CVE-2026-73208 in Dovecot Pro
Summary
by MITRE • 08/28/2026
An attacker that holds a token intended for a different purpose can authenticate, because when an OAuth2 token response does not contain a scope claim, the audience claim is used in its place and checked against the configured required scopes. These are different concepts, and the audience claim does not describe what a token is allowed to do. A token that grants no relevant permissions can be accepted because its intended recipient value happens to match a configured scope name, granting access that should have been denied. It also hides an identity provider misconfiguration where scopes are not being issued at all. Ensure the identity provider issues a scope claim for all tokens used with Dovecot, and that configured scope names do not match audience values. Update to non-vulnerable version. No publicly available exploits are known.
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Analysis
by VulDB Data Team • 08/28/2026
The vulnerability described involves a critical flaw in how an OAuth2-based authentication system handles token validation when the expected scope claim is missing from the response. In standard OAuth2 implementations, scopes define the specific permissions or access levels granted to a client application by the identity provider. These scopes are essential for enforcing fine-grained authorization policies within the relying party service. However, in this scenario, if the identity provider fails to include a scope claim in the token response, the system falls back to using the audience claim as a substitute for validation against configured required scopes. This fallback mechanism is fundamentally flawed because it conflates two distinct security concepts: who the token is intended for (audience) and what actions the token permits (scope).
The technical root cause lies in the misinterpretation of these claims during the authentication process. The audience claim identifies the recipient of the ID token or access token, ensuring that a token issued to one service cannot be used with another unrelated service. In contrast, scopes delineate the extent of access rights. By substituting the missing scope claim with the audience value for authorization checks, the system allows any token whose intended recipient matches a configured scope name to gain access. This creates a situation where an attacker possessing a valid but overly permissive or misconfigured token can authenticate successfully even if they lack the specific permissions required by the application logic. For instance, if a service is configured to require a scope named "read-only" and another service uses that same string as its audience identifier, a token intended for the second service might be accepted by the first due to this claim substitution error.
This vulnerability leads to significant operational impacts related to unauthorized access and privilege escalation. An attacker who holds a token meant for a different purpose can bypass authorization controls simply because of how claims are mapped during validation. This not only compromises the confidentiality and integrity of protected resources but also masks deeper configuration issues within the identity provider infrastructure. Specifically, it indicates that scopes are not being issued at all by the identity provider, which suggests a broader misconfiguration where proper access control policies may be entirely absent or ineffective across multiple services relying on this authentication flow.
From a threat modeling perspective, this flaw aligns with CWE-284 Improper Access Control and CWE-613 Insufficient Session Expiration, as it allows unauthorized session establishment based on flawed claim validation. It also relates to ATT&CK technique T1078 Valid Accounts, where an attacker leverages legitimate credentials or tokens that have been misconfigured or improperly validated to gain initial access. The lack of proper scope enforcement means the system fails to implement least privilege principles effectively, allowing lateral movement and data exfiltration under the guise of valid authentication.
To mitigate this vulnerability, organizations must ensure that their identity provider is correctly configured to issue explicit scope claims for all tokens used with services like Dovecot. It is imperative to audit token issuance policies to guarantee that scopes are present in every response. Furthermore, administrators should verify that configured scope names do not overlap or match audience values of other services within the ecosystem. This separation ensures that authorization decisions rely on actual permission grants rather than incidental string matches. Additionally, updating to a non-vulnerable version of the affected software is necessary if such an update exists and addresses this specific claim handling logic. Until patches are applied, monitoring for anomalous authentication patterns involving tokens with missing scope claims can help detect potential exploitation attempts in real time.