CVE-2026-77560 in Tinyauth
Summary
by MITRE • 09/21/2026
Tinyauth is an authentication and authorization server. Prior to 5.1.2, Tinyauth compares forwarded hostnames case-sensitively while reverse proxies route equivalent hostnames case-insensitively, allowing an authenticated low-privilege user to bypass per-app access controls with a differently cased hostname. The lookup in internal/service/access_controls_service.go through lookupStaticACLs and GetAccessControls, and the Docker-label fallback in internal/service/docker_service.go through GetLabels, can miss the configured app and return an empty access-control object. internal/controller/proxy_controller.go proxyHandler then treats the empty user, group, OAuth, LDAP, and IP restrictions as permissive and returns an authenticated result for an app that should exclude the user. Unauthenticated users remain subject to login, and global login-time allowlists are not bypassed. This issue is fixed in version 5.1.2.
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Analysis
by VulDB Data Team • 09/21/2026
The vulnerability identified in Tinyauth prior to version 5.1.2 stems from a fundamental mismatch between case-sensitive application logic and the case-insensitive nature of standard HTTP hostname routing protocols. As an authentication and authorization server, Tinyauth relies on accurate identification of target applications to enforce granular access controls. However, when reverse proxies route requests, they typically normalize hostnames to lowercase or handle them in a case-insensitive manner according to RFC standards. The internal service logic within Tinyauth failed to account for this normalization, performing strict case-sensitive comparisons against configured application identifiers. This discrepancy creates an opening where the system fails to correctly map incoming requests with non-standard casing to their corresponding security policies.
The technical flaw is rooted in specific code paths involving access control lookups and Docker label resolution. When a request arrives, the system invokes lookupStaticACLs and GetAccessControls within internal/service/access_controls_service.go to determine permissions. Simultaneously, it attempts to resolve application context via GetLabels in internal/service/docker_service.go using Docker-label fallback mechanisms. Because these functions perform case-sensitive matching against hostnames that may have been altered by upstream proxies or clients, they frequently fail to locate the correct configuration entry. Consequently, instead of retrieving a restrictive access control object tailored for specific users or groups, the system returns an empty access-control structure due to the lookup failure.
This erroneous return value triggers a critical logic error in the proxy handler located within internal/controller/proxy_controller.go. The design assumption that underpins this component is that an empty or null set of restrictions implies permissive access by default rather than restrictive denial. As a result, when the system fails to find matching ACLs due to hostname casing issues, it treats the absence of explicit user, group, OAuth, LDAP, and IP restrictions as permission for all actions. This allows authenticated low-privilege users who possess valid credentials but lack specific app-level permissions to bypass those controls simply by manipulating the case of the target hostname in their requests.
The operational impact is significant for environments relying on Tinyauth for fine-grained application isolation. An attacker with a standard user account can escalate privileges within specific applications that should be restricted, potentially accessing sensitive data or performing unauthorized actions. It is important to note that this vulnerability does not affect global authentication mechanisms; unauthenticated users are still required to log in, and global login-time allowlists remain intact. The bypass is strictly limited to per-application access controls after initial authentication has been successfully completed. This distinction limits the scope of the attack but remains critical for maintaining data integrity within multi-tenant or microservice architectures where strict isolation between applications is mandatory.
From a classification perspective, this vulnerability aligns with CWE-284 Improper Access Control and CWE-15 External Control of System or Configuration Setting. The failure to normalize input before comparison represents a classic logic error in security enforcement layers. In the context of the MITRE ATT&CK framework, this behavior facilitates lateral movement within an application ecosystem by allowing users to access resources they are not authorized for, potentially categorized under T1078 Valid Accounts if combined with credential theft or simply as unauthorized resource access via misconfigured permissions.
To mitigate this risk, organizations must upgrade Tinyauth to version 5.1.2 or later, where the hostname comparison logic has been corrected to handle case-insensitivity appropriately. Until an upgrade is feasible, administrators should enforce strict lowercase conventions for all hostnames in their reverse proxy configurations and application definitions to ensure consistency between the client request and internal lookup keys. Additionally, reviewing access control policies to avoid relying on empty-state defaults as permissive can provide a layer of defense-in-depth against similar logic flaws in other components. Regular audits of authentication logs should also be conducted to detect any anomalous patterns indicative of hostname casing manipulation attempts.