CVE-2026-3495: Github.com/mattermost/mattermost/server/v8 Mattermost Doesn't Escape Some Variables Vulnerability
LOWCVE-2026-3495 is a LOW (CVSS 3.8) mattermost doesn't escape some variables vulnerability (CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')) affecting Github.com/mattermost/mattermost/server/v8 libraries in the Go ecosystem. The issue involves mattermost doesn't escape some variables that could contain malicious content during error page composition. Remediation is available by upgrading affected packages to version 10.11.14, 11.5.2, 8.0.0-20260310115442-5a1ea95044d, 5.3.2-0.20260310115442-5a1ea95044dc.
CVE-2026-3495 at a Glance
Security Impact
CVSS: 3.8
The security impact metrics below are parsed directly from the CVSS v3.1 vector string (CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:L/I:L/A:N).
How to Fix CVE-2026-3495
To remediate this vulnerability, upgrade all affected Github.com/mattermost/mattermost/server/v8 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/v8 to 10.11.14Upgrade github.com/mattermost/mattermost/server/v8 to 11.5.2Upgrade github.com/mattermost/mattermost/server/v8 to 8.0.0-20260310115442-5a1ea95044dUpgrade github.com/mattermost/mattermost-server to 5.3.2-0.20260310115442-5a1ea95044dcRun the following command in your project directory to verify that all resolved dependencies use the patched release:
inspect dependencies for github.com/mattermost/mattermost/server/v8How to Check if You Are Affected
To determine whether your application is affected by CVE-2026-3495, inspect your project dependencies for Github.com/mattermost/mattermost/server/v8:
inspect dependencies for github.com/mattermost/mattermost/server/v8Analyze the terminal output to locate any version of Github.com/mattermost/mattermost/server/v8 in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 10.11.14), 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-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting'), where the product does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users. In Github.com/mattermost/mattermost/server/v8, 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-2026-3495 Affected Github.com/mattermost/mattermost/server/v8 Versions
4 component(s)| Product / Package | Ecosystem | Affected Versions | Fixed Version |
|---|---|---|---|
|
github.com/mattermost/mattermost/server/v8
|
Go |
≥ 10.11.0
< 10.11.14
|
10.11.14
|
|
github.com/mattermost/mattermost/server/v8
|
Go |
≥ 11.5.0
< 11.5.2
|
11.5.2
|
|
github.com/mattermost/mattermost/server/v8
|
Go |
< 8.0.0-20260310115442-5a1ea95044d
|
8.0.0-20260310115442-5a1ea95044d
|
|
github.com/mattermost/mattermost-server
|
Go |
< 5.3.2-0.20260310115442-5a1ea95044dc
|
5.3.2-0.20260310115442-5a1ea95044dc
|
References
Frequently Asked Questions
What is CVE-2026-3495?
Which Github.com/mattermost/mattermost/server/v8 versions are affected?
What version fixes CVE-2026-3495?
Is github.com/mattermost/mattermost/server/v8 affected?
Is github.com/mattermost/mattermost-server affected?
What is the CVSS score of CVE-2026-3495?
What CWE is associated with CVE-2026-3495?
How can I check Go dependencies for CVE-2026-3495?
Machine-readable OSV JSON
OSV v1.5 Schema
{
"schema_version" : "1.4.0",
"id" : "GHSA-jx93-pf6x-874r",
"modified" : "2026-06-01T15:00:04Z",
"published" : "2026-05-18T09:31:47Z",
"aliases" : [ "CVE-2026-3495" ],
"summary" : "Mattermost doesn't escape some variables that could contain malicious content during error page composition",
"details" : "Mattermost versions 11.5.x <= 11.5.1, 10.11.x <= 10.11.13 fail to escape some variables that could contain malicious content during error page composition which allows an attacker with access to edit some site configuration to execute some malicious code via injecting some JS as part of those values.. Mattermost Advisory ID: MMSA-2026-00622",
"severity" : [ {
"type" : "CVSS_V3",
"score" : "CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:L/I:L/A:N"
} ],
"affected" : [ {
"package" : {
"ecosystem" : "Go",
"name" : "github.com/mattermost/mattermost/server/v8"
},
"ranges" : [ {
"type" : "ECOSYSTEM",
"events" : [ {
"introduced" : "10.11.0"
}, {
"fixed" : "10.11.14"
} ]
} ]
}, {
"package" : {
"ecosystem" : "Go",
"name" : "github.com/mattermost/mattermost/server/v8"
},
"ranges" : [ {
"type" : "ECOSYSTEM",
"events" : [ {
"introduced" : "11.5.0"
}, {
"fixed" : "11.5.2"
} ]
} ]
}, {
"package" : {
"ecosystem" : "Go",
"name" : "github.com/mattermost/mattermost/server/v8"
},
"ranges" : [ {
"type" : "ECOSYSTEM",
"events" : [ {
"introduced" : "0"
}, {
"fixed" : "8.0.0-20260310115442-5a1ea95044d"
} ]
} ]
}, {
"package" : {
"ecosystem" : "Go",
"name" : "github.com/mattermost/mattermost-server"
},
"ranges" : [ {
"type" : "ECOSYSTEM",
"events" : [ {
"introduced" : "0"
}, {
"fixed" : "5.3.2-0.20260310115442-5a1ea95044dc"
} ]
} ]
} ],
"references" : [ {
"type" : "ADVISORY",
"url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-3495"
}, {
"type" : "WEB",
"url" : "https://github.com/mattermost/mattermost/commit/5a1ea95044dc2d1ca601bfe9a4c1bc17990f3872"
}, {
"type" : "PACKAGE",
"url" : "https://github.com/mattermost/mattermost"
}, {
"type" : "WEB",
"url" : "https://mattermost.com/security-updates"
} ],
"database_specific" : {
"cwe_ids" : [ "CWE-79" ],
"severity" : "LOW",
"github_reviewed" : true,
"github_reviewed_at" : "2026-06-01T15:00:03Z",
"nvd_published_at" : "2026-05-18T08:16:13Z"
}
}