CVE-2026-27659: Github.com/mattermost/mattermost/server/v8 Mattermost Doesn't Properly Validate CSRF Vulnerability

MODERATE
Advisory Identifier: GHSA-rmhw-c3xr-m3xx
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CVSS 4.6
CWE-352
Published
Advisory Summary

CVE-2026-27659 is a MODERATE (CVSS 4.6) mattermost doesn't properly validate csrf vulnerability (CWE-352: Cross-Site Request Forgery (CSRF)) affecting Github.com/mattermost/mattermost/server/v8 libraries in the Go ecosystem. The issue involves mattermost doesn't properly validate csrf tokens. Remediation is available by upgrading affected packages to version 11.4.1, 11.3.2, 11.2.3, 10.11.11.

CVE-2026-27659 at a Glance

CVE Identifier CVE-2026-27659
GHSA Identifier GHSA-rmhw-c3xr-m3xx
Severity MODERATE
CVSS v3.1 Base Score 4.6
CWE Classification CWE-352: Cross-Site Request Forgery (CSRF)
Affected Product Github.com/mattermost/mattermost/server/v8
Package Ecosystem Go
Published Date
Last Updated

Security Impact

CVSS: 4.6

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:R/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 Required (UI:R)
Scope Unchanged (S:U)
Confidentiality Low (C:L)
Integrity Low (I:L)
Availability None (A:N)

How to Fix CVE-2026-27659

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: 11.4.1 
Upgrade github.com/mattermost/mattermost/server/v8 to 11.4.1
github.com/mattermost/mattermost/server/v8 fixed: 11.3.2 
Upgrade github.com/mattermost/mattermost/server/v8 to 11.3.2
github.com/mattermost/mattermost/server/v8 fixed: 11.2.3 
Upgrade github.com/mattermost/mattermost/server/v8 to 11.2.3
github.com/mattermost/mattermost/server/v8 fixed: 10.11.11 
Upgrade github.com/mattermost/mattermost/server/v8 to 10.11.11
github.com/mattermost/mattermost/server/v8 fixed: 10.11.11 
Upgrade github.com/mattermost/mattermost/server/v8 to 10.11.11
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-2026-27659, 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 11.4.1), 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-352: Cross-Site Request Forgery (CSRF), where the web application does not, or cannot, sufficiently verify whether a request was intentionally provided by the user who sent the request, which could have originated from an unauthorized actor. 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 11.2.x <= 11.2.2, 10.11.x <= 10.11.10, 11.4.x <= 11.4.0, 11.3.x <= 11.3.1 fail to properly validate CSRF tokens in the /api/v4/access_control_policies/{policy_id}/activate endpoint, which allows an attacker to trick an admin into changing access control policy active status via a crafted request. Mattermost Advisory ID: MMSA-2026-00578

CVE-2026-27659 Affected Github.com/mattermost/mattermost/server/v8 Versions

5 component(s)
Fixed Versions:
11.4.111.3.211.2.310.11.11
Product / Package Ecosystem Affected Versions Fixed Version
github.com/mattermost/mattermost/server/v8
golang/github.com/mattermost/mattermost/server/v8
Go
≥ 11.4.0-rc1
< 11.4.1
11.4.1
github.com/mattermost/mattermost/server/v8
golang/github.com/mattermost/mattermost/server/v8
Go
≥ 11.3.0-rc1
< 11.3.2
11.3.2
github.com/mattermost/mattermost/server/v8
golang/github.com/mattermost/mattermost/server/v8
Go
≥ 11.2.0-rc1
< 11.2.3
11.2.3
github.com/mattermost/mattermost/server/v8
golang/github.com/mattermost/mattermost/server/v8
Go
≥ 10.11.0-rc1
< 10.11.11
10.11.11
github.com/mattermost/mattermost/server/v8
golang/github.com/mattermost/mattermost/server/v8
Go
≥ 8.0.0-20260105080200-d27a2195068d
< 10.11.11
10.11.11

References

NVD — CVE-2026-27659 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-27659
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-2026-27659?
CVE-2026-27659 is a security vulnerability in Github.com/mattermost/mattermost/server/v8 involving Mattermost doesn't properly validate CSRF tokens under CWE-352: Cross-Site Request Forgery (CSRF).
Which Github.com/mattermost/mattermost/server/v8 versions are affected?
Affected versions include releases prior to 11.4.1, 11.3.2, 11.2.3, 10.11.11 across affected packages: github.com/mattermost/mattermost/server/v8.
What version fixes CVE-2026-27659?
Upgrading to 11.4.1, 11.3.2, 11.2.3, 10.11.11 (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 11.4.1, 11.3.2, 11.2.3, 10.11.11. Upgrade your dependency to 11.4.1, 11.3.2, 11.2.3, 10.11.11 or later.
What is the CVSS score of CVE-2026-27659?
CVE-2026-27659 has a CVSS v3.1 base score of 4.6 (MODERATE) with vector string CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:L/I:L/A:N.
What CWE is associated with CVE-2026-27659?
CVE-2026-27659 is classified under CWE-352: Cross-Site Request Forgery (CSRF) (The web application does not, or cannot, sufficiently verify whether a request was intentionally provided by the user who sent the request, which could have originated from an unauthorized actor.).
How can I check Go dependencies for CVE-2026-27659?
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-rmhw-c3xr-m3xx",
  "modified" : "2026-03-31T23:00:00Z",
  "published" : "2026-03-25T18:31:53Z",
  "aliases" : [ "CVE-2026-27659" ],
  "summary" : "Mattermost doesn't properly validate CSRF tokens",
  "details" : "Mattermost versions 11.2.x <= 11.2.2, 10.11.x <= 10.11.10, 11.4.x <= 11.4.0, 11.3.x <= 11.3.1 fail to properly validate CSRF tokens in the /api/v4/access_control_policies/{policy_id}/activate endpoint, which allows an attacker to trick an admin into changing access control policy active status via a crafted request. Mattermost Advisory ID: MMSA-2026-00578",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:L/I:L/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/mattermost/mattermost/server/v8"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "11.4.0-rc1"
      }, {
        "fixed" : "11.4.1"
      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "Go",
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    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
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      }, {
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      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/mattermost/mattermost/server/v8"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "11.2.0-rc1"
      }, {
        "fixed" : "11.2.3"
      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/mattermost/mattermost/server/v8"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "10.11.0-rc1"
      }, {
        "fixed" : "10.11.11"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "< 10.11.1"
    }
  }, {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/mattermost/mattermost/server/v8"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "8.0.0-20260105080200-d27a2195068d"
      }, {
        "fixed" : "10.11.11"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "< 8.0.0-20260217110922-b7d4a1f1f59b"
    }
  } ],
  "references" : [ {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-27659"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/mattermost/mattermost"
  }, {
    "type" : "WEB",
    "url" : "https://mattermost.com/security-updates"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-352" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-03-31T23:00:00Z",
    "nvd_published_at" : "2026-03-25T17:16:56Z"
  }
}