CVE-2026-27656: Github.com/mattermost/mattermost server Mattermost Allows Attackers To Take Vulnerability
MODERATECVE-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
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).
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.
Upgrade github.com/mattermost/mattermost-server to 11.4.1Upgrade github.com/mattermost/mattermost-server to 11.3.2Upgrade github.com/mattermost/mattermost-server to 11.2.4Upgrade github.com/mattermost/mattermost-server to 10.11.12Upgrade github.com/mattermost/mattermost/server/v8 to 8.0.0-20260217110922-b7d4a1f1f59bRun 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-2026-27656, 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 11.4.1), 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
CVE-2026-27656 Affected Github.com/mattermost/mattermost server Versions
5 component(s)| Product / Package | Ecosystem | Affected Versions | Fixed Version |
|---|---|---|---|
|
github.com/mattermost/mattermost-server
|
Go |
≥ 11.4.0-rc1
< 11.4.1
|
11.4.1
|
|
github.com/mattermost/mattermost-server
|
Go |
≥ 11.3.0-rc1
< 11.3.2
|
11.3.2
|
|
github.com/mattermost/mattermost-server
|
Go |
≥ 11.2.0-rc1
< 11.2.4
|
11.2.4
|
|
github.com/mattermost/mattermost-server
|
Go |
≥ 10.11.0-rc1
< 10.11.12
|
10.11.12
|
|
github.com/mattermost/mattermost/server/v8
|
Go |
≥ 8.0.0-20260105080200-d27a2195068d
< 8.0.0-20260217110922-b7d4a1f1f59b
|
8.0.0-20260217110922-b7d4a1f1f59b
|
References
Frequently Asked Questions
What is CVE-2026-27656?
Which Github.com/mattermost/mattermost server versions are affected?
What version fixes CVE-2026-27656?
Is github.com/mattermost/mattermost-server affected?
Is github.com/mattermost/mattermost/server/v8 affected?
What is the CVSS score of CVE-2026-27656?
What CWE is associated with CVE-2026-27656?
How can I check Go dependencies for CVE-2026-27656?
Machine-readable OSV JSON
OSV v1.5 Schema
{
"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"
}
}