CVE-2026-6339
Received Received - Intake
Stored XSS in Mattermost via Burn-on-Read Message Reveal

Publication date: 2026-05-18

Last updated on: 2026-05-18

Assigner: Mattermost, Inc.

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
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Meta Information
Published
2026-05-18
Last Modified
2026-05-18
Generated
2026-06-10
AI Q&A
2026-05-18
EPSS Evaluated
2026-06-08
NVD
CVE-2026-6339
Affected Vendors & Products
Showing 2 associated CPEs
Vendor Product Version / Range
mattermost mattermost_server From 11.4.0 (inc) to 11.4.4 (exc)
mattermost mattermost_server From 11.5.0 (inc) to 11.5.2 (exc)
Helpful Resources
Exploitability
CWE
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KEV
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CWE ID Description
CWE-346 The product does not properly verify that the source of data or communication is valid.
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Executive Summary

This vulnerability exists in Mattermost versions 11.5.x up to 11.5.1 and 11.4.x up to 11.4.3. It occurs because the application fails to validate the X-Requested-With header on the burn-on-read reveal endpoint.

As a result, an authenticated channel member can exploit this flaw by crafting a Markdown image tag that forces the reveal of a burn-on-read message without the recipient's consent.

Impact Analysis

This vulnerability allows an authenticated user within a channel to reveal burn-on-read messages without the intended recipient's consent.

The impact is limited to the unauthorized exposure of messages that are meant to be ephemeral and only viewable by specific recipients.

According to the CVSS score of 4.3, the vulnerability has a low to medium severity with no impact on confidentiality or integrity, but it can cause availability issues.

Mitigation Strategies

The vulnerability affects Mattermost versions 11.5.x <= 11.5.1 and 11.4.x <= 11.4.3. To mitigate this vulnerability, you should upgrade Mattermost to a version later than 11.5.1 or 11.4.3 where the issue has been fixed.

Additionally, stay informed about security updates by subscribing to Mattermost's Security Bulletin and regularly checking their security updates page.

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