CAPTCHA stands for Completely Automated Public Turing test to tell Computers and Humans Apart. CAPTCHAs present visitors with tasks that are easy for humans but — in theory — difficult for automated systems: identifying objects in images, solving puzzles, clicking specific regions, or simply checking a box while behavioural signals are analysed in the background.
Common CAPTCHA implementations include Google reCAPTCHA v2 (image grid challenges), reCAPTCHA v3 (invisible, score-based), hCaptcha (similar to reCAPTCHA v2, privacy-focused), Cloudflare Turnstile (behavioural + JavaScript), and GeeTest (slide and rotate puzzles). Each has its own detection mechanism and evasion profile.
Automated CAPTCHA solving typically works through one of two approaches: human-assisted solving services that route challenges to human workers, or AI-based solvers trained on challenge types. AlterLab's Tier 5 integrates automated challenge resolution for major CAPTCHA providers. For sites that require CAPTCHA solving, the resolution latency (typically 3-30 seconds for human solving) must be accounted for in pipeline design.