11.5.0 – 11.5.1	—
mattermost	mattermost_server	>= 11.4.0 < 11.4.4	11.4.4
mattermost	mattermost_server	>= 11.5.0 < 11.5.2	11.5.2

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