GO-2026-5495 CVE-2026-28741 GHSA-m7cf-4gh2-v4qg

Mattermost doesn't validate CSRF tokens on an authentication endpoint in github.com/mattermost/mattermost-server

High Go

Severity: 8.1 / 10; High · CVSS 3.1
Exploit chance (EPSS): 0.13%; Chance of exploitation in the next 30 days. Higher than 2.52% of scored vulnerabilities.
Known exploited: No; Not in CISA's catalog
Weakness: CWE-352 Cross-Site Request Forgery (CSRF)
Fix: Available; 2 packages have a fixed version

Description

Affected versions

Go

Package	Vulnerable	Fixed in
github.com/mattermost/mattermost-server	`>= 11.3.0-rc1+incompatible, < 11.3.3+incompatible` `>= 11.4.0-rc1+incompatible, < 11.4.3+incompatible` `>= 11.5.0-rc1+incompatible, < 11.5.0+incompatible`	`11.3.3+incompatible` `11.4.3+incompatible` `11.5.0+incompatible`
github.com/mattermost/mattermost-server/v5	`all versions`	No fix yet
github.com/mattermost/mattermost-server/v6	`all versions`	No fix yet
github.com/mattermost/mattermost/server/v8	`>= 8.0.0-20250721062209-4952acea88ce, < 8.0.0-20260220133927-c29cf05d40f8`	`8.0.0-20260220133927-c29cf05d40f8`
github.com/mattermost/mattermost/server/v8	`>= 11.5.0-rc1, < 11.5.0`	`11.5.0`
github.com/mattermost/mattermost/server/v8	`>= 11.4.0-rc1, < 11.4.3`	`11.4.3`
github.com/mattermost/mattermost/server/v8	`>= 11.3.0-rc1, < 11.3.3`	`11.3.3`

How to fix

Move to the fixed version or later with your package manager, update the lockfile, and run your tests. If the package only arrives through another dependency, upgrade that dependency first. If none of its releases pulls a fixed version yet, force the version with your package manager's override.

github.com/mattermost/mattermost-server Go → `11.5.0+incompatible`

go

go get github.com/mattermost/mattermost-server@v11.5.0+incompatible && go mod tidy

github.com/mattermost/mattermost/server/v8 Go → `8.0.0-20260220133927-c29cf05d40f8`

go

go get github.com/mattermost/mattermost/server/v8@v8.0.0-20260220133927-c29cf05d40f8 && go mod tidy

Severity breakdown

CVSS 3.1 8.1

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:N

Attack vector: Network
Attack complexity: Low
Privileges required: None
User interaction: Required
Scope: Unchanged
Confidentiality: High
Integrity: High
Availability: None

Weakness

CWE-352: Cross-Site Request Forgery (CSRF)

The web application does not, or cannot, sufficiently verify whether a request was intentionally provided by the user who sent the request, which could have originated from an unauthorized actor.

How to prevent it

Architecture and Design: Use a vetted library or framework that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid [REF-1482]. For example, use anti-CSRF packages such as the OWASP CSRFGuard. [REF-330] Another example is the ESAPI Session Management control, which includes a component for CSRF. [REF-45]
Implementation: Ensure that the application is free of cross-site scripting issues (CWE-79), because most CSRF defenses can be bypassed using attacker-controlled script.
Architecture and Design: Generate a unique nonce for each form, place the nonce into the form, and verify the nonce upon receipt of the form. Be sure that the nonce is not predictable (CWE-330). [REF-332]
Architecture and Design: Identify especially dangerous operations. When the user performs a dangerous operation, send a separate confirmation request to ensure that the user intended to perform that operation.
Architecture and Design: Use the "double-submitted cookie" method as described by Felten and Zeller: When a user visits a site, the site should generate a pseudorandom value and set it as a cookie on the user's machine. The site should require every form submission to include this value as a form value and also as a cookie value. When a POST request is sent to the site, the request should only be considered valid if the form value and the cookie value are the same. Because of the same-origin policy, an attacker cannot read or modify the value stored in the cookie. To successfully submit a form on behalf of the user, the attacker would have to correctly guess the pseudorandom value. If the pseudorandom value is cryptographically strong, this will be prohibitively difficult. This technique requires Javascript, so it may not work for browsers that have Javascript disabled. [REF-331]
Architecture and Design: Do not use the GET method for any request that triggers a state change.
Implementation: Check the HTTP Referer header to see if the request originated from an expected page. This could break legitimate functionality, because users or proxies may have disabled sending the Referer for privacy reasons.

References

Advisories

Packages

github.com/mattermost/mattermost

Timeline

Apr 15, 2026 Published
Jun 25, 2026 Last updated by the source
Nov 1, 2026 Latest EPSS score

Advisory	Severity	Why it's related
GHSA-xmfh-3ccg-c9fx Mattermost has an Incorrect Authorization issue	Low	Also affects github.com/mattermost/mattermost-server
GHSA-h998-hxxj-8q83 Mattermost doesn't invalidate cached authentication state for active WebSocket connections during global session revocation	Medium	Also affects github.com/mattermost/mattermost-server
GHSA-g5vr-6pgg-74qv Mattermost doesn't enforce bot-specific permission checks on the user active status endpoint	Low	Also affects github.com/mattermost/mattermost-server
GHSA-jqgv-39mg-7c2r Mattermost doesn't authenticate Atlassian Connect installed callbacks, allowing a remote unauthenticated attacker to inject a rogue sharedSecret	Medium	Also affects github.com/mattermost/mattermost-server
GHSA-mxq2-5jpg-7474 Mattermost doesn't validate channel ownership of an existing subscription before applying edits	Medium	Also affects github.com/mattermost/mattermost-server
GHSA-2g8v-grq3-hq2g Mattermost doesn't enforce administrator authorization on the {{setDefaultInstance}} call within the {{/gitlab connect}} command handler	Medium	Also affects github.com/mattermost/mattermost-server
GHSA-6hxm-w4hv-vgvw Mattermost doesn't require role-management authorization when setting the scheme_admin flag on group syncable link and patch endpoints	High	Also affects github.com/mattermost/mattermost-server
GHSA-9p44-r552-4wp9 Mattermost doesn't sanitize the Remote Cluster API response on PATCH operations	Medium	Also affects github.com/mattermost/mattermost-server

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