Researchers discovered a critical flaw in Opera GX that allowed malicious websites to silently auto-install a browser mod (a cosmetic add-on) without any user interaction or approval, then exploit it to steal data from other sites the victim visited. By packing the mod with ~150,000 CSS rules, attackers could reconstruct sensitive information like a Gmail address character by character through a cross-site leak technique, all within seconds of the victim landing on the malicious page. Opera has patched the vulnerability in version 130.0.5847.89, rated it their highest severity (P1), and paid the maximum $5,000 bounty, though the underlying auto-install behavior had been flagged as a risk since 2023.
