OAuth2 Token Misuse in Dovecot Authentication Bypass

Executive Summary

This vulnerability involves OAuth2 token handling in Dovecot. When a token lacks a scope claim, the system incorrectly uses the audience claim instead to check permissions. This allows an attacker with a token meant for a different purpose to authenticate if the audience claim matches a configured scope name. It also masks misconfigurations where scopes are not issued by the identity provider.

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Detection Guidance

Check OAuth2 token responses for missing scope claims. Verify if audience claims are incorrectly used in place of scopes. Inspect Dovecot logs for authentication attempts using tokens without proper scope validation.

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Impact Analysis

An attacker could gain unauthorized access to systems or data protected by Dovecot if they possess a token with a matching audience claim. This could lead to data breaches, privilege escalation, or bypassing intended security controls. The impact depends on the token's intended permissions and the system's configuration.

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Compliance Impact

This vulnerability could lead to unauthorized access to sensitive data, violating principles of least privilege and data protection. It may result in non-compliance with GDPR (data protection), HIPAA (health information), or other regulations requiring strict access controls and auditability.

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Mitigation Strategies

Update Dovecot to a non-vulnerable version. Ensure the identity provider issues a scope claim for all tokens used with Dovecot. Verify configured scope names do not match audience values to prevent misconfiguration.

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