CVE-2026-5755: Denial of service via crafted TIFF file upload

Severity: medium (CVSS 6.5 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: 2026-05-22 · updated 2026-07-23
Weakness: CWE-400

Description

Mattermost versions 11.6.x <= 11.6.0, 11.5.x <= 11.5.2, 11.5.x <= 11.5.3, 11.4.x <= 11.4.4, 10.11.x <= 10.11.14 fail to validate the TIFF IFD offset in the image header before allocating memory, which allows authenticated users with file upload or posting permissions to cause a denial of service (server OOM) via uploading a crafted TIFF file or posting a URL that serves one.. Mattermost Advisory ID: MMSA-2026-00648

Affected products

Mattermost: from 10.11.0 before 10.11.15; from 11.4.0 before 11.4.5; from 11.5.0 before 11.5.3; from 11.6.0 before 11.6.1 — fixed in 10.11.15, 11.4.5, 11.5.3, 11.6.1

How to fix

Upgrade to a fixed version: Mattermost 10.11.15, 11.4.5, 11.5.3, 11.6.1. Check the vendor advisory for mitigations.

References

https://mattermost.com/security-updates

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