GHSA-j76w-p754-g2w7 Mattermost doesn't validate the response body of proxied images

• Go github.com/mattermost/mattermost-server: >= 11.5.0, < 11.5.2 → fixed in 11.5.2 • Go github.com/mattermost/mattermost-server: >= 0.0.0-20250731163400-5b955468ea1e, < 0.0.0-20260414103857-b21ef302025e → fixed in 0.0.0-20260414103857-b21ef302025e • Go github.com/mattermost/mattermost-server: >= 11.4.0, < 11.4.4 → fixed in 11.4.4

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.