How severe is CVE-2026-4054?

CVE-2026-4054 has a CVSS score of 4.3/10, rated MEDIUM. Review your exposure and patch according to your risk tolerance.

Which packages are affected by CVE-2026-4054?

CVE-2026-4054 affects the following packages: github.com/mattermost/mattermost-server (Go), github.com/mattermost/mattermost-server (Go), github.com/mattermost/mattermost-server (Go). Ecosystems affected: Go.

How do I fix CVE-2026-4054?

Update github.com/mattermost/mattermost-server to 11.5.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-4054 is resolved across your whole dependency graph.

How do I detect CVE-2026-4054 in my Go dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/mattermost/mattermost-server, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2026-4054 if there is no patch (or I can't update yet)?

Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.

Is CVE-2026-4054 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-4054 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2026-4054?

CVE-2026-4054 has an EPSS (Exploit Prediction Scoring System) score of 0.2%, placing it in the 16th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2026-4054?

CVE-2026-4054 is classified as CWE-754 (CWE-754). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2026-4054 published, and has it been updated?

CVE-2026-4054 was published on May 15, 2026 and was last updated on November 7, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐹

🐹 Go

Not in CISA KEV

MEDIUM severity

CVE-2026-4054 — mattermost-server

MEDIUM

CVE-2026-4054 is a medium-severity (CVSS 4.3) CWE-754 vulnerability in github.com/mattermost/mattermost-server. A fix is available for github.com/mattermost/mattermost-server — see the affected versions and patch details below.

SVG content served through Mattermost image proxy despite Content-Type restrictions causes client-side denial of service

Also known asGHSA-j76w-p754-g2w7GO-2026-5454

Published

May 15, 2026

Updated

Nov 7, 2026

Affected

3 pkgs

Patched

3 / 3

Exploits

None indexed

Exploitation data as of Nov 10, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

No confirmed exploitation observed yet

CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.

Exploitation and automatability from CISA’s SSVC triage for CVE-2026-4054.

EPSS Exploitation Probability

via FIRST.org ↗

0.2%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs16th percentile — riskier than 16% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

Where this sits among everything scored

Real-World Exposure

3 pkgs affected

🐹github.com/mattermost/mattermost-server🐹github.com/mattermost/mattermost-server🐹github.com/mattermost/mattermost-server

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Go packages — download data is not available via public APIs for these ecosystems.

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

Affected Packages

3 total 3 fixed

Ecosystem	Package	Vulnerable range	Fix
🐹Go	`github.com/mattermost/mattermost-server`	≥ 11.5.0&&< 11.5.2	11.5.2`go get github.com/mattermost/mattermost-server@v11.5.2`
🐹Go	`github.com/mattermost/mattermost-server`	≥ 0.0.0-20250731163400-5b955468ea1e&&< 0.0.0-20260414103857-b21ef302025e	0.0.0-20260414103857-b21ef302025e`go get github.com/mattermost/mattermost-server@v0.0.0-20260414103857-b21ef302025e`
🐹Go	`github.com/mattermost/mattermost-server`	≥ 11.4.0&&< 11.4.4	11.4.4`go get github.com/mattermost/mattermost-server@v11.4.4`

Affected Products

1 product · 3 configurations

Application

mattermost servermattermost

≥ 11.5.0 && < 11.5.2

range

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/mattermost/mattermost-server, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update github.com/mattermost/mattermost-server to 11.5.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-4054 is resolved across your whole dependency graph.
Workarounds
Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.

Frequently Asked Questions

O3 Security · Impact-Aware SCA

Is CVE-2026-4054 in your dependencies?

Find it across Go, including transitive dependencies.

Scan my dependencies How O3 SCA works

CVSS Score

4.3/10CVSS v3 base

Vector breakdown

Attack Vector

Network

Attack Complexity

Low

Privileges Required

None

User Interaction

Required

Scope

Unchanged

Confidentiality

None

Integrity

None

Availability

Low

Raw vector

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:L

CWE Classification

CWE-754CWE-754

Weaponization

No Nuclei template, Metasploit module or public proof-of-concept found.

Timeline

Disclosed

May 15, 2026

Last updated

Nov 7, 2026

Identifiers

CVE-2026-4054

GHSA-j76w-p754-g2w7

GO-2026-5454

References

Advisorygithub.com · CVE 2026 4054.json Advisorymattermost.com · security updates Advisorynvd.nist.gov · CVE 2026 4054 Packagegithub.com · mattermost

Related vulnerabilities

This CVE is medium severity; 6 of 8 other CVEs affecting mattermost-server are rated as high or higher (top: high).

CVE-2026-5139CVE-2026-8074CVE-2026-9162CVE-2026-6673CVE-2026-6062CVE-2026-8823CVE-2026-6046CVE-2026-7387

Provenance

Sources

OSV.devNVDFIRST.org (EPSS)

Last verifiedNov 7, 2026

CVE-2026-4054 — mattermost-server

Exploitation Status

No confirmed exploitation observed yet

EPSS Exploitation Probability

How urgent is this, really

Where this sits among everything scored

Real-World Exposure

Description

Affected Packages

Affected Products

Detection & mitigation playbook

Detect

Fix

Workarounds

Frequently Asked Questions

Is CVE-2026-4054 in your dependencies?