Techniques for code modification for detecting abnormal activity are described. Web code is obtained. Modified web code is generated by changing a particular programmatic element to a modified programmatic element throughout the web code. Instrumentation code is generated configured to monitor and report on one or more interactions with versions of the particular programmatic element. The instrumentation code is caused to be provided in association with the modified web code to the first client device in response to the first request from the first client device. Report data generated by the instrumentation code is received. The report data describes abnormal activity at the first client device, the abnormal activity comprising an interaction with a version of the particular programmatic element that does not exist in the modified web code. Based on the report, it is determined that the first client device is likely controlled by malware.