CVE-2026-2455: Github.com/mattermost/mattermost/server/v8 Mattermost Fails To Canonicalize IPv4-mapped Vulnerability

MODERATE
Advisory Identifier: GHSA-gqv7-j2j8-qmwq
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CVSS 4.3
CWE-918
Published
Advisory Summary

CVE-2026-2455 is a MODERATE (CVSS 4.3) mattermost fails to canonicalize ipv4-mapped vulnerability (CWE-918: Server-Side Request Forgery (SSRF)) affecting Github.com/mattermost/mattermost/server/v8 libraries in the Go ecosystem. The issue involves mattermost fails to canonicalize ipv4-mapped ipv6 addresses before reserved ip validation. Remediation is available by upgrading affected packages to version 8.0.0-20260129133647-5d787969c2d5, 5.3.2-0.20260129133647-5d787969c2d5, 10.11.11, 11.2.3, 11.3.1.

CVE-2026-2455 at a Glance

CVE Identifier CVE-2026-2455
GHSA Identifier GHSA-gqv7-j2j8-qmwq
Severity MODERATE
CVSS v3.1 Base Score 4.3
CWE Classification CWE-918: Server-Side Request Forgery (SSRF)
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:L/I:N/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 None (I:N)
Availability None (A:N)

How to Fix CVE-2026-2455

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: 8.0.0-20260129133647-5d787969c2d5 
Upgrade github.com/mattermost/mattermost/server/v8 to 8.0.0-20260129133647-5d787969c2d5
github.com/mattermost/mattermost-server fixed: 5.3.2-0.20260129133647-5d787969c2d5 
Upgrade github.com/mattermost/mattermost-server to 5.3.2-0.20260129133647-5d787969c2d5
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-2026-2455, 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 8.0.0-20260129133647-5d787969c2d5), 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-918: Server-Side Request Forgery (SSRF), where the web server receives a url or similar request from an upstream component and retrieves the contents of this url, but it does not sufficiently ensure that the request is being sent to the expected destination. 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 canonicalize IPv4-mapped IPv6 addresses before reserved IP validation which allows an attacker to perform SSRF attacks against internal services via IPv4-mapped IPv6 literals (e.g., [::ffff:127.0.0.1]).. Mattermost Advisory ID: MMSA-2026-00585

CVE-2026-2455 Affected Github.com/mattermost/mattermost/server/v8 Versions

5 component(s)
Fixed Versions:
8.0.0-20260129133647-5d787969c2d55.3.2-0.20260129133647-5d787969c2d510.11.1111.2.311.3.1
Product / Package Ecosystem Affected Versions Fixed Version
github.com/mattermost/mattermost/server/v8
golang/github.com/mattermost/mattermost/server/v8
Go
< 8.0.0-20260129133647-5d787969c2d5
8.0.0-20260129133647-5d787969c2d5
github.com/mattermost/mattermost-server
golang/github.com/mattermost/mattermost-server
Go
< 5.3.2-0.20260129133647-5d787969c2d5
5.3.2-0.20260129133647-5d787969c2d5
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-2026-2455 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-2455
Security Fix Commit FIX COMMIT
https://github.com/mattermost/mattermost/commit/5d787969c2d5ab591a9dcd61b0810475eed7a646
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-2026-2455?
CVE-2026-2455 is a security vulnerability in Github.com/mattermost/mattermost/server/v8 involving Mattermost fails to canonicalize IPv4-mapped IPv6 addresses before reserved IP validation under CWE-918: Server-Side Request Forgery (SSRF).
Which Github.com/mattermost/mattermost/server/v8 versions are affected?
Affected versions include releases prior to 8.0.0-20260129133647-5d787969c2d5, 5.3.2-0.20260129133647-5d787969c2d5, 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-2026-2455?
Upgrading to 8.0.0-20260129133647-5d787969c2d5, 5.3.2-0.20260129133647-5d787969c2d5, 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 8.0.0-20260129133647-5d787969c2d5. Upgrade your dependency to 8.0.0-20260129133647-5d787969c2d5 or later.
Is github.com/mattermost/mattermost-server affected?
Yes, github.com/mattermost/mattermost-server is affected in releases prior to 5.3.2-0.20260129133647-5d787969c2d5, 10.11.11, 11.2.3, 11.3.1. Upgrade your dependency to 5.3.2-0.20260129133647-5d787969c2d5, 10.11.11, 11.2.3, 11.3.1 or later.
What is the CVSS score of CVE-2026-2455?
CVE-2026-2455 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:L/I:N/A:N.
What CWE is associated with CVE-2026-2455?
CVE-2026-2455 is classified under CWE-918: Server-Side Request Forgery (SSRF) (The web server receives a URL or similar request from an upstream component and retrieves the contents of this URL, but it does not sufficiently ensure that the request is being sent to the expected destination.).
How can I check Go dependencies for CVE-2026-2455?
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-gqv7-j2j8-qmwq",
  "modified" : "2026-03-18T16:08:56Z",
  "published" : "2026-03-16T15:30:47Z",
  "aliases" : [ "CVE-2026-2455" ],
  "summary" : "Mattermost fails to canonicalize IPv4-mapped IPv6 addresses before reserved IP validation",
  "details" : "Mattermost versions 11.3.x <= 11.3.0, 11.2.x <= 11.2.2, 10.11.x <= 10.11.10 fail to canonicalize IPv4-mapped IPv6 addresses before reserved IP validation which allows an attacker to perform SSRF attacks against internal services via IPv4-mapped IPv6 literals (e.g., [::ffff:127.0.0.1]).. Mattermost Advisory ID: MMSA-2026-00585",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:N"
  } ],
  "affected" : [ {
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    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "8.0.0-20260129133647-5d787969c2d5"
      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/mattermost/mattermost-server"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
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      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/mattermost/mattermost-server"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
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      }, {
        "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-2026-2455"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/mattermost/mattermost/commit/5d787969c2d5ab591a9dcd61b0810475eed7a646"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/mattermost/mattermost"
  }, {
    "type" : "WEB",
    "url" : "https://mattermost.com/security-updates"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-918" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-03-18T16:08:56Z",
    "nvd_published_at" : "2026-03-16T15:16:22Z"
  }
}