Mattermost doesn't validate the response body of proxied images

CVE-2026-4054 → View on GitHub ↗ View on CVE.org ↗ View on NVD ↗

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.

Basic information

Type
reviewed
Severity
medium
Advisory on GitHub
Open advisory ↗
Repository advisory
—
Source code
Browse source ↗
Published (advisory)
2026-05-15 21:31:33 UTC
Updated
2026-05-28 19:28:49 UTC
GitHub reviewed
2026-05-28 19:28:48 UTC
NVD published
2026-05-15

EPSS Score

Score Percentile
0.10% 26.52%

CVSS Scores

Base score Version Severity Vector
4.3 3.1 —
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:L Click to expand
Attack vector (AV:N)
Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:L)
Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:N)
No account or special rights needed—anonymous or random user is enough.
User interaction (UI:R)
A real person has to do something—click, install, enable—otherwise it doesn’t land.
Scope (S:U)
Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems.
Confidentiality (C:N)
Doesn’t really leak secrets in a meaningful way.
Integrity (I:N)
Data isn’t meaningfully altered or forged.
Availability (A:L)
Might cause slowdowns, glitches, or partial disruption—not a full brick.

Identifiers

Type Value
GHSA GHSA-j76w-p754-g2w7 ↗
CVE CVE-2026-4054 ↗

CWEs

CWE id Name
CWE-754 Improper Check for Unusual or Exceptional Conditions

Affected packages (3)

Vulnerable version ranges and first patched releases as published by GitHub.

Ecosystem Package Vulnerable range First patched Vulnerable functions
go github.com/mattermost/mattermost-server >= 11.5.0, < 11.5.2 11.5.2 —
go github.com/mattermost/mattermost-server >= 0.0.0-20250731163400-5b955468ea1e, < 0.0.0-20260414103857-b21ef302025e 0.0.0-20260414103857-b21ef302025e —
go github.com/mattermost/mattermost-server >= 11.4.0, < 11.4.4 11.4.4 —

References

cvelogic Threat Intelligence