CVE-2025-12421
CWE-303Published: November 27, 2025· Updated: Jun 17, 2026
Official Description
Mattermost versions 11.0.x <= 11.0.2, 10.12.x <= 10.12.1, 10.11.x <= 10.11.4, 10.5.x <= 10.5.12 fail to to verify that the token used during the code exchange originates from the same authentication flow, which allows an authenticated user to perform account takeover via a specially crafted email address used when switching authentication methods and sending a request to the /users/login/sso/code-exchange endpoint. The vulnerability requires ExperimentalEnableAuthenticationTransfer to be enabled (default: enabled) and RequireEmailVerification to be disabled (default: disabled).
Risk Analysis
This critical vulnerability in Mattermost allows an authenticated user to take over accounts. The flaw occurs because the system fails to properly verify tokens during authentication method changes, enabling a malicious actor to craft a special email address to exploit the /users/login/sso/code-exchange endpoint. Its critical CVSS score of 9.9 indicates a severe risk to confidentiality, integrity, and availability.
No public exploit is currently known for this vulnerability. However, its network attack vector and low attack complexity suggest it could be remotely exploitable under specific configuration conditions.
Administrators should review their Mattermost configurations, specifically the ExperimentalEnableAuthenticationTransfer and RequireEmailVerification settings. Apply any available patches or upgrades for Mattermost Server to versions beyond 11.0.2, 10.12.1, 10.11.4, and 10.5.12.
Technical Analysis
CVE-2025-12421 can be exploited remotely over the network without requiring physical or adjacent access, significantly expanding the attack surface for threat actors.
Exploitation requires low privileges, which limits the exposure to scenarios where an attacker has already gained initial access.
A successful exploit results in complete confidentiality breach (data exposure), full integrity compromise (data manipulation), availability disruption (denial of service), with a CVSS base score of 9.9.
The vulnerability has a "Changed" scope, meaning successful exploitation can impact components beyond the vulnerable component itself — such as the host operating system or adjacent services.
CVSS v3.1 Vector Breakdown
Affected Vendors & Products
Exploit & PoC Resources
Official Patches & Advisories
All References (1)
Quick Facts
Related CVEs (CWE-303)
Recommended Actions
- →Apply vendor patches immediately
- →Monitor CVE-2025-12421 in threat intel feeds
- →Review IDS/IPS signatures for exploitation attempts