CVE-2026-0999: Github.com/mattermost/mattermost/server/v8 Mattermost Fails To Properly Validate Vulnerability

MODERATE
Advisory Identifier: GHSA-3c9r-7f29-qp32
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CVSS 5.4
CWE-303
Published
Advisory Summary

CVE-2026-0999 is a MODERATE (CVSS 5.4) mattermost fails to properly validate vulnerability (CWE-303: Incorrect Implementation of Authentication Algorithm) affecting Github.com/mattermost/mattermost/server/v8 libraries in the Go ecosystem. The issue involves mattermost fails to properly validate login method restrictions. Remediation is available by upgrading affected packages to version 8.0.0-20251212052346-61651b0df7ea, 5.3.2-0.20251212052346-61651b0df7ea.

CVE-2026-0999 at a Glance

CVE Identifier CVE-2026-0999
GHSA Identifier GHSA-3c9r-7f29-qp32
Severity MODERATE
CVSS v3.1 Base Score 5.4
CWE Classification CWE-303: Incorrect Implementation of Authentication Algorithm
Affected Product Github.com/mattermost/mattermost/server/v8
Package Ecosystem Go
Published Date
Last Updated

Security Impact

CVSS: 5.4

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

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

How to Fix CVE-2026-0999

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: 8.0.0-20251212052346-61651b0df7ea 
Upgrade github.com/mattermost/mattermost/server/v8 to 8.0.0-20251212052346-61651b0df7ea
github.com/mattermost/mattermost-server fixed: Latest secure release 
Upgrade github.com/mattermost/mattermost-server to Latest secure release
github.com/mattermost/mattermost-server fixed: Latest secure release 
Upgrade github.com/mattermost/mattermost-server to Latest secure release
github.com/mattermost/mattermost-server fixed: Latest secure release 
Upgrade github.com/mattermost/mattermost-server to Latest secure release
github.com/mattermost/mattermost-server fixed: 5.3.2-0.20251212052346-61651b0df7ea 
Upgrade github.com/mattermost/mattermost-server to 5.3.2-0.20251212052346-61651b0df7ea
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-2026-0999, 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 8.0.0-20251212052346-61651b0df7ea), 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-303: Incorrect Implementation of Authentication Algorithm, where the requirements for the product dictate the use of an established authentication algorithm, but the implementation of the algorithm is incorrect. 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 11.1.x <= 11.1.2, 10.11.x <= 10.11.9, 11.2.x <= 11.2.1 fail to properly validate login method restrictions which allows an authenticated user to bypass SSO-only login requirements via userID-based authentication. Mattermost Advisory ID: MMSA-2025-00548

CVE-2026-0999 Affected Github.com/mattermost/mattermost/server/v8 Versions

5 component(s)
Fixed Versions:
8.0.0-20251212052346-61651b0df7ea5.3.2-0.20251212052346-61651b0df7ea
Product / Package Ecosystem Affected Versions Fixed Version
github.com/mattermost/mattermost/server/v8
golang/github.com/mattermost/mattermost/server/v8
Go
< 8.0.0-20251212052346-61651b0df7ea
8.0.0-20251212052346-61651b0df7ea
github.com/mattermost/mattermost-server
golang/github.com/mattermost/mattermost-server
Go
≥ 11.1.0
github.com/mattermost/mattermost-server
golang/github.com/mattermost/mattermost-server
Go
≥ 10.11.0
github.com/mattermost/mattermost-server
golang/github.com/mattermost/mattermost-server
Go
≥ 11.2.0
github.com/mattermost/mattermost-server
golang/github.com/mattermost/mattermost-server
Go
< 5.3.2-0.20251212052346-61651b0df7ea
5.3.2-0.20251212052346-61651b0df7ea

References

NVD — CVE-2026-0999 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-0999
Security Fix Commit FIX COMMIT
https://github.com/mattermost/mattermost/commit/61651b0df7ea5db55d1e54f8d6fb5fce4149309c
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-2026-0999?
CVE-2026-0999 is a security vulnerability in Github.com/mattermost/mattermost/server/v8 involving Mattermost fails to properly validate login method restrictions under CWE-303: Incorrect Implementation of Authentication Algorithm.
Which Github.com/mattermost/mattermost/server/v8 versions are affected?
Affected versions include releases prior to 8.0.0-20251212052346-61651b0df7ea, 5.3.2-0.20251212052346-61651b0df7ea across affected packages: github.com/mattermost/mattermost/server/v8, github.com/mattermost/mattermost-server.
What version fixes CVE-2026-0999?
Upgrading to 8.0.0-20251212052346-61651b0df7ea, 5.3.2-0.20251212052346-61651b0df7ea (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 8.0.0-20251212052346-61651b0df7ea. Upgrade your dependency to 8.0.0-20251212052346-61651b0df7ea or later.
Is github.com/mattermost/mattermost-server affected?
Yes, github.com/mattermost/mattermost-server is affected in releases prior to Latest secure release, 5.3.2-0.20251212052346-61651b0df7ea. Upgrade your dependency to Latest secure release, 5.3.2-0.20251212052346-61651b0df7ea or later.
What is the CVSS score of CVE-2026-0999?
CVE-2026-0999 has a CVSS v3.1 base score of 5.4 (MODERATE) with vector string CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:N.
What CWE is associated with CVE-2026-0999?
CVE-2026-0999 is classified under CWE-303: Incorrect Implementation of Authentication Algorithm (The requirements for the product dictate the use of an established authentication algorithm, but the implementation of the algorithm is incorrect.).
How can I check Go dependencies for CVE-2026-0999?
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-3c9r-7f29-qp32",
  "modified" : "2026-02-19T19:34:56Z",
  "published" : "2026-02-16T12:30:24Z",
  "aliases" : [ "CVE-2026-0999" ],
  "summary" : "Mattermost fails to properly validate login method restrictions",
  "details" : "Mattermost versions 11.1.x <= 11.1.2, 10.11.x <= 10.11.9, 11.2.x <= 11.2.1 fail to properly validate login method restrictions which allows an authenticated user to bypass SSO-only login requirements via userID-based authentication. Mattermost Advisory ID: MMSA-2025-00548",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/mattermost/mattermost/server/v8"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "8.0.0-20251212052346-61651b0df7ea"
      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/mattermost/mattermost-server"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "11.1.0"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "< 11.1.3"
    }
  }, {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/mattermost/mattermost-server"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "10.11.0"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "< 10.11.10"
    }
  }, {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/mattermost/mattermost-server"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "11.2.0"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "< 11.2.2"
    }
  }, {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/mattermost/mattermost-server"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "5.3.2-0.20251212052346-61651b0df7ea"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-0999"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/mattermost/mattermost/commit/61651b0df7ea5db55d1e54f8d6fb5fce4149309c"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/mattermost/mattermost"
  }, {
    "type" : "WEB",
    "url" : "https://mattermost.com/security-updates"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-303" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-02-19T19:34:56Z",
    "nvd_published_at" : "2026-02-16T10:16:08Z"
  }
}