CVE-2026-2578: Information Disclosure via WebSocket Event When Deleting Unrevealed Burn on Read Posts

Severity: medium (CVSS 4.3 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.2% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-03-16 · updated 2026-06-17
Weakness: CWE-201

Description

Mattermost versions 11.3.x <= 11.3.0 fail to preserve the redacted state of burn-on-read posts during deletion which allows channel members to access unrevealed burn-on-read message contents via the WebSocket post deletion event.. Mattermost Advisory ID: MMSA-2026-00579

Affected products

Mattermost: from 11.3.0 before 11.3.1 — fixed in 11.3.1

How to fix

Upgrade to a fixed version: Mattermost 11.3.1. Check the vendor advisory for mitigations.

References

https://mattermost.com/security-updates

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