Client-side tracking is tracking behavior carried out in the user’s browser, using scripts, browser APIs, or other page logic rather than relying only on server logs. Instead of waiting for a request to reach the backend, the site observes what the browser can reveal: page interactions, timing, screen and device traits, canvas or audio behavior, and other signals that can be combined into a fingerprint.
This matters because client-side collection can be harder to notice, easier to spread across many pages, and more resilient than cookies alone. Even if a user clears storage or blocks some identifiers, a tracker may still recognize the browser through subtle, stable features. In attacks and privacy abuse, this can support re-identification, cross-site profiling, or covert monitoring. Defenders reduce the risk by limiting third-party scripts, auditing browser APIs in use, applying data minimization, and making tracking logic transparent and consent-based.