Exposure of Sensitive Information to an Unauthorized Actor
Opera before 12.15 does not properly block top-level domains in Set-Cookie headers, which allows remote attackers to obtain sensitive information by leveraging control of a different web site in the same top-level domain.
CWE-200 - Information Exposure
An information exposure vulnerability is categorized as an information flow (IF) weakness, which can potentially allow unauthorized access to otherwise classified information in the application, such as confidential personal information (demographics, financials, health records, etc.), business secrets, and the application's internal environment.