Monitoring and Detecting Anomalous Behavior in APIs

Many attacks against APIs don't depend on a technical vulnerability but on legitimate use taken to the extreme, such as mass scraping of user data or slow enumeration of identifiers to exploit BOLA, and that kind of abuse is only detected by observing behavior over time, not by inspecting individual requests. Effective monitoring starts by establishing a baseline of normal traffic per endpoint, client and user, and then watching for deviations such as volume spikes from a single account, sequences of IDs queried in order that suggest automation, or geographic patterns inconsistent with usual use. A valuable practice is correlating signals from different layers, for example crossing high 403 error rates for a client with attempts to access resources outside its scope, which usually indicates an active attempt to exploit authorization rather than an integration error. Without this continuous visibility, a patient attacker can operate for weeks extracting data in small volumes that individually seem harmless but together represent a massive information leak.