CVE-2025-12421Network attack vector

Account takeover via untyped token consumption

Published Nov 27, 2025 · Updated Feb 26, 2026

Improper authentication in Mattermost allows remote authenticated users to take over accounts through the SSO code-exchange endpoint. loginSSOCodeExchange passed the supplied login code to ConsumeTokenOnce without requiring the SSO code-exchange token type, so a token from another authentication flow was accepted. Exploitation requires authentication-transfer support to be enabled, email verification to be disabled, and a crafted email address during an authentication-method switch.

CVSS severity9.9
Critical
EPSS probability0.34%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Improper authentication in Mattermost allows remote authenticated users to take over accounts through the SSO code-exchange endpoint. loginSSOCodeExchange passed the supplied login code to ConsumeTokenOnce without requiring the SSO code-exchange token type, so a token from another authentication flow was accepted. Exploitation requires authentication-transfer support to be enabled, email verification to be disabled, and a crafted email address during an authentication-method switch.

The record

CVE
CVE-2025-12421
Published
Nov 27, 2025
Updated
Feb 26, 2026
Vendor
Mattermost
Product
Mattermost
Classifications
CWE-303, CWE-287, T1078
Attack vector
network
Privileges
authenticated

Timeline

How it unfolded

  1. Nov 27, 2025CVE publishedPublication date reported by the CVE source.
  2. Feb 26, 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 <=10.11.4
  2. Affected versionversion=10.12.0 <=10.12.1
  3. Affected versionversion=10.5.0 <=10.5.12
  4. Affected versionversion=11.0.0 <=11.0.2

What conditions does exploitation require?

Attack vectornetwork
Required privilegesauthenticated

What is affected?

Mattermost · Mattermostversion=10.11.0 <=10.11.4; version=10.12.0 <=10.12.1; version=10.5.0 <=10.5.12; version=11.0.0 <=11.0.2

Published CVSS scores

9.9MattermostCVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H

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
Privilegesauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-303Incorrect Implementation of Authentication Algorithm
CWE-287Improper Authentication
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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