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pmuellr is Patrick Mueller, Senior Node Engineer at NodeSource.

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Wednesday, April 11, 2007

json array exploit

Unless you've been living in a cave for the last week or so, you are familiar with JSON exploit as publicized by Fortify Software. I believe this is the same exploit I first read about on Robert Yates' blog. Original reference was from Joe Walker.

I'm not going to claim to be an expert on this particular exploit, but if I understand the situation correctly, I have a few issues with the current concerns.

  1. It's clearly not just JSON data which is suspect here. Anything which appears to be a valid (or maybe even somewhat valid) chunk of JavaScript, which someone could access via <script src=> is suspect. How many resources like this might be available at your site? How 'valid' (in terms of valid JavaScript) does it have to be? Only folks who are intimately familiar with the code of the JavaScript interpreters we use, can say for sure. Let's also not forget E4X; could your XML data be suspect?

  2. This appears to be a problem today just for Mozilla / Firefox. And I believe the reason is the advanced functionality it provides in JavaScript with coolio getter/setter capabilities. (BTW, ActionScript 3 also supports getter/setter capabilities). Is this actually a bug? Doesn't seem like it. Is there some chance this functionality could be locked down during script/src inclusion? Seems unlikely (slippery slope; what else would you lock down?). Could the enhanced functionality be turned off for user-land scripts? Probably; I don't suspect too many people are actually making use of the functionality. There's an open bug @ Mozilla on the issue that has additional thoughts.

  3. I'm clearly showing my n00biness here, but why aren't even simpler exploits possible, on all the browsers? Before doing a script/src, redefine a well-known object/function with one that does some hijacking? The security model for JavaScript is complex; so complex; too complex.

  4. Ted Husted pointed out that one way to fix this problem is to enclose your JSON in comment characters, and then strip the comments before parsing the JSON. That's great, unless I want to actually include comments inside my JSON. And assuming that JavaScript comments can't be 'recursive' (not specified at moz's js doc). Why would I want comments in my JSON? Why not? If you can have them in XML, I'm not sure why we won't someday want them in JSON. Note that I don't consider RFC 4627 to be the final statement on JSON syntax.

  5. The fact that JSON is also a syntactically valid chunk of JavaScript is smelling worse and worse all the time. I can't imagine a situation where I'd eval() JSON data in a production app; it's just so much safer to parse the data yourself. So, why don't we use this opportunity to come up with YASDN (yet another simple data notation)? JSON is great because it's readable, but I've already pointed out how we could make it even MORE readable.

  6. Isn't it about time we got a little better modularity available in JavaScript, beyond the script/src story. Which is more or less the same thing as building a C program in one big monolithic file that pulls in functionality with #include. So 1970's ...

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