Parse Server is an open source backend that can be deployed to any infrastructure that can run Node.js. Prior to 9.6.0-alpha.21 and 8.6.45, an unauthenticated attacker can crash the Parse Server process by sending a single request with deeply nested query condition operators. This terminates the server and denies service to all connected clients. Starting in version 9.6.0-alpha.21 and 8.6.45, a depth limit for query condition operator nesting has been added via the "requestComplexity.queryDepth" server option. The option is disabled by default to avoid a breaking change. To mitigate, upgrade and set the option to a value appropriate for your app. No known workarounds are available.