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Mattermost has session spoofing due to lack of single-use consumption of guest magic link tokens enforcement

Moderate severity GitHub Reviewed Published Apr 17, 2026 to the GitHub Advisory Database • Updated Apr 28, 2026

Package

gomod github.com/mattermost/mattermost-server (Go)

Affected versions

>= 10.11.0-rc1, < 10.11.13
= 11.5.0-rc1
>= 11.4.0-rc1, < 11.4.3
>= 11.3.0-rc1, < 11.3.3

Patched versions

10.11.13
11.5.0
11.4.3
11.3.3
gomod github.com/mattermost/mattermost/server/v8 (Go)
>= 8.0.0-20250721062209-4952acea88ce, < 8.0.0-20250723052842-4cb8d8940332
8.0.0-20250723052842-4cb8d8940332

Description

Mattermost versions 10.11.x <= 10.11.12, 11.5.x <= 11.5.0, 11.4.x <= 11.4.2, 11.3.x <= 11.3.2 fail to enforce atomic single-use consumption of guest magic link tokens, which allows an attacker with access to a valid magic link to establish multiple independent authenticated sessions via concurrent requests. Mattermost Advisory ID: MMSA-2026-00624.

References

Published by the National Vulnerability Database Apr 15, 2026
Published to the GitHub Advisory Database Apr 17, 2026
Reviewed Apr 28, 2026
Last updated Apr 28, 2026

Severity

Moderate

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v3 base metrics

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

CVSS v3 base metrics

Attack vector: More severe the more the remote (logically and physically) an attacker can be in order to exploit the vulnerability.
Attack complexity: More severe for the least complex attacks.
Privileges required: More severe if no privileges are required.
User interaction: More severe when no user interaction is required.
Scope: More severe when a scope change occurs, e.g. one vulnerable component impacts resources in components beyond its security scope.
Confidentiality: More severe when loss of data confidentiality is highest, measuring the level of data access available to an unauthorized user.
Integrity: More severe when loss of data integrity is the highest, measuring the consequence of data modification possible by an unauthorized user.
Availability: More severe when the loss of impacted component availability is highest.
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N

EPSS score

Exploit Prediction Scoring System (EPSS)

This score estimates the probability of this vulnerability being exploited within the next 30 days. Data provided by FIRST.
(10th percentile)

Weaknesses

Time-of-check Time-of-use (TOCTOU) Race Condition

The product checks the state of a resource before using that resource, but the resource's state can change between the check and the use in a way that invalidates the results of the check. Learn more on MITRE.

CVE ID

CVE-2026-3590

GHSA ID

GHSA-mh4x-rmrx-3hp4

Source code

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