CVE-2026-6046: Github.com/mattermost/mattermost server Mattermost Doesn't Validate That A Vulnerability

MODERATE
Advisory Identifier: GHSA-3vmp-whvv-5v9v
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CVSS 5.3
CWE-200
Published
Advisory Summary

CVE-2026-6046 is a MODERATE (CVSS 5.3) mattermost doesn't validate that a vulnerability (CWE-200: Exposure of Sensitive Information to an Unauthorized Actor) affecting Github.com/mattermost/mattermost server libraries in the Go ecosystem. The issue involves mattermost doesn't validate that a username returned during bot registration belongs to a bot account. Remediation is available by upgrading affected packages to version 11.6.1, 11.5.5, 10.11.17, 8.0.0-20260428151657-c79c3831061a.

CVE-2026-6046 at a Glance

CVE Identifier CVE-2026-6046
GHSA Identifier GHSA-3vmp-whvv-5v9v
Severity MODERATE
CVSS v3.1 Base Score 5.3
CWE Classification CWE-200: Exposure of Sensitive Information to an Unauthorized Actor
Affected Product Github.com/mattermost/mattermost server
Package Ecosystem Go
Published Date
Last Updated

Security Impact

CVSS: 5.3

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:U/C:H/I:N/A:N).

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

How to Fix CVE-2026-6046

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.

github.com/mattermost/mattermost-server fixed: 11.6.1 
Upgrade github.com/mattermost/mattermost-server to 11.6.1
github.com/mattermost/mattermost-server fixed: 11.5.5 
Upgrade github.com/mattermost/mattermost-server to 11.5.5
github.com/mattermost/mattermost-server fixed: 10.11.17 
Upgrade github.com/mattermost/mattermost-server to 10.11.17
github.com/mattermost/mattermost/server/v8 fixed: 8.0.0-20260428151657-c79c3831061a 
Upgrade github.com/mattermost/mattermost/server/v8 to 8.0.0-20260428151657-c79c3831061a
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

How to Check if You Are Affected

To determine whether your application is affected by CVE-2026-6046, inspect your project dependencies for Github.com/mattermost/mattermost server:

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

Analyze 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 11.6.1), 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-200: Exposure of Sensitive Information to an Unauthorized Actor, where the product exposes sensitive information to an actor that is not explicitly authorized to have access to that information. 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

Mattermost versions 11.6.x <= 11.6.1, 11.5.x <= 11.5.4, 10.11.x <= 10.11.15, 10.11.x <= 10.11.16 fail to validate that a username returned during bot registration belongs to a bot account, which allows an unprivileged attacker to intercept private messages sent by plugins via direct message channels by pre-registering a user account with a predictable plugin bot username. Mattermost Advisory ID: MMSA-2026-00649

CVE-2026-6046 Affected Github.com/mattermost/mattermost server Versions

4 component(s)
Fixed Versions:
11.6.111.5.510.11.178.0.0-20260428151657-c79c3831061a
Product / Package Ecosystem Affected Versions Fixed Version
github.com/mattermost/mattermost-server
golang/github.com/mattermost/mattermost-server
Go
≥ 11.6.0
< 11.6.1
All 1 versions
11.6.0
11.6.1
github.com/mattermost/mattermost-server
golang/github.com/mattermost/mattermost-server
Go
≥ 11.5.0
< 11.5.5
11.5.5
github.com/mattermost/mattermost-server
golang/github.com/mattermost/mattermost-server
Go
≥ 10.11.0
< 10.11.17
10.11.17
github.com/mattermost/mattermost/server/v8
golang/github.com/mattermost/mattermost/server/v8
Go
≥ 8.0.0-20250731163400-5b955468ea1e
< 8.0.0-20260428151657-c79c3831061a
8.0.0-20260428151657-c79c3831061a

References

Security Fix Commit FIX COMMIT
https://github.com/mattermost/mattermost/commit/aba9339a24d4b287edd77377c19901d6e341bb96
Security Fix Commit FIX COMMIT
https://github.com/mattermost/mattermost/commit/98f9778cec1e7f3d97b3d4692fb91f8e7b659972
Security Fix Commit FIX COMMIT
https://github.com/mattermost/mattermost/commit/3be10297c14d272f273d747af70728f9d03c60ec
Github.com/mattermost/mattermost server Release RELEASE
https://github.com/mattermost/mattermost/releases/tag/v10.11.16
Github.com/mattermost/mattermost server Release RELEASE
https://github.com/mattermost/mattermost/releases/tag/v11.5.5
NVD — CVE-2026-6046 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-6046
Github.com/mattermost/mattermost server Source Repository REPOSITORY
https://github.com/mattermost/mattermost/pull/36320
Github.com/mattermost/mattermost server Source Repository REPOSITORY
https://github.com/mattermost/mattermost/pull/36318
Github.com/mattermost/mattermost server Source Repository REPOSITORY
https://github.com/mattermost/mattermost/pull/36317
Github.com/mattermost/mattermost server Source Repository REPOSITORY
https://github.com/mattermost/mattermost/pull/36305
Github.com/mattermost/mattermost server Source Repository REPOSITORY
https://github.com/mattermost/mattermost/pull/36064
Security Fix Commit FIX COMMIT
https://github.com/mattermost/mattermost/commit/f706d1f01e6dfe62cab86c1d257f237daa78106a
Security Fix Commit FIX COMMIT
https://github.com/mattermost/mattermost/commit/c79c3831061a0880c0962c7d567c9e24dd35f44c
Github.com/mattermost/mattermost server Release RELEASE
https://github.com/mattermost/mattermost/releases/tag/v11.6.2
Github.com/mattermost/mattermost server Release RELEASE
https://github.com/mattermost/mattermost/releases/tag/v11.7.0
Github.com/mattermost/mattermost server Security Information SECURITY
https://mattermost.com/security-updates
Github.com/mattermost/mattermost server Source Repository REPOSITORY
https://github.com/mattermost/mattermost

Frequently Asked Questions

What is CVE-2026-6046?
CVE-2026-6046 is a security vulnerability in Github.com/mattermost/mattermost server involving Mattermost doesn't validate that a username returned during bot registration belongs to a bot account under CWE-200: Exposure of Sensitive Information to an Unauthorized Actor.
Which Github.com/mattermost/mattermost server versions are affected?
Affected versions include releases prior to 11.6.1, 11.5.5, 10.11.17, 8.0.0-20260428151657-c79c3831061a across affected packages: github.com/mattermost/mattermost-server, github.com/mattermost/mattermost/server/v8.
What version fixes CVE-2026-6046?
Upgrading to 11.6.1, 11.5.5, 10.11.17, 8.0.0-20260428151657-c79c3831061a (or the latest patched release for your respective branch) resolves the vulnerability.
Is github.com/mattermost/mattermost-server affected?
Yes, github.com/mattermost/mattermost-server is affected in releases prior to 11.6.1, 11.5.5, 10.11.17. Upgrade your dependency to 11.6.1, 11.5.5, 10.11.17 or later.
Is github.com/mattermost/mattermost/server/v8 affected?
Yes, github.com/mattermost/mattermost/server/v8 is affected in releases prior to 8.0.0-20260428151657-c79c3831061a. Upgrade your dependency to 8.0.0-20260428151657-c79c3831061a or later.
What is the CVSS score of CVE-2026-6046?
CVE-2026-6046 has a CVSS v3.1 base score of 5.3 (MODERATE) with vector string CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:N/A:N.
What CWE is associated with CVE-2026-6046?
CVE-2026-6046 is classified under CWE-200: Exposure of Sensitive Information to an Unauthorized Actor (The product exposes sensitive information to an actor that is not explicitly authorized to have access to that information.).
How can I check Go dependencies for CVE-2026-6046?
Run the dependency tree command: 'inspect dependencies for github.com/mattermost/mattermost-server'. 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-3vmp-whvv-5v9v",
  "modified" : "2026-08-24T21:39:54Z",
  "published" : "2026-06-12T18:31:59Z",
  "aliases" : [ "CVE-2026-6046" ],
  "summary" : "Mattermost doesn't validate that a username returned during bot registration belongs to a bot account",
  "details" : "Mattermost versions 11.6.x <= 11.6.1, 11.5.x <= 11.5.4, 10.11.x <= 10.11.15, 10.11.x <= 10.11.16 fail to validate that a username returned during bot registration belongs to a bot account, which allows an unprivileged attacker to intercept private messages sent by plugins via direct message channels by pre-registering a user account with a predictable plugin bot username. Mattermost Advisory ID: MMSA-2026-00649",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:N/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/mattermost/mattermost-server"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "11.6.0"
      }, {
        "fixed" : "11.6.1"
      } ]
    } ],
    "versions" : [ "11.6.0" ]
  }, {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/mattermost/mattermost-server"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "11.5.0"
      }, {
        "fixed" : "11.5.5"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 11.5.4"
    }
  }, {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/mattermost/mattermost-server"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "10.11.0"
      }, {
        "fixed" : "10.11.17"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 10.11.15"
    }
  }, {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/mattermost/mattermost/server/v8"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "8.0.0-20250731163400-5b955468ea1e"
      }, {
        "fixed" : "8.0.0-20260428151657-c79c3831061a"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-6046"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/mattermost/mattermost/pull/36320"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/mattermost/mattermost/pull/36318"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/mattermost/mattermost/pull/36317"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/mattermost/mattermost/pull/36305"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/mattermost/mattermost/pull/36064"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/mattermost/mattermost/commit/f706d1f01e6dfe62cab86c1d257f237daa78106a"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/mattermost/mattermost/commit/c79c3831061a0880c0962c7d567c9e24dd35f44c"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/mattermost/mattermost/commit/aba9339a24d4b287edd77377c19901d6e341bb96"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/mattermost/mattermost/commit/98f9778cec1e7f3d97b3d4692fb91f8e7b659972"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/mattermost/mattermost/commit/3be10297c14d272f273d747af70728f9d03c60ec"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/mattermost/mattermost/releases/tag/v10.11.16"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/mattermost/mattermost/releases/tag/v11.5.5"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/mattermost/mattermost/releases/tag/v11.6.2"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/mattermost/mattermost/releases/tag/v11.7.0"
  }, {
    "type" : "WEB",
    "url" : "https://mattermost.com/security-updates"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/mattermost/mattermost"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-200" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-08-24T21:39:13Z",
    "nvd_published_at" : "2026-06-12T17:16:26Z"
  }
}