CVE-2026-5740: Github.com/mattermost/mattermost server Mattermost Doesn't Properly Validate Msgpack-encoded Vulnerability
HIGHCVE-2026-5740 is a HIGH (CVSS 7.5) mattermost doesn't properly validate msgpack-encoded vulnerability (CWE-789: Memory Allocation with Excessive Size Value) affecting Github.com/mattermost/mattermost server libraries in the Go ecosystem. The issue involves mattermost doesn't properly validate msgpack-encoded websocket frames before memory allocation. Remediation is available by upgrading affected packages to version 11.6.1, 11.5.4, 11.4.5, 10.11.15, 8.0.0-20260410202636-17939826efa2.
CVE-2026-5740 at a Glance
Security Impact
CVSS: 7.5
The security impact metrics below are parsed directly from the CVSS v3.1 vector string (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H).
How to Fix CVE-2026-5740
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.
Upgrade github.com/mattermost/mattermost-server to 11.6.1Upgrade github.com/mattermost/mattermost-server to 11.5.4Upgrade github.com/mattermost/mattermost-server to 11.4.5Upgrade github.com/mattermost/mattermost-server to 10.11.15Upgrade github.com/mattermost/mattermost/server/v8 to 8.0.0-20260410202636-17939826efa2Run the following command in your project directory to verify that all resolved dependencies use the patched release:
inspect dependencies for github.com/mattermost/mattermost-serverHow to Check if You Are Affected
To determine whether your application is affected by CVE-2026-5740, inspect your project dependencies for Github.com/mattermost/mattermost server:
inspect dependencies for github.com/mattermost/mattermost-serverAnalyze 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 11.6.1), 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-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, 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-2026-5740 Affected Github.com/mattermost/mattermost server Versions
5 component(s)| Product / Package | Ecosystem | Affected Versions | Fixed Version |
|---|---|---|---|
|
github.com/mattermost/mattermost-server
|
Go |
≥ 11.6.0
< 11.6.1
All 1 versions
11.6.0
|
11.6.1
|
|
github.com/mattermost/mattermost-server
|
Go |
≥ 11.5.0
< 11.5.4
|
11.5.4
|
|
github.com/mattermost/mattermost-server
|
Go |
≥ 11.4.0
< 11.4.5
|
11.4.5
|
|
github.com/mattermost/mattermost-server
|
Go |
≥ 10.11.0
< 10.11.15
|
10.11.15
|
|
github.com/mattermost/mattermost/server/v8
|
Go |
< 8.0.0-20260410202636-17939826efa2
|
8.0.0-20260410202636-17939826efa2
|
References
Frequently Asked Questions
What is CVE-2026-5740?
Which Github.com/mattermost/mattermost server versions are affected?
What version fixes CVE-2026-5740?
Is github.com/mattermost/mattermost-server affected?
Is github.com/mattermost/mattermost/server/v8 affected?
What is the CVSS score of CVE-2026-5740?
What CWE is associated with CVE-2026-5740?
How can I check Go dependencies for CVE-2026-5740?
Machine-readable OSV JSON
OSV v1.5 Schema
{
"schema_version" : "1.4.0",
"id" : "GHSA-w9m8-p4cc-4qj9",
"modified" : "2026-06-29T22:52:00Z",
"published" : "2026-05-26T13:30:15Z",
"aliases" : [ "CVE-2026-5740" ],
"summary" : "Mattermost doesn't properly validate msgpack-encoded WebSocket frames before memory allocation",
"details" : "Mattermost versions 11.6.x <= 11.6.0, 11.5.x <= 11.5.3, 11.4.x <= 11.4.4, 10.11.x <= 10.11.14 fail to properly validate msgpack-encoded WebSocket frames before memory allocation which allows an unauthenticated remote attacker to crash the server process and cause a full service outage for all users via a crafted binary WebSocket message sent to the public WebSocket endpoint.. Mattermost Advisory ID: MMSA-2026-00647",
"severity" : [ {
"type" : "CVSS_V3",
"score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"
} ],
"affected" : [ {
"package" : {
"ecosystem" : "Go",
"name" : "github.com/mattermost/mattermost-server"
},
"ranges" : [ {
"type" : "ECOSYSTEM",
"events" : [ {
"introduced" : "11.6.0"
}, {
"fixed" : "11.6.1"
} ]
} ],
"versions" : [ "11.6.0" ]
}, {
"package" : {
"ecosystem" : "Go",
"name" : "github.com/mattermost/mattermost-server"
},
"ranges" : [ {
"type" : "ECOSYSTEM",
"events" : [ {
"introduced" : "11.5.0"
}, {
"fixed" : "11.5.4"
} ]
} ]
}, {
"package" : {
"ecosystem" : "Go",
"name" : "github.com/mattermost/mattermost-server"
},
"ranges" : [ {
"type" : "ECOSYSTEM",
"events" : [ {
"introduced" : "11.4.0"
}, {
"fixed" : "11.4.5"
} ]
} ]
}, {
"package" : {
"ecosystem" : "Go",
"name" : "github.com/mattermost/mattermost-server"
},
"ranges" : [ {
"type" : "ECOSYSTEM",
"events" : [ {
"introduced" : "10.11.0"
}, {
"fixed" : "10.11.15"
} ]
} ]
}, {
"package" : {
"ecosystem" : "Go",
"name" : "github.com/mattermost/mattermost/server/v8"
},
"ranges" : [ {
"type" : "ECOSYSTEM",
"events" : [ {
"introduced" : "0"
}, {
"fixed" : "8.0.0-20260410202636-17939826efa2"
} ]
} ]
} ],
"references" : [ {
"type" : "ADVISORY",
"url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-5740"
}, {
"type" : "WEB",
"url" : "https://github.com/mattermost/mattermost/commit/17939826efa20a97f087b3d390ec5136df350bae"
}, {
"type" : "PACKAGE",
"url" : "https://github.com/mattermost/mattermost"
}, {
"type" : "WEB",
"url" : "https://mattermost.com/security-updates"
} ],
"database_specific" : {
"cwe_ids" : [ "CWE-789" ],
"severity" : "HIGH",
"github_reviewed" : true,
"github_reviewed_at" : "2026-06-29T22:52:00Z",
"nvd_published_at" : "2026-05-22T11:16:23Z"
}
}