CVE-2026-6333: SSRF via Host Header Spoofing in Custom Slash Commands

Severity: medium (CVSS 5.0 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.1% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-05-18 · updated 2026-10-14
Weakness: CWE-918

Description

Mattermost versions 11.5.x <= 11.5.1, 10.11.x <= 10.11.13 fail to validate the Host header when constructing response URLs for custom slash commands which allows an authenticated attacker to redirect slash command responses to an attacker-controlled server via a spoofed Host header.. Mattermost Advisory ID: MMSA-2026-00582

Affected products

Mattermost: from 10.11.0 before 10.11.14; from 11.5.0 before 11.5.2 — fixed in 10.11.14, 11.5.2

How to fix

Upgrade to a fixed version: Mattermost 10.11.14, 11.5.2. Check the vendor advisory for mitigations.

References

https://mattermost.com/security-updates

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