CVE-2026-6340: Github.com/mattermost/mattermost/server/v8 Mattermost Doesn't Validate 7zip Archive Vulnerability
MODERATECVE-2026-6340 is a MODERATE (CVSS 4.3) mattermost doesn't validate 7zip archive 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 doesn't validate 7zip archive structure before processing. Remediation is available by upgrading affected packages to version 11.5.2, 10.11.14, 11.4.4, 8.0.0-20260325191733-fb11968f8798, 5.3.2-0.20260325191733-fb11968f8798.
CVE-2026-6340 at a Glance
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).
How to Fix CVE-2026-6340
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.
Upgrade github.com/mattermost/mattermost/server/v8 to 11.5.2Upgrade github.com/mattermost/mattermost/server/v8 to 10.11.14Upgrade github.com/mattermost/mattermost/server/v8 to 11.4.4Upgrade github.com/mattermost/mattermost/server/v8 to 8.0.0-20260325191733-fb11968f8798Upgrade github.com/mattermost/mattermost-server to 5.3.2-0.20260325191733-fb11968f8798Run 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/v8How to Check if You Are Affected
To determine whether your application is affected by CVE-2026-6340, inspect your project dependencies for Github.com/mattermost/mattermost/server/v8:
inspect dependencies for github.com/mattermost/mattermost/server/v8Analyze 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 11.5.2), 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
CVE-2026-6340 Affected Github.com/mattermost/mattermost/server/v8 Versions
5 component(s)| Product / Package | Ecosystem | Affected Versions | Fixed Version |
|---|---|---|---|
|
github.com/mattermost/mattermost/server/v8
|
Go |
≥ 11.5.0
< 11.5.2
|
11.5.2
|
|
github.com/mattermost/mattermost/server/v8
|
Go |
≥ 10.11.0
< 10.11.14
|
10.11.14
|
|
github.com/mattermost/mattermost/server/v8
|
Go |
≥ 11.4.0
< 11.4.4
|
11.4.4
|
|
github.com/mattermost/mattermost/server/v8
|
Go |
< 8.0.0-20260325191733-fb11968f8798
|
8.0.0-20260325191733-fb11968f8798
|
|
github.com/mattermost/mattermost-server
|
Go |
< 5.3.2-0.20260325191733-fb11968f8798
|
5.3.2-0.20260325191733-fb11968f8798
|
References
Frequently Asked Questions
What is CVE-2026-6340?
Which Github.com/mattermost/mattermost/server/v8 versions are affected?
What version fixes CVE-2026-6340?
Is github.com/mattermost/mattermost/server/v8 affected?
Is github.com/mattermost/mattermost-server affected?
What is the CVSS score of CVE-2026-6340?
What CWE is associated with CVE-2026-6340?
How can I check Go dependencies for CVE-2026-6340?
Machine-readable OSV JSON
OSV v1.5 Schema
{
"schema_version" : "1.4.0",
"id" : "GHSA-cjm8-jxpw-g43m",
"modified" : "2026-06-01T15:09:30Z",
"published" : "2026-05-18T09:31:47Z",
"aliases" : [ "CVE-2026-6340" ],
"summary" : "Mattermost doesn't validate 7zip archive structure before processing",
"details" : "Mattermost versions 11.5.x <= 11.5.1, 10.11.x <= 10.11.13, 11.4.x <= 11.4.3 fail to validate 7zip archive structure before processing which allows an authenticated attacker to cause server memory exhaustion and denial of service via uploading a specially crafted 7zip file with excessive folder declarations.. Mattermost Advisory ID: MMSA-2026-00573",
"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" : "11.5.0"
}, {
"fixed" : "11.5.2"
} ]
} ]
}, {
"package" : {
"ecosystem" : "Go",
"name" : "github.com/mattermost/mattermost/server/v8"
},
"ranges" : [ {
"type" : "ECOSYSTEM",
"events" : [ {
"introduced" : "10.11.0"
}, {
"fixed" : "10.11.14"
} ]
} ]
}, {
"package" : {
"ecosystem" : "Go",
"name" : "github.com/mattermost/mattermost/server/v8"
},
"ranges" : [ {
"type" : "ECOSYSTEM",
"events" : [ {
"introduced" : "11.4.0"
}, {
"fixed" : "11.4.4"
} ]
} ]
}, {
"package" : {
"ecosystem" : "Go",
"name" : "github.com/mattermost/mattermost/server/v8"
},
"ranges" : [ {
"type" : "ECOSYSTEM",
"events" : [ {
"introduced" : "0"
}, {
"fixed" : "8.0.0-20260325191733-fb11968f8798"
} ]
} ]
}, {
"package" : {
"ecosystem" : "Go",
"name" : "github.com/mattermost/mattermost-server"
},
"ranges" : [ {
"type" : "ECOSYSTEM",
"events" : [ {
"introduced" : "0"
}, {
"fixed" : "5.3.2-0.20260325191733-fb11968f8798"
} ]
} ]
} ],
"references" : [ {
"type" : "ADVISORY",
"url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-6340"
}, {
"type" : "WEB",
"url" : "https://github.com/mattermost/mattermost/commit/fb11968f8798925c7b75711025bc5f991124ba26"
}, {
"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" : "2026-06-01T15:09:30Z",
"nvd_published_at" : "2026-05-18T08:16:14Z"
}
}