SVG content served through Mattermost image proxy despite Content-Type restrictions causes client-side denial of service
Mattermost versions 11.5.x <= 11.5.1, 10.11.x <= 10.11.13, 11.4.x <= 11.4.3 Fail to validate the response body of proxied images, which allows a remote attacker to enact client-side DoS via an SVG file served from an attacker-controlled origin under a non-SVG Content-Type header (e.g. image/png) embedded in an og:image meta tag or Markdown image link.. Mattermost Advisory ID: MMSA-2026-00630
Problem Types
| Type | CWE ID | Description |
|---|
| CWE | CWE-754 | CWE-754: Improper Check for Unusual or Exceptional Conditions |
Type: CWE
Description: CWE-754: Improper Check for Unusual or Exceptional Conditions
Metrics
| Version | Base score | Base severity | Vector |
|---|
| 3.1 | 4.3 | MEDIUM | CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:L |
Version: 3.1
Base score: 4.3
Base severity: MEDIUM
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:L