[haberman]

Your threat model seems to be that anyone who sends input to a sandboxed app should be assumed to have full control of it. But the apps frequently process input without wanting to give the sender of that input access to all its resources.

By your analysis, Gmail could implement a feature that lets a sender run arbitrary JavaScript in the recipient's browser, and this would have no security impact as long as the JavaScript sandbox was not escaped. But in reality this would be a huge breach, because there are valuable things inside the sandbox that attackers should not have access to.

Put another way, this wouldn't help defend Chrome from NaCl, but it would help defend the NaCl app from it's clients. This would be in Google's interest to implement because it would make the platform more attractive to developers.

[tptacek]

That seems like an uncommon scenario for the kinds of things NACL gets used for. These are intrinsically server-controlled applications, when they're applications at all (my real sense of what NACL is about is "another tool to migrate away from things like Flash and plugins; ie: a platform for video codecs).

I see your point. I guess you're saying, there could be a photo editing app in which Alice can send pictures to Bob, and Mallory might send a malicious picture to Alice that coerces her client into betraying all its photos.