A vulnerability has been identified in Mendix Runtime V10 (All versions only if the basic authentication mechanism is used by the application), Mendix Runtime V10.12 (All versions only if the basic authentication mechanism is used by the application), Mendix Runtime V10.6 (All versions only if the basic authentication mechanism is used by the application), Mendix Runtime V8 (All versions < V8.18.32 only if the basic authentication mechanism is used by the application), Mendix Runtime V9 (All versions only if the basic authentication mechanism is used by the application). The authentication mechanism of affected applications contains an observable response discrepancy vulnerability when validating usernames. This could allow unauthenticated remote attackers to distinguish between valid and invalid usernames.