CoStar Group is trading at $43.63, up 51.65% in after-hours trading, after closing at $28.77 on September 22, 2026; no clear catalyst has been identified. - No credible company-specific announcement, earnings release, acquisition, or analyst action was found after the close. - The latest company release was Homes.com’s August housing-market report, published at 9:00 AM on September 22. - The broader market was mixed, offering no clear macro explanation for the outsized move.