CVE-2026-27656: Github.com/mattermost/mattermost server Mattermost Allows Attackers To Take Vulnerability

MODERATE
Advisory Identifier: GHSA-fg35-5rf6-qg3g
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CVSS 5.7
CWE-303
Published
Advisory Summary

CVE-2026-27656 is a MODERATE (CVSS 5.7) mattermost allows attackers to take vulnerability (CWE-303: Incorrect Implementation of Authentication Algorithm) affecting Github.com/mattermost/mattermost server libraries in the Go ecosystem. The issue involves mattermost allows attackers to take over arbitrary user accounts via overly permissive substring matching flaw. Remediation is available by upgrading affected packages to version 11.4.1, 11.3.2, 11.2.4, 10.11.12, 8.0.0-20260217110922-b7d4a1f1f59b.

CVE-2026-27656 at a Glance

CVE Identifier CVE-2026-27656
GHSA Identifier GHSA-fg35-5rf6-qg3g
Severity MODERATE
CVSS v3.1 Base Score 5.7
CWE Classification CWE-303: Incorrect Implementation of Authentication Algorithm
Affected Product Github.com/mattermost/mattermost server
Package Ecosystem Go
Published Date
Last Updated

Security Impact

CVSS: 5.7

The security impact metrics below are parsed directly from the CVSS v3.1 vector string (CVSS:3.1/AV:N/AC:H/PR:H/UI:R/S:U/C:H/I:H/A:N).

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

How to Fix CVE-2026-27656

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: 11.4.1 
Upgrade github.com/mattermost/mattermost-server to 11.4.1
github.com/mattermost/mattermost-server fixed: 11.3.2 
Upgrade github.com/mattermost/mattermost-server to 11.3.2
github.com/mattermost/mattermost-server fixed: 11.2.4 
Upgrade github.com/mattermost/mattermost-server to 11.2.4
github.com/mattermost/mattermost-server fixed: 10.11.12 
Upgrade github.com/mattermost/mattermost-server to 10.11.12
github.com/mattermost/mattermost/server/v8 fixed: 8.0.0-20260217110922-b7d4a1f1f59b 
Upgrade github.com/mattermost/mattermost/server/v8 to 8.0.0-20260217110922-b7d4a1f1f59b
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-2026-27656, 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 11.4.1), 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-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, 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 11.4.x <= 11.4.0, 11.3.x <= 11.3.1, 11.2.x <= 11.2.3, 10.11.x <= 10.11.11 fail to properly validate user identity in the OpenID {{IsSameUser()}} comparison logic, which allows an attacker to take over arbitrary user accounts via an overly permissive substring matching flaw in the user discovery flow. Mattermost Advisory ID: MMSA-2026-00590

CVE-2026-27656 Affected Github.com/mattermost/mattermost server Versions

5 component(s)
Fixed Versions:
11.4.111.3.211.2.410.11.128.0.0-20260217110922-b7d4a1f1f59b
Product / Package Ecosystem Affected Versions Fixed Version
github.com/mattermost/mattermost-server
golang/github.com/mattermost/mattermost-server
Go
≥ 11.4.0-rc1
< 11.4.1
11.4.1
github.com/mattermost/mattermost-server
golang/github.com/mattermost/mattermost-server
Go
≥ 11.3.0-rc1
< 11.3.2
11.3.2
github.com/mattermost/mattermost-server
golang/github.com/mattermost/mattermost-server
Go
≥ 11.2.0-rc1
< 11.2.4
11.2.4
github.com/mattermost/mattermost-server
golang/github.com/mattermost/mattermost-server
Go
≥ 10.11.0-rc1
< 10.11.12
10.11.12
github.com/mattermost/mattermost/server/v8
golang/github.com/mattermost/mattermost/server/v8
Go
≥ 8.0.0-20260105080200-d27a2195068d
< 8.0.0-20260217110922-b7d4a1f1f59b
8.0.0-20260217110922-b7d4a1f1f59b

References

NVD — CVE-2026-27656 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-27656
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-2026-27656?
CVE-2026-27656 is a security vulnerability in Github.com/mattermost/mattermost server involving Mattermost allows attackers to take over arbitrary user accounts via overly permissive substring matching flaw under CWE-303: Incorrect Implementation of Authentication Algorithm.
Which Github.com/mattermost/mattermost server versions are affected?
Affected versions include releases prior to 11.4.1, 11.3.2, 11.2.4, 10.11.12, 8.0.0-20260217110922-b7d4a1f1f59b across affected packages: github.com/mattermost/mattermost-server, github.com/mattermost/mattermost/server/v8.
What version fixes CVE-2026-27656?
Upgrading to 11.4.1, 11.3.2, 11.2.4, 10.11.12, 8.0.0-20260217110922-b7d4a1f1f59b (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 11.4.1, 11.3.2, 11.2.4, 10.11.12. Upgrade your dependency to 11.4.1, 11.3.2, 11.2.4, 10.11.12 or later.
Is github.com/mattermost/mattermost/server/v8 affected?
Yes, github.com/mattermost/mattermost/server/v8 is affected in releases prior to 8.0.0-20260217110922-b7d4a1f1f59b. Upgrade your dependency to 8.0.0-20260217110922-b7d4a1f1f59b or later.
What is the CVSS score of CVE-2026-27656?
CVE-2026-27656 has a CVSS v3.1 base score of 5.7 (MODERATE) with vector string CVSS:3.1/AV:N/AC:H/PR:H/UI:R/S:U/C:H/I:H/A:N.
What CWE is associated with CVE-2026-27656?
CVE-2026-27656 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-2026-27656?
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-fg35-5rf6-qg3g",
  "modified" : "2026-03-31T05:19:17Z",
  "published" : "2026-03-25T18:31:53Z",
  "aliases" : [ "CVE-2026-27656" ],
  "summary" : "Mattermost allows attackers to take over arbitrary user accounts via overly permissive substring matching flaw",
  "details" : "Mattermost versions 11.4.x <= 11.4.0, 11.3.x <= 11.3.1, 11.2.x <= 11.2.3, 10.11.x <= 10.11.11 fail to properly validate user identity in the OpenID {{IsSameUser()}} comparison logic, which allows an attacker to take over arbitrary user accounts via an overly permissive substring matching flaw in the user discovery flow. Mattermost Advisory ID: MMSA-2026-00590",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:H/PR:H/UI:R/S:U/C:H/I:H/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/mattermost/mattermost-server"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "11.4.0-rc1"
      }, {
        "fixed" : "11.4.1"
      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/mattermost/mattermost-server"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "11.3.0-rc1"
      }, {
        "fixed" : "11.3.2"
      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/mattermost/mattermost-server"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "11.2.0-rc1"
      }, {
        "fixed" : "11.2.4"
      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/mattermost/mattermost-server"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "10.11.0-rc1"
      }, {
        "fixed" : "10.11.12"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "< 10.11.2"
    }
  }, {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/mattermost/mattermost/server/v8"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "8.0.0-20260105080200-d27a2195068d"
      }, {
        "fixed" : "8.0.0-20260217110922-b7d4a1f1f59b"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-27656"
  }, {
    "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" : "2026-03-31T05:19:17Z",
    "nvd_published_at" : "2026-03-25T17:16:56Z"
  }
}