CVE-2026-10819: Mattermost Server Denial of Service via Animated GIF Emoji Upload

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: 2026-07-27 · updated 2026-08-03
Weakness: CWE-409

Description

Mattermost versions 11.6.x <= 11.6.5, 10.11.x <= 10.11.20, 11.8.x <= 11.8.1, 11.7.x <= 11.7.4 fail to limit the number of frames and enforce the file size cap on animated GIF uploads, which allows an authenticated attacker to cause a denial of service via a crafted animated GIF uploaded as a custom emoji.. Mattermost Advisory ID: MMSA-2026-00695

Affected products

Mattermost: from 10.11.0 before 10.11.21; from 11.6.0 before 11.6.6; from 11.7.0 before 11.7.5; from 11.8.0 before 11.8.2 — fixed in 10.11.21, 11.6.6, 11.7.5, 11.8.2

How to fix

Upgrade to a fixed version: Mattermost 10.11.21, 11.6.6, 11.7.5, 11.8.2. Check the vendor advisory for mitigations.

References

https://mattermost.com/security-updates

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