CVE-2026-6333: Github.com/mattermost/mattermost/server/v8 Mattermost Doesn't Validate The Host Vulnerability
LOWCVE-2026-6333 is a LOW (CVSS 3.5) mattermost doesn't validate the host vulnerability (CWE-918: Server-Side Request Forgery (SSRF)) affecting Github.com/mattermost/mattermost/server/v8 libraries in the Go ecosystem. The issue involves mattermost doesn't validate the host header when constructing response urls for custom slash command. Remediation is available by upgrading affected packages to version 11.5.2, 10.11.14, 8.0.0-20260325160634-e738016c5920, 5.3.2-0.20260325160634-e738016c5920.
CVE-2026-6333 at a Glance
Security Impact
CVSS: 3.5
The security impact metrics below are parsed directly from the CVSS v3.1 vector string (CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:C/C:N/I:L/A:N).
How to Fix CVE-2026-6333
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 11.5.2Upgrade github.com/mattermost/mattermost/server/v8 to 10.11.14Upgrade github.com/mattermost/mattermost/server/v8 to 8.0.0-20260325160634-e738016c5920Upgrade github.com/mattermost/mattermost-server to 5.3.2-0.20260325160634-e738016c5920Run 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-2026-6333, 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 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-918: Server-Side Request Forgery (SSRF), where the web server receives a url or similar request from an upstream component and retrieves the contents of this url, but it does not sufficiently ensure that the request is being sent to the expected destination. 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-2026-6333 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 |
≥ 11.5.0
< 11.5.2
|
11.5.2
|
|
github.com/mattermost/mattermost/server/v8
|
Go |
≥ 10.11.0
< 10.11.14
|
10.11.14
|
|
github.com/mattermost/mattermost/server/v8
|
Go |
< 8.0.0-20260325160634-e738016c5920
|
8.0.0-20260325160634-e738016c5920
|
|
github.com/mattermost/mattermost-server
|
Go |
< 5.3.2-0.20260325160634-e738016c5920
|
5.3.2-0.20260325160634-e738016c5920
|
References
Frequently Asked Questions
What is CVE-2026-6333?
Which Github.com/mattermost/mattermost/server/v8 versions are affected?
What version fixes CVE-2026-6333?
Is github.com/mattermost/mattermost/server/v8 affected?
Is github.com/mattermost/mattermost-server affected?
What is the CVSS score of CVE-2026-6333?
What CWE is associated with CVE-2026-6333?
How can I check Go dependencies for CVE-2026-6333?
Machine-readable OSV JSON
OSV v1.5 Schema
{
"schema_version" : "1.4.0",
"id" : "GHSA-vqp5-2mrp-qqxg",
"modified" : "2026-06-01T15:29:47Z",
"published" : "2026-05-18T09:31:48Z",
"aliases" : [ "CVE-2026-6333" ],
"summary" : "Mattermost doesn't validate the Host header when constructing response URLs for custom slash command",
"details" : "Mattermost versions 11.5.x <= 11.5.1, 10.11.x <= 10.11.13 fail to validate the Host header when constructing response URLs for custom slash commands which allows an authenticated attacker to redirect slash command responses to an attacker-controlled server via a spoofed Host header.. Mattermost Advisory ID: MMSA-2026-00582",
"severity" : [ {
"type" : "CVSS_V3",
"score" : "CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:C/C:N/I:L/A:N"
} ],
"affected" : [ {
"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" : "10.11.0"
}, {
"fixed" : "10.11.14"
} ]
} ]
}, {
"package" : {
"ecosystem" : "Go",
"name" : "github.com/mattermost/mattermost/server/v8"
},
"ranges" : [ {
"type" : "ECOSYSTEM",
"events" : [ {
"introduced" : "0"
}, {
"fixed" : "8.0.0-20260325160634-e738016c5920"
} ]
} ]
}, {
"package" : {
"ecosystem" : "Go",
"name" : "github.com/mattermost/mattermost-server"
},
"ranges" : [ {
"type" : "ECOSYSTEM",
"events" : [ {
"introduced" : "0"
}, {
"fixed" : "5.3.2-0.20260325160634-e738016c5920"
} ]
} ]
} ],
"references" : [ {
"type" : "ADVISORY",
"url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-6333"
}, {
"type" : "WEB",
"url" : "https://github.com/mattermost/mattermost/commit/e738016c592045e14bf926eafaeda6f8521def6d"
}, {
"type" : "PACKAGE",
"url" : "https://github.com/mattermost/mattermost"
}, {
"type" : "WEB",
"url" : "https://mattermost.com/security-updates"
} ],
"database_specific" : {
"cwe_ids" : [ "CWE-918" ],
"severity" : "LOW",
"github_reviewed" : true,
"github_reviewed_at" : "2026-06-01T15:29:47Z",
"nvd_published_at" : "2026-05-18T09:16:23Z"
}
}