Opera before 12.01 on Windows and UNIX, and before 11.66 and 12.x before 12.01 on Mac OS X, does not properly escape characters in DOM elements, which makes it easier for remote attackers to bypass cross-site scripting (XSS) protection mechanisms via a crafted HTML document.

Opera before 12.01 allows remote attackers to cause a denial of service (application crash) via a crafted web site, as demonstrated by the Lenovo "Shop now" page.

Opera before 12.01 on Windows and UNIX, and before 11.66 and 12.x before 12.01 on Mac OS X, ignores some characters in HTML documents in unspecified circumstances, which makes it easier for remote attackers to conduct cross-site scripting (XSS) attacks via a crafted document.