CVE-2026-93849: Mattermost allows authenticated users to trigger excessive memory allocation via crafted msgpack binary WebSocket frames
Mattermost versions 11.9.x <= 11.9.1, 11.8.x <= 11.8.5, 11.7.x <= 11.7.10, 11.10.x <= 11.10.1 Mattermost fail to apply allocation limits to the byte-string decode paths in its WebSocket msgpack decoder, which allows an authenticated remote attacker to cause excessive server memory allocation via a crafted binary WebSocket frame declaring a Bin32 payload length of up to 4,294,967,295 bytes.. Mattermost Advisory ID: MMSA-2026-00768
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
Mattermostto a version that resolves this vulnerability.Fixed in 11.11.0
Event History
Frequently Asked Questions
Who can exploit this issue?
An attacker must be authenticated to Mattermost and able to send crafted binary WebSocket frames to the server. The attack is remote and does not require user interaction.
What is the expected impact of successful exploitation?
A crafted msgpack frame can declare a Bin32 payload length of up to 4,294,967,295 bytes, causing excessive server memory allocation. The stated impact is availability loss; no confidentiality or integrity impact is identified.
Which Mattermost releases are affected?
Affected releases are Mattermost 11.9.x through 11.9.1, 11.8.x through 11.8.5, 11.7.x through 11.7.10, and 11.10.x through 11.10.1.