CVE 2015-3753
WebKit in Apple Safari before 6.2.8, 7.x before 7.1.8, and 8.x before 8.0.8, as used in iOS before 8.4.1 and other products, does not properly perform taint checking for CANVAS elements, which allows remote attackers to bypass the Same Origin Policy and obtain sensitive image data by leveraging a redirect to a data:image resource.
See the 
CVE page on Mitre.org
for more details.
