CVE-2026-20796: Github.com/mattermost/mattermost server Mattermost Doesn't Properly Validate Channel Vulnerability

LOW
Advisory Identifier: GHSA-2xf7-hmf6-p64j
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CVSS 3.1
CWE-367
Published
Advisory Summary

CVE-2026-20796 is a LOW (CVSS 3.1) mattermost doesn't properly validate channel vulnerability (CWE-367: Time-of-check Time-of-use (TOCTOU) Race Condition) affecting Github.com/mattermost/mattermost server libraries in the Go ecosystem. The issue involves mattermost doesn't properly validate channel membership at the time of data retrieval. Remediation is available by upgrading affected packages to version 10.11.10.

CVE-2026-20796 at a Glance

CVE Identifier CVE-2026-20796
GHSA Identifier GHSA-2xf7-hmf6-p64j
Severity LOW
CVSS v3.1 Base Score 3.1
CWE Classification CWE-367: Time-of-check Time-of-use (TOCTOU) Race Condition
Affected Product Github.com/mattermost/mattermost server
Package Ecosystem Go
Published Date
Last Updated

Security Impact

CVSS: 3.1

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:L/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 Low (C:L)
Integrity None (I:N)
Availability None (A:N)

How to Fix CVE-2026-20796

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: 10.11.10 
Upgrade github.com/mattermost/mattermost-server to 10.11.10
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-2026-20796, 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 10.11.10), 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-367: Time-of-check Time-of-use (TOCTOU) Race Condition, where the product checks the state of a resource before using that resource, but the resource's state can change between the check and the use in a way that invalidates the results of the check. 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 <= 10.11.9 fail to properly validate channel membership at the time of data retrieval which allows a deactivated user to learn team names they should not have access to via a race condition in the /common_teams API endpoint.. Mattermost Advisory ID: MMSA-2025-00549

CVE-2026-20796 Affected Github.com/mattermost/mattermost server Versions

1 component(s)
Fixed Versions:
10.11.10
Product / Package Ecosystem Affected Versions Fixed Version
github.com/mattermost/mattermost-server
golang/github.com/mattermost/mattermost-server
Go
≥ 10.11.0
< 10.11.10
10.11.10

References

NVD — CVE-2026-20796 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-20796
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-2026-20796?
CVE-2026-20796 is a security vulnerability in Github.com/mattermost/mattermost server involving Mattermost doesn't properly validate channel membership at the time of data retrieval under CWE-367: Time-of-check Time-of-use (TOCTOU) Race Condition.
Which Github.com/mattermost/mattermost server versions are affected?
Affected versions include releases prior to 10.11.10 across affected packages: github.com/mattermost/mattermost-server.
What version fixes CVE-2026-20796?
Upgrading to 10.11.10 (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 10.11.10. Upgrade your dependency to 10.11.10 or later.
What is the CVSS score of CVE-2026-20796?
CVE-2026-20796 has a CVSS v3.1 base score of 3.1 (LOW) with vector string CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:N.
What CWE is associated with CVE-2026-20796?
CVE-2026-20796 is classified under CWE-367: Time-of-check Time-of-use (TOCTOU) Race Condition (The product checks the state of a resource before using that resource, but the resource's state can change between the check and the use in a way that invalidates the results of the check.).
How can I check Go dependencies for CVE-2026-20796?
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-2xf7-hmf6-p64j",
  "modified" : "2026-02-13T20:55:54Z",
  "published" : "2026-02-13T12:31:21Z",
  "aliases" : [ "CVE-2026-20796" ],
  "summary" : "Mattermost doesn't properly validate channel membership at the time of data retrieval",
  "details" : "Mattermost versions 10.11.x <= 10.11.9 fail to properly validate channel membership at the time of data retrieval which allows a deactivated user to learn team names they should not have access to via a race condition in the /common_teams API endpoint.. Mattermost Advisory ID: MMSA-2025-00549",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/mattermost/mattermost-server"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "10.11.0"
      }, {
        "fixed" : "10.11.10"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 10.11.9"
    }
  } ],
  "references" : [ {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-20796"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/mattermost/mattermost"
  }, {
    "type" : "WEB",
    "url" : "https://mattermost.com/security-updates"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-367" ],
    "severity" : "LOW",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-02-13T20:55:54Z",
    "nvd_published_at" : "2026-02-13T11:16:10Z"
  }
}