CVE-2026-15814: Uploading a crafted image causes excessive memory allocation in the Mattermost Server

Severity: medium (CVSS 6.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.4% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-09-14 · updated 2026-10-07
Weakness: CWE-409

Description

Mattermost versions 11.9.x <= 11.9.0, 11.8.x <= 11.8.4, 11.7.x <= 11.7.7, 10.11.x <= 10.11.22 fail to limit the amount of memory allocated when decoding uploaded image files which allows an authenticated user to cause excessive server memory consumption and potential denial of service via uploading a specially crafted image as a profile picture, channel file attachment, team icon, or custom brand image. Mattermost Advisory ID: MMSA-2026-00719

Affected products

Mattermost: from 10.11.0 before 10.11.23; from 11.7.0 before 11.7.8; from 11.8.0 before 11.8.5; from 11.9.0 before 11.9.1 — fixed in 10.11.23, 11.7.8, 11.8.5, 11.9.1

How to fix

Upgrade to a fixed version: Mattermost 10.11.23, 11.7.8, 11.8.5, 11.9.1. Check the vendor advisory for mitigations.

References

https://mattermost.com/security-updates

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