CVE-2026-3495: Unescaped variables during error page composition

Severity: medium (CVSS 4.8 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-79

Description

Mattermost versions 11.5.x <= 11.5.1, 10.11.x <= 10.11.13 fail to escape some variables that could contain malicious content during error page composition which allows an attacker with access to edit some site configuration to execute some malicious code via injecting some JS as part of those values.. Mattermost Advisory ID: MMSA-2026-00622

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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