Today, Opera released a patch which attempts to address one of the issue I reported them in February, 2013.
The most likely attack is that a remote attacker may trick a user to visit a specially crafted web page; and to perform undisclosed operation (social engineering). As a result, a malicious executable file may be planted on the user's computer.
Either social engineering or a second bug is required to plant malware, so it's reasonable to assign moderate severity to the issue.
Another, probably less likely attack vector includes that an attacker has physical access to the machine. When the file downloads are blocked by security policy it may be easily possible for the attacker to bypass it and plant malware. This is a risk, specially in a corporate environment.
Firefox (Bug 845880), Chrome (Issue 177980), and Safari (Follow-up: 260250675) are affected.
Internet Explorer 10 is not affected. I concluded they recognized this issue internally and implemented defense.
I haven't verified Opera's patch yet.
Technical description scheduled to be posted later this year.