CVE-2026-4054Network attack vector

Client-side denial of service via mislabeled SVG

Published May 15, 2026 · Updated May 15, 2026

Improper image validation in Mattermost server releases allows remote attackers to interrupt a user's client through a proxied SVG. The image proxy trusts an origin's non-SVG Content-Type without validating the response bytes, so SVG content reaches the client despite type restrictions. Affected releases are 10.11.0 through 10.11.13, 11.4.0 through 11.4.3, and 11.5.0 through 11.5.1; the user must view the resulting link preview or Markdown image.

CVSS severity4.3
Medium
EPSS probability0.24%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Improper image validation in Mattermost server releases allows remote attackers to interrupt a user's client through a proxied SVG. The image proxy trusts an origin's non-SVG Content-Type without validating the response bytes, so SVG content reaches the client despite type restrictions. Affected releases are 10.11.0 through 10.11.13, 11.4.0 through 11.4.3, and 11.5.0 through 11.5.1; the user must view the resulting link preview or Markdown image.

The record

CVE
CVE-2026-4054
Published
May 15, 2026
Updated
May 15, 2026
Vendor
Mattermost
Product
Mattermost
Classifications
CWE-754, T1499
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. May 15, 2026CVE publishedPublication date reported by the CVE source.
  2. May 15, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=10.11.0 <=10.11.13
  2. Affected versionversion=11.4.0 <=11.4.3
  3. Affected versionversion=11.5.0 <=11.5.1

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Mattermost · Mattermostversion=10.11.0 <=10.11.13; version=11.4.0 <=11.4.3; version=11.5.0 <=11.5.1

Published CVSS scores

6.5NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H
4.3Mattermost, Inc.CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:L

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-754Improper Check for Unusual or Exceptional Conditions
T1499Endpoint Denial of Service

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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