Abstract: Devices and techniques are generally described for fraud detection. In various examples, a first plurality of requests are received over a first time period. In at least some examples, the first plurality of requests may be requests to access a first service. A hierarchical data representation comprising an ordered set of values may be determined for each request of the first plurality of requests. A first subset of the first plurality of requests may be determined. The hierarchical data representation associated with each request of the first subset may include fewer than a threshold number of value substitutions relative to each other hierarchical data representation of the first subset of the first plurality of requests. Access to the first service may be prevented by subsequent requests associated with the first subset of the first plurality of requests.