GHSA-xvcx-mgpc-5xh3: github.com/mattermost/mattermost/server/v8 (CVE-2026-6339)

Mattermost doesn't validate the X-Requested-With header on the burn-on-read reveal endpoint

CVSS 4.3 MediumEPSS 0.0011 · 1st percentile

Published by the GitHub Advisory Database at 18 May 2026, 09:31 UTC. agentprime has not analysed this yet, so whether it affects any app is not known.

Package	Ecosystem	Affected	Fixed in
github.com/mattermost/mattermost/server/v8	Go	>=11.5.0, <11.5.2 \|\| >=11.4.0, <11.4.4 \|\| <8.0.0-20260327001745-7a339a6438f5	11.5.2, 11.4.4, 8.0.0-20260327001745-7a339a6438f5
github.com/mattermost/mattermost-server	Go	<5.3.2-0.20260327001745-7a339a6438f5	5.3.2-0.20260327001745-7a339a6438f5

Identifiers

GHSA-xvcx-mgpc-5xh3 · CVE-2026-6339 · GO-2026-5769

Weakness (CWE)

CWE-346

Machine-written summary

Summary written by Claude Haiku from the source advisory. Not reviewed by a person.

Mattermost versions 11.4.x up to 11.4.3 and 11.5.x up to 11.5.1 do not validate the X-Requested-With header on the burn-on-read reveal endpoint. An authenticated channel member can use a crafted Markdown image to force a burn-on-read message to be revealed without the recipient's consent. Upgrade to 11.4.4 or 11.5.2.

Written 7 Nov 2026.

Not analysed yet

We haven’t published our analysis of this Advisory yet. Once a person has reviewed it, this page will say what’s vulnerable, what has to be true for your app to be affected, and how to check.

Severity and exploitation

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

CISA KEV Not listed.
EPSS 0.0011, the 1st percentile, on 11 Nov 2026: FIRST’s estimate of the chance of exploitation in the next 30 days.

Timeline

31 Oct 2026, 15:06 UTC First seen by agentprime
31 Oct 2026, 15:06 UTC Confirmed by OSV

Sources

Facts on this page come from the GitHub Advisory Database (CC-BY 4.0), OSV and the other sources listed, as published. We don’t reproduce full advisory descriptions, which can contain exploit code.