CVE-2026-4054: Mattermost doesn't validate the response body of proxied images

May 15, 2026 (updated May 28, 2026)

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.

References

github.com/advisories/GHSA-j76w-p754-g2w7
mattermost.com/security-updates
nvd.nist.gov/vuln/detail/CVE-2026-4054

Code Behaviors & Features

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