More than a security problem
It's not just a security problem: There's more than just web browsers using http. Any automated tool that pulls over HTTP could potentially be broken by this sort of injection because it is expecting the remote end to respond the way it always has -- without injected code. It may have hardcoded offsets, etc. The average person (cough, or even more inept, the average lawmaker) of course doesn't ever think about this but most of the communication on the internet is automated, not interactive. We're wiring in toasters, cameras, cars... we're wiring in all our public utilities infrastructure too. And a lot of the time, those things are accessed over http -- you think your wifi box is the only thing with that? Try again: The chemical factory four blocks from where you live probably has similar web management interfaces. Or the sewage processing plant. The internet of things is here, and the most common cause of failure isn't hackers or terrorists -- and it never was. Failure usually doesn't come to the screams of "Death to America!" or "Gimmie your money, bitch!"... it's to the quiet sound of "Oops."
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Network neutrality isn't about bandwidth or protecting competition and all that jazz... it's about protecting the integrity of the communications themselves. When you start screwing with the content, you're making assumptions. Comcast is assuming that only web browsers live on port 80. Engineers know better.
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Enlightened engineers are hoping the world comes to its senses and bans this sort of thing before something bad happens. Which is adorable. The more practical of us are just waiting out the inevitable death, destruction, and mayhem that's going to happen eventually, because we know that lawmakers are like children: No matter how many times you tell them not to touch the hot stove, they're not gonna listen, so you just sit back and let them burn their hands... not because you hate the kid, but because that's just the nature of what a child is.
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So, a word of advice: If Comcast serves internet in your area and you live next to the sewage processing plant, invest in a pair of rubber wading boots.