---
id: "CVE-2026-93849"
kind: "vulnerability"
aliases: ["CVE-2026-93849"]
title: "Mattermost: crafted WebSocket frame forces excessive server memory allocation"
severity: "medium"
cvss: 6.5
epss: null
exploited: false
status: "open"
category: "package"
products: []
ecosystems: []
tags: ["mattermost", "package", "self-hosted", "websocket"]
not_affected_if: ["patched-version"]
published_at: "2026-10-26T15:16:46+00:00"
first_seen_at: "2026-10-26T16:02:26+00:00"
updated_at: "2026-10-26T16:03:51+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-93849", "first_seen_at": "2026-10-26T16:02:26+00:00"}]
---

# Mattermost: crafted WebSocket frame forces excessive server memory allocation

Mattermost's WebSocket msgpack decoder does not limit byte-string allocations, so a crafted binary frame declaring a Bin32 length of up to 4,294,967,295 bytes makes the server allocate huge amounts of memory. An authenticated attacker can exhaust server memory and cause denial of service.

_Summary, check and fix written by a model from the sources; labelled as such._

## How to check

- Run the Mattermost server version check (for example `mattermost version` or the System Console version page) and compare against the affected ranges: 11.7.x up to 11.7.10, 11.8.x up to 11.8.5, 11.9.x up to 11.9.1, 11.10.x up to 11.10.1
- Confirm whether the instance is reachable by users who hold a login account

## Fix

- Upgrade Mattermost to a release newer than the affected ranges: `11.7.11` or later on the 11.7 line, `11.8.6` or later on 11.8, `11.9.2` or later on 11.9, `11.10.2` or later on 11.10 (fixed versions inferred from the affected ranges; confirm against MMSA-2026-00768)

## Timeline

- 2026-10-26T16:02:26+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-93849

## From the sources

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

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_Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai_