CVE-2026-4054: Github.com/mattermost/mattermost server Mattermost Doesn't Validate The Response Vulnerability
MODERATECVE-2026-4054 is a MODERATE (CVSS 4.3) mattermost doesn't validate the response vulnerability (CWE-754: Improper Check for Unusual or Exceptional Conditions) affecting Github.com/mattermost/mattermost server libraries in the Go ecosystem. The issue involves mattermost doesn't validate the response body of proxied images. Remediation is available by upgrading affected packages to version 11.5.2, 0.0.0-20260414103857-b21ef302025e, 11.4.4.
CVE-2026-4054 at a Glance
Security Impact
CVSS: 4.3
The security impact metrics below are parsed directly from the CVSS v3.1 vector string (CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:L).
How to Fix CVE-2026-4054
To remediate this vulnerability, upgrade all affected Github.com/mattermost/mattermost server dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.
Upgrade github.com/mattermost/mattermost-server to 11.5.2Upgrade github.com/mattermost/mattermost-server to 0.0.0-20260414103857-b21ef302025eUpgrade github.com/mattermost/mattermost-server to 11.4.4Run the following command in your project directory to verify that all resolved dependencies use the patched release:
inspect dependencies for github.com/mattermost/mattermost-serverHow to Check if You Are Affected
To determine whether your application is affected by CVE-2026-4054, inspect your project dependencies for Github.com/mattermost/mattermost server:
inspect dependencies for github.com/mattermost/mattermost-serverAnalyze the terminal output to locate any version of Github.com/mattermost/mattermost server in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 11.5.2), your application is affected.
Note: Verify declared build manifests and deployed runtime container environments.
Technical Details
Vulnerability Mechanism & Technical Analysis
This vulnerability is classified under CWE-754: Improper Check for Unusual or Exceptional Conditions, where the product does not check or incorrectly checks for unusual or exceptional conditions that are not expected to occur frequently during day to day operation of the product. In Github.com/mattermost/mattermost server, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.
Source Advisory Statement
CVE-2026-4054 Affected Github.com/mattermost/mattermost server Versions
3 component(s)| Product / Package | Ecosystem | Affected Versions | Fixed Version |
|---|---|---|---|
|
github.com/mattermost/mattermost-server
|
Go |
≥ 11.5.0
< 11.5.2
|
11.5.2
|
|
github.com/mattermost/mattermost-server
|
Go |
≥ 0.0.0-20250731163400-5b955468ea1e
< 0.0.0-20260414103857-b21ef302025e
|
0.0.0-20260414103857-b21ef302025e
|
|
github.com/mattermost/mattermost-server
|
Go |
≥ 11.4.0
< 11.4.4
|
11.4.4
|
References
Frequently Asked Questions
What is CVE-2026-4054?
Which Github.com/mattermost/mattermost server versions are affected?
What version fixes CVE-2026-4054?
Is github.com/mattermost/mattermost-server affected?
What is the CVSS score of CVE-2026-4054?
What CWE is associated with CVE-2026-4054?
How can I check Go dependencies for CVE-2026-4054?
Machine-readable OSV JSON
OSV v1.5 Schema
{
"schema_version" : "1.4.0",
"id" : "GHSA-j76w-p754-g2w7",
"modified" : "2026-05-28T19:28:48Z",
"published" : "2026-05-15T21:31:33Z",
"aliases" : [ "CVE-2026-4054" ],
"summary" : "Mattermost doesn't validate the response body of proxied images",
"details" : "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.",
"severity" : [ {
"type" : "CVSS_V3",
"score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:L"
} ],
"affected" : [ {
"package" : {
"ecosystem" : "Go",
"name" : "github.com/mattermost/mattermost-server"
},
"ranges" : [ {
"type" : "ECOSYSTEM",
"events" : [ {
"introduced" : "11.5.0"
}, {
"fixed" : "11.5.2"
} ]
} ]
}, {
"package" : {
"ecosystem" : "Go",
"name" : "github.com/mattermost/mattermost-server"
},
"ranges" : [ {
"type" : "ECOSYSTEM",
"events" : [ {
"introduced" : "0.0.0-20250731163400-5b955468ea1e"
}, {
"fixed" : "0.0.0-20260414103857-b21ef302025e"
} ]
} ]
}, {
"package" : {
"ecosystem" : "Go",
"name" : "github.com/mattermost/mattermost-server"
},
"ranges" : [ {
"type" : "ECOSYSTEM",
"events" : [ {
"introduced" : "11.4.0"
}, {
"fixed" : "11.4.4"
} ]
} ]
} ],
"references" : [ {
"type" : "ADVISORY",
"url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-4054"
}, {
"type" : "PACKAGE",
"url" : "https://github.com/mattermost/mattermost"
}, {
"type" : "WEB",
"url" : "https://mattermost.com/security-updates"
} ],
"database_specific" : {
"cwe_ids" : [ "CWE-754" ],
"severity" : "MODERATE",
"github_reviewed" : true,
"github_reviewed_at" : "2026-05-28T19:28:48Z",
"nvd_published_at" : "2026-05-15T19:17:04Z"
}
}