CVE-2026-25783: Github.com/mattermost/mattermost/server/v8 Mattermost Fails To Properly Validate Vulnerability

MODERATE
Advisory Identifier: GHSA-2v3w-6g35-5f9v
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CVSS 4.3
CWE-1287
Published
Advisory Summary

CVE-2026-25783 is a MODERATE (CVSS 4.3) mattermost fails to properly validate vulnerability (CWE-1287: Improper Validation of Specified Type of Input) affecting Github.com/mattermost/mattermost/server/v8 libraries in the Go ecosystem. The issue involves mattermost fails to properly validate user-agent header tokens. Remediation is available by upgrading affected packages to version 8.0.0-20260129181235-1346cf529aef, 5.3.2-0.20260129181235-1346cf529aef, 10.11.11, 11.2.3, 11.3.1.

CVE-2026-25783 at a Glance

CVE Identifier CVE-2026-25783
GHSA Identifier GHSA-2v3w-6g35-5f9v
Severity MODERATE
CVSS v3.1 Base Score 4.3
CWE Classification CWE-1287: Improper Validation of Specified Type of Input
Affected Product Github.com/mattermost/mattermost/server/v8
Package Ecosystem Go
Published Date
Last Updated

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:L/UI:N/S:U/C:N/I:N/A:L).

Attack Vector Network (AV:N)
Attack Complexity Low (AC:L)
Privileges Required Low (PR:L)
User Interaction None (UI:N)
Scope Unchanged (S:U)
Confidentiality None (C:N)
Integrity None (I:N)
Availability Low (A:L)

How to Fix CVE-2026-25783

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.

github.com/mattermost/mattermost/server/v8 fixed: 8.0.0-20260129181235-1346cf529aef 
Upgrade github.com/mattermost/mattermost/server/v8 to 8.0.0-20260129181235-1346cf529aef
github.com/mattermost/mattermost-server fixed: 5.3.2-0.20260129181235-1346cf529aef 
Upgrade github.com/mattermost/mattermost-server to 5.3.2-0.20260129181235-1346cf529aef
github.com/mattermost/mattermost-server fixed: 10.11.11 
Upgrade github.com/mattermost/mattermost-server to 10.11.11
github.com/mattermost/mattermost-server fixed: 11.2.3 
Upgrade github.com/mattermost/mattermost-server to 11.2.3
github.com/mattermost/mattermost-server fixed: 11.3.1 
Upgrade github.com/mattermost/mattermost-server to 11.3.1
Verify Direct and Transitive Dependencies

Run 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/v8

How to Check if You Are Affected

To determine whether your application is affected by CVE-2026-25783, inspect your project dependencies for Github.com/mattermost/mattermost/server/v8:

inspect dependencies for github.com/mattermost/mattermost/server/v8

Analyze 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 8.0.0-20260129181235-1346cf529aef), 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-1287: Improper Validation of Specified Type of Input, where the product receives input that is expected to be of a certain type, but it does not validate or incorrectly validates that the input is actually of the expected type. 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

Mattermost versions 11.3.x <= 11.3.0, 11.2.x <= 11.2.2, 10.11.x <= 10.11.10 fail to properly validate User-Agent header tokens which allows an authenticated attacker to cause a request panic via a specially crafted User-Agent header. Mattermost Advisory ID: MMSA-2026-00586

CVE-2026-25783 Affected Github.com/mattermost/mattermost/server/v8 Versions

5 component(s)
Fixed Versions:
8.0.0-20260129181235-1346cf529aef5.3.2-0.20260129181235-1346cf529aef10.11.1111.2.311.3.1
Product / Package Ecosystem Affected Versions Fixed Version
github.com/mattermost/mattermost/server/v8
golang/github.com/mattermost/mattermost/server/v8
Go
< 8.0.0-20260129181235-1346cf529aef
8.0.0-20260129181235-1346cf529aef
github.com/mattermost/mattermost-server
golang/github.com/mattermost/mattermost-server
Go
< 5.3.2-0.20260129181235-1346cf529aef
5.3.2-0.20260129181235-1346cf529aef
github.com/mattermost/mattermost-server
golang/github.com/mattermost/mattermost-server
Go
≥ 10.11.0-rc1
< 10.11.11
10.11.11
github.com/mattermost/mattermost-server
golang/github.com/mattermost/mattermost-server
Go
≥ 11.2.0-rc1
< 11.2.3
11.2.3
github.com/mattermost/mattermost-server
golang/github.com/mattermost/mattermost-server
Go
≥ 11.3.0-rc1
< 11.3.1
11.3.1

References

NVD — CVE-2026-25783 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-25783
Security Fix Commit FIX COMMIT
https://github.com/mattermost/mattermost/commit/1346cf529aef0672c39a56ec10d1b8a9c8fb387d
Github.com/mattermost/mattermost/server/v8 Source Repository REPOSITORY
https://github.com/mattermost/mattermost
Github.com/mattermost/mattermost/server/v8 Security Information SECURITY
https://mattermost.com/security-updates

Frequently Asked Questions

What is CVE-2026-25783?
CVE-2026-25783 is a security vulnerability in Github.com/mattermost/mattermost/server/v8 involving Mattermost fails to properly validate User-Agent header tokens under CWE-1287: Improper Validation of Specified Type of Input.
Which Github.com/mattermost/mattermost/server/v8 versions are affected?
Affected versions include releases prior to 8.0.0-20260129181235-1346cf529aef, 5.3.2-0.20260129181235-1346cf529aef, 10.11.11, 11.2.3, 11.3.1 across affected packages: github.com/mattermost/mattermost/server/v8, github.com/mattermost/mattermost-server.
What version fixes CVE-2026-25783?
Upgrading to 8.0.0-20260129181235-1346cf529aef, 5.3.2-0.20260129181235-1346cf529aef, 10.11.11, 11.2.3, 11.3.1 (or the latest patched release for your respective branch) resolves the vulnerability.
Is github.com/mattermost/mattermost/server/v8 affected?
Yes, github.com/mattermost/mattermost/server/v8 is affected in releases prior to 8.0.0-20260129181235-1346cf529aef. Upgrade your dependency to 8.0.0-20260129181235-1346cf529aef or later.
Is github.com/mattermost/mattermost-server affected?
Yes, github.com/mattermost/mattermost-server is affected in releases prior to 5.3.2-0.20260129181235-1346cf529aef, 10.11.11, 11.2.3, 11.3.1. Upgrade your dependency to 5.3.2-0.20260129181235-1346cf529aef, 10.11.11, 11.2.3, 11.3.1 or later.
What is the CVSS score of CVE-2026-25783?
CVE-2026-25783 has a CVSS v3.1 base score of 4.3 (MODERATE) with vector string CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L.
What CWE is associated with CVE-2026-25783?
CVE-2026-25783 is classified under CWE-1287: Improper Validation of Specified Type of Input (The product receives input that is expected to be of a certain type, but it does not validate or incorrectly validates that the input is actually of the expected type.).
How can I check Go dependencies for CVE-2026-25783?
Run the dependency tree command: 'inspect dependencies for github.com/mattermost/mattermost/server/v8'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.
Machine-readable OSV JSON OSV v1.5 Schema
Normalized schema output
View Raw JSON 
{
  "schema_version" : "1.4.0",
  "id" : "GHSA-2v3w-6g35-5f9v",
  "modified" : "2026-03-17T19:59:39Z",
  "published" : "2026-03-16T15:30:42Z",
  "aliases" : [ "CVE-2026-25783" ],
  "summary" : "Mattermost fails to properly validate User-Agent header tokens",
  "details" : "Mattermost versions 11.3.x <= 11.3.0, 11.2.x <= 11.2.2, 10.11.x <= 10.11.10 fail to properly validate User-Agent header tokens which allows an authenticated attacker to cause a request panic via a specially crafted User-Agent header. Mattermost Advisory ID: MMSA-2026-00586",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/mattermost/mattermost/server/v8"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "8.0.0-20260129181235-1346cf529aef"
      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/mattermost/mattermost-server"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "5.3.2-0.20260129181235-1346cf529aef"
      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/mattermost/mattermost-server"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "10.11.0-rc1"
      }, {
        "fixed" : "10.11.11"
      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/mattermost/mattermost-server"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "11.2.0-rc1"
      }, {
        "fixed" : "11.2.3"
      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/mattermost/mattermost-server"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "11.3.0-rc1"
      }, {
        "fixed" : "11.3.1"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-25783"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/mattermost/mattermost/commit/1346cf529aef0672c39a56ec10d1b8a9c8fb387d"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/mattermost/mattermost"
  }, {
    "type" : "WEB",
    "url" : "https://mattermost.com/security-updates"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-1287" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-03-17T19:59:39Z",
    "nvd_published_at" : "2026-03-16T14:18:23Z"
  }
}