CVE-2026-3590Network attack vector

Guest-session multiplication via concurrent magic-link redemption

Published Apr 15, 2026 · Updated Apr 15, 2026

A TOCTOU race in affected Mattermost Server releases allows remote attackers to create multiple guest sessions from one magic link. Guest magic-link redemption checks and consumes a nominally single-use token without atomic enforcement, so concurrent requests can each obtain an independent authenticated session. Exploitation requires a valid, unexpired guest magic link and enabled guest magic-link login; resulting access remains limited to the invited guest's authorized channels.

CVSS severity6.5
Medium
EPSS probability0.14%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

A TOCTOU race in affected Mattermost Server releases allows remote attackers to create multiple guest sessions from one magic link. Guest magic-link redemption checks and consumes a nominally single-use token without atomic enforcement, so concurrent requests can each obtain an independent authenticated session. Exploitation requires a valid, unexpired guest magic link and enabled guest magic-link login; resulting access remains limited to the invited guest's authorized channels.

The record

CVE
CVE-2026-3590
Published
Apr 15, 2026
Updated
Apr 15, 2026
Vendor
Mattermost
Product
Mattermost
Classifications
CWE-367, T1078
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Apr 15, 2026CVE publishedPublication date reported by the CVE source.
  2. Apr 15, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=10.11.0 <=10.11.12
  2. Affected versionversion=11.3.0 <=11.3.2
  3. Affected versionversion=11.4.0 <=11.4.2
  4. Affected versionversion=11.5.0 <=11.5.0

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Mattermost · Mattermostversion=10.11.0 <=10.11.12; version=11.3.0 <=11.3.2; version=11.4.0 <=11.4.2; version=11.5.0 <=11.5.0

Published CVSS scores

6.5Mattermost, Inc.CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-367Time-of-check Time-of-use (TOCTOU) Race Condition
T1078Valid Accounts

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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