Billion laughs

It isn't obvious at first glance that this small xml file actually expands to billion "lols". You really have to give the bad guys credit for ingenuity.

Probably should rename this to "billion reposts."<p>Can we move beyond this simple issue and discuss more complicated aspects of security on HN?

So, how much memory would a real-world parser actually consume given this file? I'd try it, but I had to RMA my workstation's motherboard yesterday, leaving me with a machine that only has 3GB, which is the obvious minimum for a full expansion. But I could imagine an XML parser might use UCS-2 internally, inflating this to 6GB. Or, some parsers might be clever and not attempt a full expansion.

Is there a legitimate use case for being able to recursively define entities like that?

I have to say, i love this, crazy, for a language that is really for transferring data.<p>I guess you could do this with YAML too?

This doesn't work with my sed-based XML parsers. :(

so, how do I protect myself against this?