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How the CVE Works:
This vulnerability exploits a flaw in the OpenID Connect (OIDC) authentication flow of Mattermost. The application fails to properly validate the `state` parameter returned by the OIDC identity provider (IdP). An authenticated attacker with team creation privileges can initiate an OAuth login for a target victim user. By manipulating the authentication request, the attacker can cause the Mattermost server to associate the returning OAuth authorization code and session with their own attacker-controlled user account instead of the intended victim’s. This is achieved by intercepting or crafting the OAuth callback so that the server processes it in the context of the attacker’s active session. A successful exploit requires specific conditions: email verification disabled, OIDC enabled, and the attacker controlling two linked SSO users where one has never logged into Mattermost before, allowing account linkage hijacking.
DailyCVE Form:
Platform: Mattermost Team Edition
Version: <=11.0.3
Vulnerability: OAuth Account Takeover
Severity: Critical
Date: 2025-11-27
Prediction: Patch 2025-12-11
What Undercode Say:
Analytics:
Check Mattermost version curl -s http://localhost:8065/api/v4/system/ping | jq .version Grep for OAuth endpoint patterns in source grep -r "oauth/authorize" /opt/mattermost/ grep -r "state.validation" /opt/mattermost/server/ Simulate OIDC callback (conceptual) curl -X POST 'http://target/mm/oauth/openid/callback' \ -H 'Cookie: MMAUTHTOKEN=<attacker_token>' \ --data 'code=<stolen_auth_code>&state=<malicious_state>'
How Exploit:
1. Attacker with team creation rights logs in.
- Initiates OIDC login for a victim SSO user (who has never logged into Mattermost).
3. Captures or predicts OAuth `state` parameter.
- Tricks system into linking victim’s SSO identity to attacker’s Mattermost session via flawed state validation in callback.
- Attacker gains full access to the victim’s Mattermost account and data.
Protection from this CVE:
- Immediately upgrade to Mattermost versions patched for CVE-2025-12419.
2. Enable email verification for account creation.
- Implement strict OAuth `state` parameter validation using cryptographically secure nonces.
4. Review and audit all OIDC integrations.
- Apply principle of least privilege for team creation roles.
Impact:
Complete account takeover, unauthorized data access, privilege escalation, potential data exfiltration, and session hijacking.
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Sources:
Reported By: nvd.nist.gov
Extra Source Hub:
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