CVE-2026-27769: Github.com/mattermost/mattermost server Mattermost Doesn't Validate Whether Users Vulnerability

LOW
Advisory Identifier: GHSA-mxxh-fmjq-j6x4
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CVSS 2.7
CWE-862
Published
Advisory Summary

CVE-2026-27769 is a LOW (CVSS 2.7) mattermost doesn't validate whether users vulnerability (CWE-862: Missing Authorization) affecting Github.com/mattermost/mattermost server libraries in the Go ecosystem. The issue involves mattermost doesn't validate whether users were correctly owned by the correct connected workspace. Remediation is available by upgrading affected packages to version 8.0.0-20260316060126-bc1a2b34b1f9.

CVE-2026-27769 at a Glance

CVE Identifier CVE-2026-27769
GHSA Identifier GHSA-mxxh-fmjq-j6x4
Severity LOW
CVSS v3.1 Base Score 2.7
CWE Classification CWE-862: Missing Authorization
Affected Product Github.com/mattermost/mattermost server
Package Ecosystem Go
Published Date
Last Updated

Security Impact

CVSS: 2.7

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:N/I:L/A:N).

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

How to Fix CVE-2026-27769

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: 8.0.0-20260316060126-bc1a2b34b1f9 
Upgrade github.com/mattermost/mattermost-server to 8.0.0-20260316060126-bc1a2b34b1f9
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-2026-27769, 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 8.0.0-20260316060126-bc1a2b34b1f9), 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-862: Missing Authorization, where the product does not perform an authorization check when an actor attempts to access a resource or perform an action. 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 10.11.x <= 10.11.12 fail to validate whether users were correctly owned by the correct Connected Workspace which allows a malicious remote server connected using the Conntexted Workspaces feature to change the displayed status of local users via the Connected Workspaces API. Mattermost Advisory ID: MMSA-2026-00603.

CVE-2026-27769 Affected Github.com/mattermost/mattermost server Versions

1 component(s)
Fixed Versions:
8.0.0-20260316060126-bc1a2b34b1f9
Product / Package Ecosystem Affected Versions Fixed Version
github.com/mattermost/mattermost-server
golang/github.com/mattermost/mattermost-server
Go
≥ 8.0.0-20250721062209-4952acea88ce
< 8.0.0-20260316060126-bc1a2b34b1f9
8.0.0-20260316060126-bc1a2b34b1f9

References

NVD — CVE-2026-27769 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-27769
Github.com/mattermost/mattermost server Source Repository REPOSITORY
https://github.com/mattermost/mattermost
Github.com/mattermost/mattermost server Security Information SECURITY
https://mattermost.com/security-updates

Frequently Asked Questions

What is CVE-2026-27769?
CVE-2026-27769 is a security vulnerability in Github.com/mattermost/mattermost server involving Mattermost doesn't validate whether users were correctly owned by the correct Connected Workspace under CWE-862: Missing Authorization.
Which Github.com/mattermost/mattermost server versions are affected?
Affected versions include releases prior to 8.0.0-20260316060126-bc1a2b34b1f9 across affected packages: github.com/mattermost/mattermost-server.
What version fixes CVE-2026-27769?
Upgrading to 8.0.0-20260316060126-bc1a2b34b1f9 (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 8.0.0-20260316060126-bc1a2b34b1f9. Upgrade your dependency to 8.0.0-20260316060126-bc1a2b34b1f9 or later.
What is the CVSS score of CVE-2026-27769?
CVE-2026-27769 has a CVSS v3.1 base score of 2.7 (LOW) with vector string CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:N/I:L/A:N.
What CWE is associated with CVE-2026-27769?
CVE-2026-27769 is classified under CWE-862: Missing Authorization (The product does not perform an authorization check when an actor attempts to access a resource or perform an action.).
How can I check Go dependencies for CVE-2026-27769?
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-mxxh-fmjq-j6x4",
  "modified" : "2026-04-28T23:02:16Z",
  "published" : "2026-04-17T15:31:17Z",
  "aliases" : [ "CVE-2026-27769" ],
  "summary" : "Mattermost doesn't validate whether users were correctly owned by the correct Connected Workspace",
  "details" : "Mattermost versions 10.11.x <= 10.11.12 fail to validate whether users were correctly owned by the correct Connected Workspace which allows a malicious remote server connected using the Conntexted Workspaces feature to change the displayed status of local users via the Connected Workspaces API. Mattermost Advisory ID: MMSA-2026-00603.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:N/I:L/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/mattermost/mattermost-server"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "8.0.0-20250721062209-4952acea88ce"
      }, {
        "fixed" : "8.0.0-20260316060126-bc1a2b34b1f9"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-27769"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/mattermost/mattermost"
  }, {
    "type" : "WEB",
    "url" : "https://mattermost.com/security-updates"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-862" ],
    "severity" : "LOW",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-04-28T23:02:16Z",
    "nvd_published_at" : "2026-04-15T11:16:33Z"
  }
}