Broken Gates: Seven Solver Services and Six LLM Browser Agents Defeat CAPTCHA Defenses at Negligible Cost

arXiv 2607.18659·high signal

A systematic re-evaluation of hCaptcha, reCaptcha v2, reCaptcha v3, and Cloudflare Turnstile against six LLM browser-agent configurations (cloud-hosted, self-hosted, AI-assisted, browser-extension) and seven commercial solver services. Challenge-based defenses showed near-perfect bypass at negligible cost; non-interactive reCaptcha v3 resisted better but still fell once execution-environment authenticity was matched. The paper's key isolation: execution-environment authenticity, not agent behavior, is what actually determines whether a defense holds — meaning behavioral bot detection is the wrong layer to invest in.