CVE-2026-26246: Github.com/mattermost/mattermost/server/v8 Mattermost Fails To Bound Memory Vulnerability

MODERATE
Advisory Identifier: GHSA-44mv-jq72-gj49
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CVSS 4.3
CWE-789
Published
Advisory Summary

CVE-2026-26246 is a MODERATE (CVSS 4.3) mattermost fails to bound memory vulnerability (CWE-789: Memory Allocation with Excessive Size Value) affecting Github.com/mattermost/mattermost/server/v8 libraries in the Go ecosystem. The issue involves mattermost fails to bound memory allocation when processing psd image files. Remediation is available by upgrading affected packages to version 8.0.0-20260115183946-38b413a27604, 5.3.2-0.20260115183946-38b413a27604, 10.11.11, 11.2.3, 11.3.1.

CVE-2026-26246 at a Glance

CVE Identifier CVE-2026-26246
GHSA Identifier GHSA-44mv-jq72-gj49
Severity MODERATE
CVSS v3.1 Base Score 4.3
CWE Classification CWE-789: Memory Allocation with Excessive Size Value
Affected Product Github.com/mattermost/mattermost/server/v8
Package Ecosystem Go
Published Date
Last Updated

Security Impact

CVSS: 4.3

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

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

How to Fix CVE-2026-26246

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: 8.0.0-20260115183946-38b413a27604 
Upgrade github.com/mattermost/mattermost/server/v8 to 8.0.0-20260115183946-38b413a27604
github.com/mattermost/mattermost-server fixed: 5.3.2-0.20260115183946-38b413a27604 
Upgrade github.com/mattermost/mattermost-server to 5.3.2-0.20260115183946-38b413a27604
github.com/mattermost/mattermost-server fixed: 10.11.11 
Upgrade github.com/mattermost/mattermost-server to 10.11.11
github.com/mattermost/mattermost-server fixed: 11.2.3 
Upgrade github.com/mattermost/mattermost-server to 11.2.3
github.com/mattermost/mattermost-server fixed: 11.3.1 
Upgrade github.com/mattermost/mattermost-server to 11.3.1
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-2026-26246, 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 8.0.0-20260115183946-38b413a27604), 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-789: Memory Allocation with Excessive Size Value, where the product allocates memory based on an untrusted, large size value, but it does not ensure that the size is within expected limits, allowing arbitrary amounts of memory to be allocated. 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 11.3.x <= 11.3.0, 11.2.x <= 11.2.2, 10.11.x <= 10.11.10 fail to bound memory allocation when processing PSD image files which allows an authenticated attacker to cause server memory exhaustion and denial of service via uploading a specially crafted PSD file. Mattermost Advisory ID: MMSA-2026-00572

CVE-2026-26246 Affected Github.com/mattermost/mattermost/server/v8 Versions

5 component(s)
Fixed Versions:
8.0.0-20260115183946-38b413a276045.3.2-0.20260115183946-38b413a2760410.11.1111.2.311.3.1
Product / Package Ecosystem Affected Versions Fixed Version
github.com/mattermost/mattermost/server/v8
golang/github.com/mattermost/mattermost/server/v8
Go
< 8.0.0-20260115183946-38b413a27604
8.0.0-20260115183946-38b413a27604
github.com/mattermost/mattermost-server
golang/github.com/mattermost/mattermost-server
Go
< 5.3.2-0.20260115183946-38b413a27604
5.3.2-0.20260115183946-38b413a27604
github.com/mattermost/mattermost-server
golang/github.com/mattermost/mattermost-server
Go
≥ 10.11.0-rc1
< 10.11.11
10.11.11
github.com/mattermost/mattermost-server
golang/github.com/mattermost/mattermost-server
Go
≥ 11.2.0-rc1
< 11.2.3
11.2.3
github.com/mattermost/mattermost-server
golang/github.com/mattermost/mattermost-server
Go
≥ 11.3.0-rc1
< 11.3.1
11.3.1

References

NVD — CVE-2026-26246 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-26246
Security Fix Commit FIX COMMIT
https://github.com/mattermost/mattermost/commit/38b413a27604e8721fbe008f8ec4b4e6c47ad4f0
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-2026-26246?
CVE-2026-26246 is a security vulnerability in Github.com/mattermost/mattermost/server/v8 involving Mattermost fails to bound memory allocation when processing PSD image files under CWE-789: Memory Allocation with Excessive Size Value.
Which Github.com/mattermost/mattermost/server/v8 versions are affected?
Affected versions include releases prior to 8.0.0-20260115183946-38b413a27604, 5.3.2-0.20260115183946-38b413a27604, 10.11.11, 11.2.3, 11.3.1 across affected packages: github.com/mattermost/mattermost/server/v8, github.com/mattermost/mattermost-server.
What version fixes CVE-2026-26246?
Upgrading to 8.0.0-20260115183946-38b413a27604, 5.3.2-0.20260115183946-38b413a27604, 10.11.11, 11.2.3, 11.3.1 (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 8.0.0-20260115183946-38b413a27604. Upgrade your dependency to 8.0.0-20260115183946-38b413a27604 or later.
Is github.com/mattermost/mattermost-server affected?
Yes, github.com/mattermost/mattermost-server is affected in releases prior to 5.3.2-0.20260115183946-38b413a27604, 10.11.11, 11.2.3, 11.3.1. Upgrade your dependency to 5.3.2-0.20260115183946-38b413a27604, 10.11.11, 11.2.3, 11.3.1 or later.
What is the CVSS score of CVE-2026-26246?
CVE-2026-26246 has a CVSS v3.1 base score of 4.3 (MODERATE) with vector string CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L.
What CWE is associated with CVE-2026-26246?
CVE-2026-26246 is classified under CWE-789: Memory Allocation with Excessive Size Value (The product allocates memory based on an untrusted, large size value, but it does not ensure that the size is within expected limits, allowing arbitrary amounts of memory to be allocated.).
How can I check Go dependencies for CVE-2026-26246?
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-44mv-jq72-gj49",
  "modified" : "2026-04-27T14:19:12Z",
  "published" : "2026-03-16T15:30:42Z",
  "aliases" : [ "CVE-2026-26246" ],
  "summary" : "Mattermost fails to bound memory allocation when processing PSD image files",
  "details" : "Mattermost versions 11.3.x <= 11.3.0, 11.2.x <= 11.2.2, 10.11.x <= 10.11.10 fail to bound memory allocation when processing PSD image files which allows an authenticated attacker to cause server memory exhaustion and denial of service via uploading a specially crafted PSD file. Mattermost Advisory ID: MMSA-2026-00572",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/mattermost/mattermost/server/v8"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "8.0.0-20260115183946-38b413a27604"
      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/mattermost/mattermost-server"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "5.3.2-0.20260115183946-38b413a27604"
      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/mattermost/mattermost-server"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "10.11.0-rc1"
      }, {
        "fixed" : "10.11.11"
      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/mattermost/mattermost-server"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "11.2.0-rc1"
      }, {
        "fixed" : "11.2.3"
      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/mattermost/mattermost-server"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "11.3.0-rc1"
      }, {
        "fixed" : "11.3.1"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-26246"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/mattermost/mattermost/commit/38b413a27604e8721fbe008f8ec4b4e6c47ad4f0"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/mattermost/mattermost"
  }, {
    "type" : "WEB",
    "url" : "https://mattermost.com/security-updates"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-789" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-03-17T19:59:53Z",
    "nvd_published_at" : "2026-03-16T14:18:26Z"
  }
}