CVE-2026-6339
Published:May 18, 2026
Updated:June 11, 2026
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 Packages
https://github.com/mattermost/mattermost.git (GITHUB):
Affected version(s) >=v11.5.0 <v11.5.2Fix Suggestion:
Update to version v11.5.2https://github.com/mattermost/mattermost.git (GITHUB):
Affected version(s) >=v11.4.0 <v11.4.4Fix Suggestion:
Update to version v11.4.4github.com/mattermost/mattermost (GO):
Affected version(s) >=v11.4.0 <v11.4.4Fix Suggestion:
Update to version v11.4.4github.com/mattermost/mattermost/server/v8 (GO):
Affected version(s) >=v8.0.0-20230429093631-aac4f2337933 <v8.0.0-20260327001745-7a339a6438f5Fix Suggestion:
Update to version v8.0.0-20260327001745-7a339a6438f5github.com/mattermost/mattermost (GO):
Affected version(s) >=v11.5.0 <v11.5.2Fix Suggestion:
Update to version v11.5.2Related Resources (4)
Do you need more information?
Contact UsCVSS v4
Base Score:
5.3
Attack Vector
NETWORK
Attack Complexity
LOW
Attack Requirements
NONE
Privileges Required
LOW
User Interaction
NONE
Vulnerable System Confidentiality
NONE
Vulnerable System Integrity
NONE
Vulnerable System Availability
LOW
Subsequent System Confidentiality
NONE
Subsequent System Integrity
NONE
Subsequent System Availability
NONE
CVSS v3
Base Score:
4.3
Attack Vector
NETWORK
Attack Complexity
LOW
Privileges Required
LOW
User Interaction
NONE
Scope
UNCHANGED
Confidentiality
NONE
Integrity
NONE
Availability
LOW
Weakness Type (CWE)
Origin Validation Error
EPSS
Base Score:
0.02